The Southern Pine Beetle
Chapter 2: Life History and Habits
Thomas L. Payne – Department of Entomology, Texas A. & M. University, and Texas Agricultural Experiment Station, College Station, TX.
The southern pine beetle (SPB) — Dendroctonus frontalis Zimmermann (Coleoptera: Scolytidae) — is the most destructive insect pest of pine forests in 13 Southeastern States and in parts of Mexico and Central America. This is a well-worn statement but nonetheless richly deserved and quite accurate. The beetle ideally represents the definition of its genus — killer of trees.
The southern pine beetle is one of more than 12 American species of Dendroctonus. It is a primary bark beetle pest, attacking several coniferous species throughout its range. The SPB is an aggressive tree killer that can attack and overcome healthy, vigorous trees when its populations are large (epidemic). But its success is somewhat limited when its populations are quite low (endemic) and attacks are confined to weakened or dying trees, host material attacked by other insects, particularly the omnipresent Ips species, or even downed timber.
Most outbreaks are of relatively short duration (e.g., 2 to 3 years). This fact has led to the belief that the beetle is cyclical in nature, particularly since major epidemics seem to occur about every 10 years. In fact, a feature that differentiates SPB somewhat from other species of Dendroctonus is its decided periodicity in the level of activity where outbreaks have recurred over the years. MacAndrews (1926 unpublished) summed up the situation aptly: "It is either abundant, killing up to 50 percent of the stands of pine over large areas and killing out groups of pine here and there throughout the country, or so rare during the intervening years that it is difficult even to make collections."
Somewhere within the beetle’s range, epidemic populations may be found almost every year. And beetle activity fluctuates significantly in local areas and across the range of the insect (fig. 2-1). In Texas, for example, infestation levels have fluctuated dramatically over the last 20 years but not necessarily on a typical 10-year cycle (fig. 2-2).
Single Year Map of Outbreaks
|Figure 2-1 – Distribution of southern pine beetle infestations in the United States from 1960 through 1996.|
For more detailed outbreak distribution information please see: A History of Southern Pine Beetle Outbreaks In The Southeastern United States
Figure 2-2 – Number of southern pine beetle infestations detected in Texas, 1958-1979 (after Texas Forest Service 1978).
Adult SPB attack the living host tree by boring through the bark and feeding upon the phloem tissue, where they also oviposit for the next generation. Their ability to overtake host trees is due, in part, to their mass attack on trees over a relatively short period of time. At times, such behavior makes it possible for them to overcome even the most resistant host. Also, the beetle produces multiple overlapping generations each year throughout its range — a fact that adds to its effectiveness as a destructive pest.
Long before formal records of its damage were kept, accounts suggest that the southern pine beetle plagued virgin southern yellow pine forests over large areas in the late 1700’s and early 1800’s. Price and Dogget (1978) found accounts, from Moravian settlers and others dating back to 1750, describing the destruction of vast amounts of pine timber due to the "mischief" of what appears to have been bark beetles. Oldtimers in east Texas report that early in this century settlers used beetle infestations to clear the land for pasture. First they hit the trees with the back of an ax and then leaned infested "sticks" against them (J.P. Vité personal communication).
No doubt some of the earliest accounts reported damage due to more than one species of bark beetle: pre-nineteenth century observers did not have the benefit of Dr. Charles Zimmermann’s initial description of the species (1868). However, it is most probable that SPB were involved in many of these outbreaks.
Early accounts of tree mortality caused by the southern pine beetle are fragmentary, but one can still determine its general impact. For example, St. George and Beal (1929) reported that in a single outbreak, timber valued at $2 million was destroyed and that timber killed by SPB from 1891 to 1929 had a value of at least $50 million. Records compiled from sketchy data by Price and Doggett (1978) for 1882 to 1960 showed that the SPB was responsible for killing over 200,000 cords and 500 million board feet of timber.
Since 1960, more accurate records have been kept on the damage caused by the beetle in the Southeast. Data compiled from 1960 to 1978 for 12 States (Alabama, Arkansas, Florida, Georgia, Kentucky, Louisiana, Mississippi, North and South Carolina, Tennessee, Texas, and Virginia) show an estimated total volume of timber killed of nearly 9 million cords and 3 billion board feet. This loss has been valued at more than $225 million. Fluctuations in the size of the infested areas and volume killed each year over that time period have been quite striking.
Considerable destruction by the beetle has also occurred in Mexico and Honduras; however, documentation of losses in these countries is much less complete than in the United States. Fox et al. (1964) reported that an infestation in Honduras extended over 4.9 million acres (2 million ha) from 1962 through 1964. Devastation of pine forests in Mexico and Central America by the SPB is undoubtedly much more extensive than available records indicate.
As would be expected, the significance of the southern pine beetle as a forest pest has stimulated much concern and numerous investigations. Hopkins (1909b) carried out a monumental study to describe aspects of the biology and behavior of the beetle. Since that time, other studies have examined the problem from all angles (see reviews by Thatcher 1960, Dixon and Osgood 1961, Coulson et al. 1972b).
Although we have learned a lot about the pest, its host, and associates, we have not come up with effective means for dealing with the beetle on a long-term basis. It is not surprising, then, that our need for an integrated pest management system — a system that would incorporate detailed knowledge of the pest, its host, and the environment as a functional component of overall forest management — became apparent in the early 1970’s. At that time a massive outbreak occurred in 10 Southeastern States. Under the USDA Expanded Southern Pine Beetle Research and Applications Program (ESPBRAP), knowledge of the life history and habits of the SPB has been greatly expanded. This chapter, a blend of previous and new knowledge, explains our current understanding of the life history and habits of the southern pine beetle.
Zimmermann originally described the southern pine beetle in 1868, placing it in the family Hylurgidae under the tribe Hylurgi. He synonymized it with Bostrichus frontalis Fabr. in his description. This was later corrected by Le Conte (1876). SPB was placed in the family Scolytidae. In 1963, Stephen Wood synonymized Dendroctonus arizonicus Hopkins, which occurred in Arizona and New Mexico, and Dendroctonus mexicanus Hopkins, which occurred in Mexico, with Dendroctonus frontalis Zimmermann. Later Rose (1966 unpublished) suggested distinct differences between D. frontalis and D. mexicanus based to a large extent on host preferences, and recommended that further investigations be undertaken.
