Mountain Pine Beetle Biological Control

Dendroctonus ponderosae Hopkins

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: Throughout the pine forests of Alberta, British Columbia, the western United States, and Mexico.

Plant hosts and damage: Principal hosts: Pinus albicaulis, Pinus contorta var. latifolia, Pinus lambertiana, Pinus monticola, Pinus nigra, Pinus ponderosa, Pinus sylvestris. Other pines are also recorded as hosts. Adults bore into bark, create egg galleries between the bark and the wood, and oviposit. Larvae feed between the bark and the wood. Heavy infestations kill the host tree.

Natural Enemies: Natural enemies of this species include woodpeckers, predaceous and parasitic insects (Table 5), and nematodes (Reid 1963, Rasmussen 1976).

Table 5. Invertebrate natural enemies of Dendroctonus ponderosae

¹Rasmussen 1976
²Schmid 1969
³Furniss and Carolin 1977

Pest Status: This beetle ranks first in destructiveness among the bark beetles in the western United States (Furniss and Carolin 1977). It was responsible for tremendous losses in several outbreaks in the 20th century, including the loss of 15 billion board feet in Idaho and Montana from 1911 to 1935. In lodgepole pine, this species infests mature forests, often over large areas. In ponderosa, western white, and sugar pines, group killing often on a large scale can occur in both mature forests and in young overstocked stands. This beetle is epidemic almost continuously somewhere in the West on one or more of its principal hosts. During endemic infestations, the beetle tends to attack weaker, less vigorous trees. During epidemic infestations, healthy trees are also attacked. Over most of its range, the beetle is univoltine. In the more northerly regions, it may require 2 years to complete a single generation. In parts of California, two or more generations may develop in a single year.

Natural factors that affect the abundance of this species include subzero winter temperatures, nematodes, and various natural enemies. As stand susceptibility to the beetle increases, the suppression by natural controls is overcome by beetle reproduction, and outbreaks may develop. Harvesting susceptible stands before outbreaks or relieving stand stress (as by thinning) may be of value in preventing an outbreak. Control before an outbreak begins is important because of the large areas over which measures must be applied once an outbreak is underway. Direct control measures against outbreaks, such as felling, burning, and related approaches, are considered uneconomical. The development of pheromone traps may assist in control or at least detection of incipient outbreaks.

Spacing within stands may have considerable bearing on the susceptibility of trees to this pest. Cochran and Barrett (1993) demonstrated marked increases in pine mortality due to this beetle as stand density increased. Wide spacing increased average tree volumes, increased mean diameters, and reduced the probability of mortality without sacrificing gross cubic volume potential.

Biological Control: Survival of this pest and the role of natural enemies have been evaluated in both endemic and epidemic populations. Beetle survival was significantly greater in endemic (3.7%) than in epidemic (1.4%) or post-epidemic (0.5%) infestations (Amman 1984). Parasitoids and predators accounted for 8% of beetle mortality in endemic populations, 33% in epidemic populations, and 4% in post-epidemic populations. The dolichopodid predator Medetera aldrichii (13%) and woodpeckers (15%) accounted for the greatest amount of predation in endemic populations in that study.

Schmid, in a series of studies, evaluated the role of several predators on this beetle. The asillid fly Laphria gilva took 1% of adults (Schmid 1969). The clerid beetle Enoclerus sphegeus fed on D. ponderosae in the laboratory (Schmid 1970). Each clerid adult killed about one adult of D. ponderosae per day, and each larval clerid killed about 25 larvae, pupae or teneral adults of D. ponderosae during development. It was estimated that adult clerids consumed less than 1% of the adult bark beetle population and that the larvae killed 5-11% of the brood. Larval M. aldrichii entered galleries of the bark beetle through the entrance holes made by the adult scolytids (Schmid 1971), and probably caused a major part of the bark beetle mortality between August and the following May. Amman (1972) concluded from laboratory studies of the clerid Thanasimus undatulus that each clerid larva would consume between 18 and 43 larvae of scolytid, depending on the size and abundance of the prey.

The natural enemies known for this species are likely most important in limiting or controlling populations in the endemic phase. In outbreak phase, natural enemies appear less able to exert sufficient limits on the population.

Recommendations: The opportunities for enhancing biological control of this species may be limited. There is a diverse natural enemy fauna already attacking this species. Biological control by augumentation has been suggested as a possible tactic against this beetle. Research into the possibility of rearing some of the more common predaceous beetles that attack bark beetles may lead to alternative approaches to management of incipient infestations. Following studies of spot infestations of this beetle, Borden (1993) suggested that spot infestations can be depended upon to expand and form subsequent outbreaks if not treated before adult beetles emerge. In an unrelated report, Moeck and Safranyik (1983) suggested that the main tactic for increasing biological control of this pest should be inundative releases of native clerids against low intensity outbreaks. Employing inundation against spot infestations may prove successful, but no studies have been reported. The major hurdles to overcome will be the cost of production and release, and the technology of mass rearing numbers sufficient to treat the areas involved.

Many management options are available during bark beetle infestations. Among these are various forms of harvesting, destruction, or removal of the infested or at-risk trees. Care should be exercised to conduct such operations with a clear understanding of the biology of the natural enemies involved. Many natural enemies of bark beetles use kairomonal cues to locate their hosts or prey (Chatelain and Schenk 1984), and will attack beetle infestations in trees and logs. Untimely removal of infested trees and logs could remove large numbers of natural enemies from the system. Similarly, pheromone traps can attract and kill large numbers of natural enemies that are using these chemical cues to locate host-infested trees. Careful integration of management options, based on the status of the forest stands at risk, the biology of the pest species, and the responses of the natural enemies, will be necessary to optimize the reduction of the beetle population while retaining the beneficial effect of the natural enemy population (Nebecker 1989).

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