Western Pine Beetle Biological Control

Dendroctonus brevicomis LeConte

From: Bellows, Thomas S. ,Carol Meisenbacher, and Richard C. Reardon, 1998, Biological Control of Arthropod Forest Pests of the Western United States: A Review and Recommendations, USDA, FS, FHTET-96-21.

Origin: North America.

Range in North America: Arizona, British Columbia, California, Colorado, Idaho, Montana, Nebraska, New Mexico, Oregon, Utah, Washington.

Plant host and damage: Pinus ponderosa, Pinus coulteri. Adults bore into bark, create egg galleries in the bark, and oviposit. Larvae feed between the bark and the wood. Heavy infestations kill the host tree.

Natural Enemies: Natural enemies known from D. brevicomis include vertebrate predators, invertebrate predators and parasitoids (Table 4).

Table 4. Natural enemies reported from Dendroctonus brevicomis

¹ Otvos 1965
² DeMars et al. 1986
³ Goheen et al. 1985
⁴ Stephen and Dahlsten 1976

Pest Status: In endemic populations this beetle breeds in overmature, windfall, or root-rotted trees (Keen and Salman 1942, Cobb et al. 1974), or in trees weakened by drought or fires. Under epidemic conditions, the species will attack and kill apparently vigorous trees of all ages, although trees under 15 cm diameter are seldom attacked. The beetle does not breed in limbs. Extensive damage of up to 90% of a forest stand has been recorded. From 1921-1937, 18.6 billion board feet of lumber were destroyed; this infestation was related to drought in the 1920's and 1930's.

Adults are active from late spring and early summer until the advent of cold weather. Females produce from one to three broods. There are from one to two generations annually in the northern part of this species' range, and from two to four in the southern portion of its range (Furniss and Carolin 1977). Attacked trees are often also attacked by other bark beetles, including Ips spp. and other Dendroctonus spp.

Management of this species has usually focused on reducing the impact of epidemics, usually by felling infested trees and either harvesting the lumber or destroying the brood in some other way.

Biological Control: Dendroctonus brevicomis is attacked by several invertebrate predators and parasitoids (Table 4), and ecotypically similar fauna make up the predatory and parasitic communities of bark beetles throughout the world. In California, over 90% of the predators feeding in broods of D. brevicomis were either E. lecontei or Temnochila chlorodia (Berryman 1967). A study of the laboratory biology of E. lecontei, the most common predator of this beetle, was conducted by Berryman (1967), who found 0.26 prey consumed per predator per day, a rate confirmed by field studies (Berryman 1967). Berryman (1966) reported a maximum oviposition by E. lecontei of over 1,000 eggs by a single female over 90 days.

Many of these natural enemies are attracted to infested trees (or to trees at high risk of infestation) as early as are the bark beetles (DeMars et al. 1986, Byers 1988). In a study by Stephen and Dahlsten (1976), predators that feed on adults and larvae arrived first, and parasitoids arrived later, coincident with the appearance of their later larval host stages. Attraction of predators to infested or at-risk trees is attributed to sensitivity to chemicals exuded either by the tree or by the attacking bark beetles. Sensitivity to various component chemicals of beetle aggregation pheromones have been examined in several predators (Tilden et al. 1983, Byers 1988, Payne 1989).

The impact of various management strategies on natural enemies of bark beetles can be considerable. Swezey and Dahlsten (1983) found, on logs treated with lindane, 89% reduction in emergence of predators and 80% reduction in the ratio of emerging predators to emerging bark beetle. In a comparative study of three insecticides, chlorpyrifos was equally toxic to D. brevicomis, E. lecontei, and T. chlorodia; lindane was more toxic to E. lecontei and T. chlorodia than to the bark beetle; and carbaryl was relatively nontoxic to E. lecontei compared with the other two species.

Removal to trap or bait trees can have a differential impact on pests and natural enemies, as natural enemies often emerge after the bark beetles. Removal of infested timber after the emergence of bark beetles but before the emergence of natural enemies could contribute to an imbalance in natural enemy and pest populations.

Vetebrate predators of this pest include birds. Otvos (1965) found, in decreasing order of importance, the woodpeckers Dendrocopus villosus, Dendrocopus pubescens, Dendrocopus albolarvatus, and Dryocopus pileatus preying on D. brevicomis (see also Berryman et al. 1970). Otvos (1965) noted additional bird species (creepers, nuthatches and flycatchers) thought likely to prey on this pest. Predation by woodpeckers reduced the population of D. brevicomis by direct consumption of 31.8% of the insects in the two generations studied. In addition, there was an increase in parasitoid densities by a factor of 3.85. This increase in parasitoid density was attributed to the removal of bark by the birds, so that parasitoids with short ovipositors (Cecidostiba spp. and Roptrocerus spp.) were more effective.

Recommendations: Natural variations in the density of this bark beetle in its principal host, Pinus ponderosa, appear more closely related to variations in tree stand health and vigor than to natural enemy action. The pest species is attacked by a diverse group of invertebrate predators and parasitoids and birds, which together appear likely to contribute considerable mortality to populations of the bark beetle in most situations. Conditions favorable to epidemics are often related to increase in numbers of susceptible trees (such as through drought or windfall). During epidemics, sanitation may play an important role in removing or reducing populations of the beetle, but care must be taken not to differentially remove natural enemies.

Little additional biological control work appears practical at this time for this species. Some consideration might be given to the possible impact of predators of other bark beetles, but the potential for increasing the natural mortality of D. brevicomis may be limited. The introduction of exotic bark beetle predators against native bark beetles has been attempted, but has not yet proved successful (Moser 1989, Van Driesche et al. 1996).

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