After years of controversy about the beetle’s taxonomy, Vité et al. (1974) provided conclusive evidence, based upon biological and biochemical studies, that D. frontalis and D. mexicanus are two separate species. They found that beetles from Texas, Virginia, and Arizona differed significantly from the Mexican species in aspects of external morphology, gallery construction, host species preferences, structure of the male seminal rod, and in pheromone production. In addition, the Texas beetles failed to breed with the Mexican beetles. Their findings prompted Wood (1974) to reinstate D. mexicanus as a valid species. Furthermore, their findings have been corroborated by subsequent efforts. Lanier (1977 unpublished) showed through breeding experiments and karyotype analyses that D. frontalis from the southeastern United States, Arizona, and Mexico are conspecific; whereas D. mexicanus is chromosomally distinct and reproductively isolated from them. He confirmed the seminal rod differences between the species and that where D. frontalis occurs in Mexico, it is found on host species different from D. mexicanus. This finding was also reported by Hendrichs (1977 unpublished).
There has been confusion as to the distribution of the southern pine beetle in the United States (i.e., was it in Arizona?) and Central America (i.e., was it in Mexico, or was the beetle D. mexicanus?). The studies of Vité et al. (1974, 1975) and the revision by Wood (1974), coupled with the surveys of Lanier (1977 unpublished) and Hendrichs (1977 unpublished), have provided an accurate account of the present distribution of the beetle (fig. 2-3).
The southern pine beetle occurs in North America south of a line from New Jersey to central Arizona, south in Central America to northern Nicaragua. It has also been reported in Delaware, Pennsylvania, Ohio, New Jersey, Indiana, Illinois, and Missouri (St. George and Beal, 1929). Vité (1974) noted that under the present concept of the geographical distribution of the beetle, two large areas are involved — the southern and southeastern United States, where the distribution is continuous and roughly coincides with the distribution of loblolly pine (Pinus taeda L.), and an area ranging from Arizona to Honduras, where the populations are not so continuous, being interrupted by the Isthmus of Tehuantepec and Guatemala.
Figure 2-3 – Present known distribution of the southern
pine beetle (after Hendrichs 1977 unpublished).
Anderson, Berisford, and Kimmich (1979) found significant differences in the electrophoretic analyses of six genes in beetles from Texas, Georgia, Virginia, Arizona, and Mexico. Although D. frontalis populations occur in Arizona and Mexico, they appear to have become genetically differentiated from each other. Genetic evidence supports the possibility that the disjunct Mexican and Arizonian populations of the beetle diverged from the main body of the species in the southern and southeastern United States.
The southern pine beetle has been reported to attack and kill all pine species in its range (Hopkins 1909b, St. George and Beal 1929, Dixon and Osgood 1961). In the Southeastern States it prefers loblolly and shortleaf pine (P. echinata Mill.) but has successfully colonized pitch pine (P. rigida Mill.), Virginia pine (P. virginiana Mill.), table-mountain pine (P. pungens Lamb.), eastern white pine (P. strobes L.), longleaf pine (P. palustris Mill.), spruce pine (P. glabra Walt.), slash pine (P. elliotti Engel.), as well as red spruce (Picea rubens Sarg.) and Norway spruce (P. abies L.). SPB has also attacked and killed Japanese red pine (P. densiflora Sieb. and Zucc.), red pine (P. resinosa Ait.), and pond pine (P. serotina Michx.). In Arizona and New Mexico, the SPB has been reported only from ponderosa pine (P. ponderosae Laws.) (Hopkins 1909b, Wood 1963). However, more recent investigations revealed that its attacks are limited to Apache pine (P. engelmannii Carr.) (Vité et al. 1974, 1975; Lanier 1977 unpublished; Hendrichs 1977 unpublished). Exceptional hosts (e.g., P. strobus or Picea spp.) are occasionally attacked in a "spill over" during an epidemic in the preferred host types. Such exotic species do not support epidemics, though (J.P. Vité personal communication).
In the northeastern part of its range in Mexico, the southern pine beetle attacks P. teocote Schiede and Deppe on the gulf side of the Sierra Madre Orientale in Nuevo León (Vité et al. 1974, Lanier 1977 unpublished, Hendrichs 1977 unpublished). In southern Mexico, it is found at lower elevations coinciding with the range of P. oocarpa Schiede, on the gulf side of the Sierra Madre Orientale and the plateaus of Chiapas, as well as the Pacific slopes of Chiapas, the Sierra Madres del Sur, and the Sierra Madre Occidentale (Lanier 1977 unpublished and Hendrichs 1977 unpublished). SPB has also been found in Pringle pine (P. pringlei Shaw), in Guerrero on the Pacific coast (Hendrichs 1977 unpublished).
In Honduras the SPB occurs at lower elevations coincident with P. oocarpa but has also been reported in P. pseudostrobus Lindl. (Vité et al. 1974, 1975; Hendrichs 1977 unpublished). It has been found only in P. oocarpa in Nicaragua (Vité et al. 1974, 1975). The beetle has been reported from El Salvador, but the host species was not given (Hendrichs 1977 unpublished). The presence of the beetle in P. oocarpa along the Guatemalan border of Mexico and in Honduras and Nicaragua suggests its presence in Guatemala, since the host is abundant; however, the SPB has yet to be reported from that country.
The southern pine beetle is a multivoltine species with a complete metamorphosis consisting of the egg, larval, pupal, and adult stages. Detailed descriptions of the life stages were presented by Hopkins (1909b), and have been subsequently added to by others (references in Thatcher 1960 and Dixon and Osgood 1961).
Figure 2-4A – Life stages of the
southern pine beetle: egg.
Figure 2-4B – Life stages of the
southern pine beetle: larva.
Figure 2-4C – Life stages of the
southern pine beetle: pupa.
Figure 2-4D – Life stages of the
southern pine beetle: callow adult.
Figure 2-4E – Life stages of the
southern pine beetle: mature adult.
The egg is slightly oblong to oval with rounded ends (fig. 2-4A). It is opaque, pearly white, and shiny, measuring about 1.5 mm long by 1 mm wide. The egg stage lasts from 3 to 11 days, at a temperature range of 30 degrees to 15 degrees C and as long as 34 days at temperatures as low as 10 degrees C (Gagne 1980 unpublished).
The larva is a subcylindrical, wrinkled, legless grub with 3 thoracic and 10 abdominal segments (fig. 2-4B). It is yellowish white in color. Upon emergence from the egg, the larva is curved and approximately 2 mm long. Its head is prominent, having well-developed mouthparts with the mandibles stout and dark. In fact, the mandibles begin to show through the egg covering approximately 1 day before eclosion. The head and last abdominal segment are clothed with a few long, white hairs. The mature larva is 5 to 7 mm long. Its body is essentially straight, with the head a reddish color, and with frontal elevations or tubercles and a few long hairs. The mandibles are reddish black with obscure antennae situated in depressions just above the bases of the mandibles.
Fronk (1947) investigated the larval instars using head capsule measurements and Dyar’s Law, and determined that the SPB has four instars with the following ranges in head width: 1st instar — 0.294-0.336 mm; 2nd instar — 0.378-0.504 mm; 3rd instar — 0.547-0.672 mm; 4th instar — 0.756-0.960 mm. Fronk pointed out that the largest individuals of one instar may be larger than the smallest individuals of the next larger instar. Goldman and Franklin (1977) and Mizell and Nebeker (1979) also found four larval instars in their investigations. Fronk’s ratio of increase for instar growth (1.39) fell within the range found by Mizell and Nebeker (1.34-1.44). The overall larval stage lasts from 15 to 40 days, over a temperature range of 25 degrees to 15 degrees C (Gagne 1980 unpublished). Individual larval stages have durations ranging from 7 to 13 days (Fronk 1947).
The pupa has the general color of the larva (yellowish white) and is fragile. It has the form of the adult, but with the wing pads and legs folded beneath and the abdominal segments exposed (fig. 2-4C). Fleshy tubercles and spines are present on the posterior edges of its second and seventh abdominal segments. The front of the head has a groove. Pupae range in size from 3 to 4 mm in length. The pupal stage lasts 5 to 17 days, over a temperature range of 30 degrees to 15 degrees C (Gagne 1980 unpublished).
New callow adults are yellowish white (fig. 2-4D). They change from this color to yellowish brown to reddish brown, finally becoming dark brown approximately 1 week before the adult is ready to emerge from the host tree. This stage lasts from 6 to 14 days, over a temperature range of 30 degrees to 15 degrees C (Gagne 1980 unpublished).
The adult SPB is cylindrical and somewhat stout to elongated (fig. 2-4E). It is 2 to 4 mm in length and brownish to black in color. The head is broad and prominent, with well-developed chewing mouthparts and median elevations forming a distinct frontal groove. The front of the head is coarsely punctured and channeled in both sexes. The elevations, or tubercles, are rougher and more acute on the male, while the middle front of the female’s head is more convex and shiny. The back of the head is thickly covered with very fine punctures.
The eyes are compound, round to oval and are situated behind the base of each antenna. The antennae are seven-segmented, consisting of the basal pedicel, elongated scape, four-segmented funicle, and an enlarged club.
The prothorax is shiny and slightly narrowed toward the head. Its surface is thinly covered with different-sized punctures and a relatively smooth, distinct dorsal line. The elytra have fine to coarse rubosites between rows of obscure to distinct punctures. The elytral declivity is convex. Females are distinguished from males by the presence of a transverse, rather broad elevated ridge, called a mycangium, on the anterior pronotum. Males lack the mycangium but have a distinct frontal groove, and elevations or tubercles on the head are more distinct.
The duration from egg to adult ranges from 26 to 54 days, depending upon the season (Thatcher 1960, 1967). The beetle may have as few as three generations per year in the northern part of its range (North Carolina, Virginia) and as many as seven to nine generations per year in the southern parts of its range (Texas, Honduras) (Thatcher 1960). However, the subject of discrete generations in the higher numbers has been questioned, due to the overlapping of successive generations (see Chapter 5).
MacAndrews (1926 unpublished) presented the following early predictive model by which to determine the time of successive generations: "The emergence of the first generation was correlated with the opening of the blossoms of the flame-colored azalea (Rhododendron calendulaceum). That of the second with the opening of mountain laurel (Kalmia latifolia) blossoms. The third with sourwood (Oxydendrum aboreum) blossoms."
Factors Influencing Development
There are several abiotic and biotic factors that influence the development of the beetle through its life stages. Temperature probably represents the greatest single abiotic influence and generally affects the developmental rates of the various stages as well as their behavior (Fronk 1947, Bremer 1967 unpublished, White and Franklin 1976, Gagne 1980 unpublished; see Chapter 5). Parasites and predators of the various life stages, as well as competitors for the beetle’s food supply, have significant effects on the life stages of the beetle (Dixon and Payne 1979b, Birch et al. 1980, T.D. Paine personal communication; see Chapters 3 and 5). These probably represent the more important biotic influences, along with tree physiology and site and stand parameters, which affect host susceptibility (Hodges et al. 1979; see Chapter 6).
The life cycle of the southern pine beetle can be characterized as a sequence of behavior components that culminate in propagation of the species. The sequence begins with the emergence of brood adults from their host trees. They fly from the host tree where they developed to a new host three, where they bore through the bark and start constructing galleries in the phloem-cambium tissues. Just prior to or at the onset of the boring activity, the adults release pheromones (secondary attractants). Perception of the pheromones, as well as host odors released from the freshly wounded tree, stimulates aggregation on the tree by other SPB in the area. As these beetles attack the tree, they also release pheromones, which, along with host odors, attract more beetles. As a result of this aggregation behavior, the tree is successfully attacked, mating takes place, egg galleries are constructed, eggs are deposited, broods develop, and adults emerge to attack new host trees.
Although biological systems generally defy precise behavioral classifications, the activities of the SPB can be broadly classified in terms of host selection, aggregation, colonization, reemergence and emergence, dispersal, and overwintering (Wood 1972, Vité and Francke 1976).
Conceptually, host selection has been attributed to the efforts of beetles that initially attack susceptible host trees. They are commonly referred to as "pioneer" beetles (Borden 1974). Pioneers are essential, for they must successfully establish a focal point for the next generation. For the SPB, females are responsible for host selection. The females must locate suitable host trees without the aid of secondary attractants and thus are the first to become established in new host trees. Male SPB enter the picture only after the females have selected and successfully attacked a host and secondary attraction has been initiated.
Several investigators have indicated that beetle populations behave differently in the winter, spring, summer, and fall (Thatcher and Pickard 1967, Franklin 1970a, Hedden and Billings 1977, Billings 1979; see Chapter 5). This is consistent with the seasonal behavior of other organisms. SPB disperse in the fall, so that by winter the populations are often scattered throughout the forest in single trees and small infestations. Some beetles may also remain in small groups of infested trees in larger spots. These populations remain dispersed until spring, and development proceeds at a slow rate. Some infestations are associated with lightning-struck trees (Hodges and Pickard 1971). In general, overwintering spots seem insignificant because of their small size and widespread distribution, the lack of spot growth, and very slow crown discoloration. However, with the arrival of warm spring weather, the picture often changes dramatically. During March through May, the emergence and flight of brood adults lead to the initiation and growth of larger infestations. During the summer months, infested trees deteriorate more rapidly, brood development accelerates, and the beetles remain within the infestations, contributing to spot growth.
Fall- and spring-dispersing SPB are likely to be true pioneer beetles in that they select suitable host trees in the uninfested surrounding forest without the benefit of secondary attractants. By comparison, summer-emerging beetles are likely to be continually affected by the presence of secondary attractants coming from newly attacked trees at the edges of active infestations. Host selection might seldom occur during the summer, when continued emergence and reemergence prevents the collapse of aggregation within existing spots (Gara 1967). These SPB can overcome tree resistance and continually attack new trees over time. The beetles generally attack new host trees near the old ones.
Overwintering beetles generally do not develop at a rate that would provide massive populations to attack large numbers of new host trees. Furthermore, colder temperatures greatly reduce emergence and any subsequent flight. Because of prolonged development times and the absence of favorable weather conditions through the winter, secondary attractants are less likely to be available near suitable host trees. Beetles emerging during winter and early spring would have trouble finding newly attacked trees. As a result, they would be stimulated to disperse and engage in host selection rather than attack trees within the infestation areas where they developed (Gara 1967). When temperatures are low, beetles may not fly but simply migrate to and attack unattacked portions of the same trees in which they developed (Thatcher and Pickard 1964).
Fat content. — The fat content of emerging beetles may be an important factor in the seasonal behavior of SPB. Fat content is commonly used as a measure of the energy available for flight and subsequent colonization.
Spring- and fall-emerging beetles in Texas have significantly more fat than those emerging in the summer and winter, and thus are better equipped for dispersal. Female beetles have a higher fat content than males (Hedden and Billings 1977). This should be expected, however, since females are responsible for host selection, aggregation, and reproduction.
|Figure 2-5A – Seasonal variation in pheromone content
of the female southern pine beetle - Frontalin.
|Figure 2-5B – Seasonal variation in pheromone content
of the female southern pine beetle - trans Verbenol.
Primary Attraction or Random Landing?
Figure 2-6 – Seasonal variation in response to an
attractant (frontalin, verbenone, and turpentine) by
southern pine beetles in the laboratory (1975-1977).
There are two main hypotheses about how beetles locate and select hosts. Some investigators have proposed primary attraction via olfactory stimuli as the means by which the beetle accomplishes host selection. "Primary" is used to reflect that the phenomenon takes place as a result of some stimulus released from the host tree before any beetle visits it. That is, the host tree does not provide a source of "secondary attraction" via beetle-produced volatiles. It has been hypothesized that pioneer beetles are attracted to susceptible hosts by changes in the volatile compounds resulting from deterioration of the plant tissues (Person 1931, Heikkenen 1977). This phenomenon has been shown for species of ambrosia beetle (e.g., Moeck 1970); however, definitive experiments have not been carried out that confirm the primary attraction phenomenon for SPB.
Random landing by dispersing beetles has been proposed as another means the SPB uses to locate and select its host, guided only by its strong preference to land on vertical objects (Gara, Vité, and Cramer 1965). Hypothetically, beetles land at random on both host and nonhost trees. Once on a host tree, female beetles bite the outer bark in response to chemical stimuli there (Thomas, Richmond, and Bradley 1979). If the SPB female identifies a suitable host, she initiates boring activity, and the aggregation phase of the beetle’s life cycle begins. If the host is unsuitable, she flies on to another tree.
Once a few beetles have selected a susceptible host tree, secondary attraction begins. As a result, other beetles begin to aggregate on the tree. This phase of the beetle’s life cycle is critical: it enables the insects to arrive on the host tree in sufficient numbers and over a short enough period of time to overcome the natural resistance of the tree. It is unlikely that a single beetle could successfully colonize a tree since the resin pressure would usually pitch it out. With multiple attacks, however, the tree becomes weakened, and continuing attacks result in successful colonization.
We do not know how many successful attacks it takes to initiate aggregation behavior. Theoretically, one beetle could initiate secondary attraction. We do know, however, that the process is heavily dependent upon the perception of both beetle- and host-tree-produced volatiles and their effects on the flying beetles (Payne 1979).
In general, the olfactory organs of insects are located on the antennae; this is the case with SPB. It is possible to investigate the beetle’s olfactory sense at the single-cell and whole antenna (electroantennogram) levels (fig. 2-7).
All of the structures (sensilla) that perceive odor are found on the distal segment of the antenna — the club (Dickens and Payne 1978a). The location and arrangement of the olfactory sensilla on the club are well adapted for the beetle’s needs. Most sensilla are located within the sensory bands, which encircle the club. Each club has hundreds of olfactory sensilla, and the cuticle of each individual sensillum is perforated with thousands of pores that collect the important air-borne molecules of pheromone and host odor from the environment surrounding the beetle. Ultimately the molecule-bound information is transferred through the central nervous system of the beetle and changed into a behavioral response.
Figure 2-7 – Schematic of olfactory sensillum and
whole antenna showing sensilla distribution (A)
electroantennogram (EAG), and (B) single-cell
recording techniques (after Payne 1979).
Several compounds have been isolated and identified from the beetle, host tree, associated microorganisms, and the beetle-host tree system (Appendix, table 1). All of the compounds have not been evaluated, but a few have significant effects on the beetle and are believed to play a role in its aggregation behavior.
Frontalin. — Frontalin is considered the primary aggregation pheromone of the southern pine beetle (Kinzer et al. 1969, Payne et al. 1978a). It is found in the hindguts of newly emerged female beetles (Costar and Vité 1972) and probably is released when they make contact with suitable host trees (Renwick and Vité 1969). In fact, by the time the female has fed, the level of frontalin has declined significantly (Coster and Vité 1972).
The pheromone is naturally synthesized in a ratio of 15 percent positive to 85 percent negative of its enantiomeric forms (Stewart et al. 1977). The beetle responds significantly more to the negative than the positive form; however, it responds as well to the racemic mixture of the two forms as to the negative form (Payne et al., unpublished).
By itself, frontalin attracts flying beetles of both sexes (Payne et al. 1978a). But in the presence of host odor, its effect is greatly enhanced (Kinzer et al. 1969; Payne et al. 1978a). About three times as many males are attracted to the pheromone as females. This predominantly male response may be due to testing procedures and the tendency of males to orient closer to the pheromone source than females (Hughes 1976). However, the entire pheromone complement of the beetle-tree system, including frontalin, causes male and female beetles to aggregate on host trees in a nearly 1:1 ratio (Coster et al. 1977a).
It is likely that frontalin functions primarily in close-range communication to keep individual SPB close together so that they are present in sufficient numbers to overcome the resistance of host trees. Frontalin probably does not function over long distances (Payne et al. 1978b, Johnson and Coster 1978). Along with trans-verbenol and host odor (i.e. –pinene), frontalin may promote close-range communication on the surface of the host tree, since in closely related species it has been shown to stimulate male beetles to produce an "attractive chirp" known to occur when the male is near the entrance hole of a female (Rudinsky 1973, Rudinsky et al. 1974).
Alpha-pinene. — Alpha-pinene has been singled out as the most significant host tree odor in the behavioral chemical complex of the SPB (Renwick and Vité 1969). By itself, the terpene is not attractive to field populations, nor is any other host tree odor. However, it does synergize the attractiveness of frontalin in aggregation beetles on host trees (Kinzer et al. 1969). The SPB probably does not rely on this terpene alone as its input from the host tree. In fact, turpentine tends to be a more effective synergist (Payne et al. 1978a), a fact suggesting that the host tree signal is not embodied in one compound. Although -pinene does not attract flying beetles, it is arrestive to walking beetles (McCarty et al. 1980). In combination with frontalin, it may serve to aid beetles in orientation on the surface of the host.
Alpha-pinene has been proposed to function as an arrestant in combination with frontalin (Renwick and Vité 1970, Payne 1973). That is, the pheromone attracts beetles to the tree, and the host tree odor arrests their flight so they land. The terpene has been shown to arrest beetles on nonsticky traps baited with frontalin, whereas beetles that responded to frontalin alone did not remain on the trap (J.A.A. Renwick and J.P. Vité personal communication).
Trans-verbenol. — Female beetles produce trans-verbenol (Renwick 1967). It is naturally synthesized in a ratio of 60 percent positive and 40 percent negative of its enantiomeric forms (Plummer et al. 1976). However, the behavioral effects of the enantiomers have not been determined. Trans-verbenol can synergize the attractiveness of frontalin (Kinzer et al. 1969, Payne et al. 1978a) and has been proposed as a substitute for host odors as resin exudation ceases (Renwick and Vité 1969). Trans-verbenol may also have an arresting effect on beetle flight (Dickens and Payne 1978b). On the host tree, it probably aids in close-range communication between the sexes, since in combination with frontalin and -pinene it was shown to elicit the attractant chirp from males (Rudinsky 1973; Rudinsky et al. 1974).
The compound is found in the hindgut (Renwick 1967), frass, and volatiles from SPB-infested host material (R.M. Silverstein and J.R. West personal communication). The level of trans-verbenol in the hindgut is influenced by exposure of the beetle to vapors of -pinene (Hughes 1973; Renwick, Hughes and Ty 1973). The biological significance of this apparent chemostimulated synthesis is unknown since newly emerged, unfed females contain up to 75 percent more trans-verbenol in their hindguts than do females that have entered the host, fed, and thus become greatly exposed to resin vapors (Coster and Vité 1972). Exposure of males to -pinene stimulated synthesis of trans-verbenol, which under other circumstances is not synthesized in that sex (Renwick et al. 1973).
Verbenone. — Verbenone is produced essentially by males and is found in the hindgut (Renwick 1967), as well as in the frass and volatiles from SPB-infested host material (R.M. Silverstein and J.R. West personal communication). The pheromone is also found in female beetles but in very small amounts.
Verbenone is believed to affect beetle behavior in several ways (Rudinsky 1973). At lower concentrations it affects beetles attracted to host trees by reducing the number of males and thereby balancing the sex ratio more toward 1:1 (Renwick and Vité 1969, Payne et al. 1978a). In higher amounts it tends to inhibit the aggregation of both males and females on host trees. In contrast, when released in very small amounts by the female, verbenone is believed to synergize the attractant pheromone mixture (frontalin, trans-verbenol, and host odor) in close-range orientation of males to the entrance holes of females (Rudinsky 1973). Experimentally, low concentrations of the pheromone have elicited attractant chirps from males. At higher concentrations, those believed to be principally associated with the male, verbenone elicits "rivalry chirps" from males.
Endo-brevicomin. — Endo-brevicomin is produced, in very small amounts, in the hindgut of the male beetle only (Pitmal et al. 1969) throughout most of its range. However, in Arizona-Honduras beetles, endo-brevicomin is found in greater amounts (Vité et al. 1974). It inhibits the response of both male and female SPB to attractive host trees and thus facilitates attacks on other new trees (Vité and Renwick 1971, Payne et al. 1978a). The pheromone may also contribute to male competition on the host tree since it has been shown to elicit rivalry chirps (Rudinsky et al. 1974).
Myrtenol. — Myrtenol is produced by both male and female beetles and is found in their hindguts (Hughes 1973, Renwick et al. 1973). In laboratory tests, it synergized the attractant mixture of frontalin and trans-verbenol, causing males to stop near the source of the pheromones (Rudinsky et al. 1974). When released by the female, myrtenol may have a similar function as that proposed for verbenone in helping males find the entrance holes of females.
Role of microorganisms. — The microorganisms associated with the southern pine beetle may be responsible in part for the ultimate composition of the behavioral chemical system that regulates its behavior. Mycangial fungi in female beetles, for example, are capable of oxidizing trans-verbenol to verbenone (Brand et al. 1976). The significance of this phenomenon in the behavior of the beetle is not known; however, both of the pheromones are important. A basidiomycete in the mycangium produces the compounds isoamyl alcohol, 6-methyl-5-hepten-2-one and 6-methyl-5-hepten-2-ol (Brand and Barras 1977). The behavioral significance of these compounds has not been determined; however, isoamyl alcohol does enhance the attractiveness of a pheromone mixture in laboratory bioassays. In fact, isoamyl acetate, 2-phenyl-ethanol, and 2-phenylethyl acetate — metabolites of three yeasts isolated from the beetle — are highly effective in synergizing unattractive concentrations of the attractant mixture of frontalin, trans-verbenol, and host odor (Brand et al. 1977). By themselves, the metabolites are unattractive. The metabolites are not attractive to field populations either, suggesting that they may function in close-range olfactory behavior of the beetle on the host tree.
It is unlikely that the behavioral chemical system of the southern pine beetle has been completely described. Many compounds have been isolated from the SPB and the beetle-host tree system, but few have been identified. All remain to be evaluated for their roles in the life cycle of the beetle.
Olfactory Receptor System
The beetle’s antennal olfactory receptor system uses available sense cells efficiently, in that several of the behavioral chemicals interact on some of the same receptors (Dickens and Payne 1977). At first it may appear that such a situation would prevent the beetle from determining if it should respond to an attractant or to an inhibitor if both stimulate the same receptor. But the beetle’s olfactory sense is quite sophisticated and can readily sense the difference.
The ability of the beetle to decipher the complex olfactory messages in its environment depends on the number and specificity of the receptors it has for various behavioral chemicals. Whether or not a pheromone will elicit a behavioral response is in part dependent upon the number of receptors stimulated. The apparent differences that exist in the number of receptors for the different compounds provide the beetle with the flexibility and versatility to perceive the chemicals and translate the information into behavioral responses. In addition, the rate at which the receptors recover from stimulation adds to the SPB’s versatility in perceiving the behavioral chemicals and the messages they carry.
From a simplistic view, the olfactory receptor system segregates the behavioral chemicals as attractants, inhibitors, and synergists. Although both male and female beetles respond to the same compound, their receptor systems differ (Dickens and Payne 1977). The attractants, inhibitors, and synergists form three discrete groups in the female receptor system (fig. 2-8). Receptors for frontalin form an all-inclusive group, while verbenone and endo-brevicomin form a second mutually exclusive group, which occupies 85 percent of the receptors for frontalin. Receptors for they synergists -pinene and trans-verbenol form a third group, which occupies between 33 and 48 percent of the frontalin receptors.
Figure 2-8 – Female southern pine beetle olfactory receptor system. Mean percent interaction of pheromones and host terpenes with frontalin acceptors. Width of columns represents X ± SE for each compound with the exception of frontalin (Dickens and Payne 1977).
Figure 2-9 – Male southern pine
beetle olfactory receptor system.
The olfactory receptor system of the male beetle differs considerably from that of the female (fig. 2-9). The inhibitors occupy 66 to 76 percent of the receptors for frontalin. The synergists occupy 44 to 68 percent of the receptors. The overlap of verbenone with both the synergist and inhibitor groups may have implications in the multifunctional characteristics of the pheromone (Rudinsky 1973, Rudinsky et al. 1974).
Southern pine beetles have the largest number of receptors for the attractant frontalin; all of the other compounds, both pheromones and host odors, share them, although not all of them. For example, the inhibitors endo-brevicomin and verbenone can stimulate 66 to 85 percent of the receptors, depending on whether the beetle is male or female. The synergists trans-verbenol and -pinene, on the other hand, can react with 33 to 68 percent of the receptors. This does not mean that when a beetle is smelling frontalin it cannot also smell another compound. Certainly not, or how could -pinene, or trans-verbenol for that matter, synergize the effect of frontalin?
In nature, the beetle is not likely to come in contact with such a concentration of any one compound (except possibly a host odor) that all of the receptors for the material would be occupied at the same time. More likely, the beetle has many receptors constantly receiving olfactory signals from different odors, such that the resulting behavior comes from an integration in the central nervous system of all of the information from those receptors (Payne 1979). Therefore, when the beetle’s receptors are receiving primarily frontalin and synergist stimulation, the pattern of signals arriving in the central nervous system elicits aggregation behavior. This phenomenon is continuous, and as the qualitative and quantitative characteristics of the odor stimuli change, different behaviors result.
As the concentration of attractant decreases and the concentration of inhibitors increases, changes occur in the pattern of the incoming signals to the central nervous system and in the resulting behavior. Receptors once stimulated by frontalin, or possibly a synergist, now become increasingly stimulated by endo-brevicomin and verbenone. The responding beetles become deterred from the attacked tree, and switching behavior (switching to another host tree) results. As the concentration of attractant begins to increase from the newly attacked trees, receptors once occupied by inhibitors are stimulated by the attractants and the beetle responds with aggregation behavior. All along, stimuli arriving in the central nervous system from other senses (e.g., sound and vision) become integrated with those arriving from the olfactory receptors and subsequently influence the ultimate behavioral response.
Our current understanding of the sequence of events in the aggregation phase of the SPB’s life cycle leaves us with an incomplete picture. But despite this fact, we can still begin to understand the events taking place in the interactions of the beetle and the host tree.
After selecting and attacking a suitable host tree, a female immediately begins to release the aggregation pheromone frontalin (Kinzer et al. 1969; Renwick and Vité 1969, 1970). Frontalin, along with host tree odors, attracts large numbers of male and female beetles to the tree. Males predominate. The initial attack and aggregation occur on the midbole of the tree (fig. 2-10) (Coster et al. 1977a, Fargo et al. 1979). As more females arrive, more pheromone is released and more beetles are subsequently attracted. Attacks then begin to spread from the midbole to the upper and lower areas of the bole (fig. 2-10). During the winter, however, additional attacks may be limited more to the upper bole of the tree (Thatcher and Pickard 1964). Aggregation behavior follows a diurnal pattern in summer, with peak flight activitiy at 5 p.m. (fig. 2-11) (Vité, Gara, and von Scheller 1964; Coster et al. 1977a and b). A bimodal pattern may occur in the spring when peak flight occurs at around 10 a.m. and 5 p.m.
Figure 2-10 – Height distribution of southern pine
beetles on host trees during the aggregation phase
(after Coster et al. 1977a).
Figure 2-11 – Diurnal distribution of southern pine
beetles on host trees during the aggregation phase
(after Coster et al. 1977b).
As each female arrives, she selects a position on the bark (usually a crevice), initiates boring, and releases frontalin. As long as the tree resists attack by exuding resin, the female continues to release the pheromone. But once she begins to feed, pheromone production declines and stops. During this activity the female may stridulate, sending off a series of chirps when near another female. Such signaling may have an intraspecific spacing function during the selection of entrance sites and may be caused by a chemostimulus (Rudinsky and Michael 1973).
Generally, at the point where each female enters the tree, a characteristic pitch tube forms as a result of the severing of resin ducts by the boring beetle (fig. 2-12). Pitch tube formation depends upon what condition the host tree is in and whether pitch flow has ceased as a result of other earlier attacks. Those beetles arriving late in the aggregation phase are less likely to stimulate pitch tube formation since the resin pressure in the tree has been reduced by beetles that attacked earlier.
Besides frontalin, the females also release trans-verbenol immediately upon landing on the host (Renwick and Vité 1969, 1970). Trans-verbenol enhances the aggregating effect of frontalin (Renwick and Vite 1969, Payne et al. 1978a). Once they have entered the tree, the females are believed to release small amounts of verbenone. This substance enhances the attractive effects of the pheromones and host odors in orienting males to the entrance holes once they have landed on the bark (Rudinsky 1973, Rudinsky et al. 1974).
When the males land on the host, they begin to search, presumably for the entrance hole of a female beetle. The males move over the bark, investigating crevices, entrance holes, and pitch tubes as they encounter them (Bunt 1979
Figure 2-12 – Characteristic pitch tubes on host
tree mass attacked by the southern pine beetle.
unpublished). Most of the males search in an upward direction from where they land on the bark. Some males orient directly to an entrance hole or pitch tube, and thus exhibit chemoklinotaxis. Others search randomly. Upon contact with an entrance hole, the male circles the hole, pokes its head and thorax inside, clears away frass, and sometimes swims in the resin, if there is any. In the female’s entrance hole, males frequently give off an audible sound or attractant chirp as a "presence-announcing" stridulation (Barr 1969, Rudinsky 1973), which is believed to be stimulated by female pheromones (Rudinsky 1973, Rudinsky et al. 1974).
In some cases, the male encounters another male while searching the bark for a female’s entrance hole (Bunt 1979 unpublished). Then the males may simply resume searching or drop from the host; however, direct combat can occur. Fighting most frequently takes place when confrontation occurs at a female’s entrance hole. Males may give off a "rivalry chirp." In any event, when combat ensues the males butt heads and generally the larger of the two drives the other away to search for another female.
When the male finally locates and enters the female’s entrance hole, he begins to release verbenone, which balances the sex ratio of responding beetles by reducing the response of males to females (Renwick 1969, 1970; Payne et al. 1978a). As higher amounts of verbenone are released, the response of both sexes is inhibited. In addition to verbenone, the males also release endo-brevicomin, which reduces the attraction of both males and females to the host tree (Vité and Renwick 1971, Payne et al. 1978a). As the population of males on the tree increases, so does the amount of verbenone and endo-brevicomin being released. As a result more and more males and females are deterred from the host, and the phenomenon of "switching" takes place (Gara and Coster 1968). The focus of aggregation and attack by the beetles is switched to an adjacent host tree, and the dynamic process begins all over.
Generally, only those trees within a critical distance of the attacked host are likely to come under attack by switching populations (Gara and Coster 1968). In large spots the shifting of attack can take place rapidly; and under the local influence of the aggregation pheromones, beetle attack may occur on more than one tree before the mass attack is complete on an individual tree. However, in small spots the pheromones are less profuse, and the attack remains focused on a single tree at a time. As the level of attraction increases on an adjacent tree, so does the focus of fight, landing, and boring activity. The success of the switching activity is to some extent dependent upon the proximity of adjacent host trees. The closer trees are to one another, the likelier it is that switching will take place and adjacent trees will be colonized (Gara and Coster 1968, Johnson and Coster 1978). (see Chapter 5 for a detailed discussion of infestation growth and proliferation.)
The aggregation phase is by far the most dynamic aspect of the life cycle of the beetle. During the warmer months of the year, when beetle populations are most active, the entire aggregation phase — including initial attack, mass attack, and switching — may be completed within 10 days (fig. 2-13) (Coster et al. 1977a, Fargo et al. 1979). In fact, in most cases a host tree can be completely mass attacked within 3 to 5 days after the first pioneer female lands on its bark. The rapid increase in beetle numbers aggregating on and mass attacking a tree, following by an equally rapid decline, can be attributed for the most part to the relative amounts of behavioral chemicals present over the aggregation and attacking period.
Figure 2-13 – Simplistic model of role of behavioral chemicals during the
aggregation phase (revised after Renwick and Vité 1969, Coster et al. 1977a).
Once the male joins the female on the host tree, mating occurs and the colonization phase of the beetle’s life cycle begins
Figure 2-14 – Stages of host tree colonization by the southern pine beetle.
The southern pine beetle is monogamous, and copulation takes place in the nuptial chamber formed in the inner bark by the female once resin flow has stopped (fig. 2-14) (Hopkins 1909b, Thatcher 1960). The nuptial chamber is somewhat shoe shaped and is formed directly opposite the entrance from the outer bark (MacAndrews 1926 unpublished).
Often, when resin flow is profuse, both the female and male work for some time in order to excavate an entrance hole and keep it open (Hopkins 1899). In some cases, they fail and become entombed in a resin-filled entrance hole or initial egg gallery. When the resin flow is persistent, the female may excavate a preliminary gallery that proceeds upward and laterally, often for some distance, in the outer area of the inner bark before the inner bark is completely penetrated. Generally these galleries have a short, curved form, lack beetles, and are packed with frass and hardened resin.
The southern pine beetle’s mating behavior has been observed under laboratory conditions (Yu and Tsao 1967). When the male reaches the female in the gallery, he moves to her posterior end and begins to remove the frass she has made. When that job is completed, he backs out of the gallery entirely or to a widened area of the gallery, turns around and backs in. When he meets the female, he mates with her end to end. In laboratory experiments, a single female mated with up to six different males when they were presented separately at the entrance hole. This suggests that although the SPB is monogamous and generally only one male and one female are found in a gallery, a given female could mate with more than one male.
Once the female has mated, she begins to construct an S-shaped or serpentine egg gallery (fig. 2-14) (Hopkins 1899, MacAndrews 1926 unpublished, Thatcher 1960). As debris (frass) accumulates in the gallery, the female pushes it back with her legs and packs it, using her abdomen like a scoop. The male follows the female and helps her remove boring particles from the area of current activity. However, the male contributes little to the female’s activities if she was previously mated (Yu and Tsao 1967). The female moves back and forth, packing pieces of bark down with her head and putting fallen pieces in place with her mouthparts. She keeps a space of approximately 15 to 25 mm clear of frass (Hopkins 1899).
The gallery is mined in the cambium diagonally across the grain of the wood and sometimes lightly scores the sapwood. It is always continuous, never branches, and forms a long, winding track such that as the host tree becomes heavily infested, individual galleries crisscross each other. Widened areas may be formed in the gallery wall to afford space for beetles to turn around (T.L. Wagner personal communication). Single galleries range from 10 to 24 cm in length.
When the egg gallery is approximately 2 to 3 cm long, the female begins to cut individual egg niches in the walls of the gallery. An egg is deposited in each niche and held in place by a thin wall of fine, tightly packed borings (Fronk 1947). Eggs are deposited at irregular intervals along the gallery at a rate of up to 30 per gallery (Lashomb and Nebeker 1979, T.L. Wagner personal communication).
Parent adults begin to emerge 1 to 3 days after mass attack, mating, and egg deposition (fig. 2-14) (Coulson et al. 1978). The scattered holes that appear on the bark surface at this stage were thought to be ventilation holes, before the extent of reemergence was recognized (T.L. Wagner personal communication). Depending on when a given adult entered the host during the aggregation phase, reemergence continues for 16 to 20 days. Once the parent adults have left a host, their role in colonization of that tree is over. But they continue to play a vital role in the dynamics of the infestation, because they remain capable of receiving olfactory signals, attacking new hosts, producing pheromones, mating, and laying eggs (Franklin 1970a, Coulson et al. 1978, Telfer 1979 unpublished, Cooper and Stephen 1979. see Chapter 5 for details.).
The eggs hatch in 2 to 9 days after being laid (Fronk 1947, Gagne 1980 unpublished). The emerging first-instar larva begins to bite and subsequently enters the cambium layer of the host. Initially it makes a fine, threadlike, gently winding gallery a few centimeters long in the cambium and perpendicular to the adult gallery. Then it enters the inner bark, where it spends most of its larval period. As the larva molts to each successive stage, the gallery enlarges (fig. 2-14). Some initial galleries are completely hidden in the phloem tissue. Others are exposed early or late in larval development. When nearly mature, the larva bores to the outer area of the inner bark; and in the fourth instar it bores to the outer, dead bark (Goldman and Franklin 1977).
Upon reaching the outer bark, the fully mature larva forms an oblong pupal cell (fig. 2-14). Occasionally, pupal cells are formed in a widened area of the larval mine within the inner bark, but normally they occur in the outer bark. Once the cell is formed, the mature larva transforms into the pupal stage.
The mature pupa transforms into a callow adult and remains in the pupal cell as the hardening and darkening process of the cuticle takes place. During this time, the adult changes from yellowish tan to reddish brown to its final color of black-brown (fig. 2-14).
Emergence, Dispersal, and Overwintering
Once the adult southern pine beetle has fully developed, it constructs an exit hole from the pupal cell by boring directly through the outer bark, leaving a clear-cut, open hole behind (fig. 2-14). If conditions are not correct, however, the adult may remain under the bark for some time. Generally this delay in emergence is associated with colder air temperature (Kinn 1978).
Emergence does not take place all at once. A few beetles emerge initially, followed by a larger number, and then a declining number over an extended period of time (see Chapter 5).
Environmental conditions affect beetle dispersal. During the winter, emerging beetles may not disperse, but instead reattack the same tree (Thatcher and Pickard 1964). Generally, though, emerged beetles leave the host tree and, depending upon the time of the year, either aggregate on adjacent trees under attack or leave the previously established center of attraction and find a suitable new host tree elsewhere (see Chapter 5).
The southern pine beetle overwinters in all life stages (MacAndrews 1926 unpublished). Mature larvae, pupae, and adults overwinter in the corky outer bark, while young larvae and eggs are found in the inner bark. The beetle does not go through a diapause. Development of all stages continues throughout the year, slowing considerably in the winter and accelerating in the spring and summer (Thatcher 1967).
Our knowledge of the life history and habits of the southern pine beetle has increased tremendously since the mid-1700’s, when the Moravians were remarking on its "mischief." The extensive works of Hopkins, MacAndrews, St. George and Beal, Fronk, and others in the early 1900’s have provided us with a good basic understanding of the general biology of the beetle, from which more detailed studies have been launched.
In the ensuing years, detailed studies have been carried out at both the basic and applied levels. Information has been gathered on the beetle’s biology and physiology, its interaction with the host, the influence of pathogens, parasites, predators, and associates on its populations, silvicultural influences, and chemical control. Since the late 1950’s, significant advances have been made in our understanding of the behavior of the beetle. Valuable insights were gained on the aggregation behavior of the beetle and the role of behavioral chemicals.
All of these efforts paved the way for much of the progress made in the Expanded Southern Pine Beetle Research and Applications Program in increasing our understanding of the beetle’s behavior and in our efforts to develop behavioral chemicals for use in pest management. Through the Program, information has continued to be collected and synthesized to provide us with a fuller understanding of the life history and habits of the beetle and insights into how we might manipulate its populations as part of forest management.
The author wishes to thank Dr. J.P. Vité Direktor, Forstzoologisches Institut, Universitat Freiburg, West Germany, for his critical review of this chapter, and Mrs. P.D. Billings for assistance in constructing the table.
This paper is a contribution from the Texas Agricultural Experiment Station (TAES #15693).
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