Parasites and predators of Dendroctonus ponderosae Hopkins (Coleoptera : Scolytidae) in ponderosa pine

Abstract

The bark beetle Dendroctonus ponderosae Hopkins (Coleoptera: Scolytidae) has caused severe mortality in recent decades to young stands of ponderosa pine in the western United States. Insect parasites and predators, believed to be of importance in the population dynamics of the beetle, were the subject of the thesis research, conducted in eastern Oregon. An annotated catalog of parasites and predators of D. ponderosae found in ponderosa pine is given. Parasites encountered were few both in number of species and relative abundance, and generally were limited in distribution to the upper boles of infested trees. As a result, they were considered of little importance as mortality factors. Medetera aldrichii Wheeler (Diptera: Dolichopodida) was the most abundant and widely distributed predator collected from infested ponderosa pine. The coleopteran predators Enoclerus sphegeus Fabricius, E. lecontei (Wolcott) (Coleoptera: Cleridae) and Temnochila virescens Mannerheim (Coleoptera: Ostomidae) were found in relatively low numbers. Other insect species present in sufficient numbers to Influence bark beetle brood survival, if predaceous, included Lonchaea sp. (Diptera: Lonchaeidae); Rhizophagus sculpturatus Mannerheim (Coleoptera: Rhizophagidae); Aulonium longum LeConte (Coleoptera: Colyiidae); Othnius sp. (Coleoptera: Othniidae); Corticeus sp. (Coleoptera: Tenebrionidae) and species of Staphylinidae (order Coleoptera). Five infestations, representing three distinct levels in a bark beetle gradation (enzootic, epizootic, decline from epizootic), were sampled to determine if differences in species composition and abundance of natural enemies existed among infestation levels. No differences in species composition of known predators nor of the more common associated insects possibly predaceous in habit were found. M. aldrichii was the only predatory species to exhibit a significant numerical (density dependent) response to increasing bark beetle infestation level. On basis of this attribute and its relative abundance, this species was considered by far the most important insect enemy of D. ponderosae in young ponderosa pine. Results suggest that M. aldrichii, as a natural regulatory agent of D. ponderosae, exerts little influence in preventing bark beetle outbreaks in ponderosa pine but may significantly accelerate population decline. Since all other predatory insects showed no evident response to host density increases, their combined influence on bark beetle broods would be greatest at low population levels. The spatial distribution of the more common predators within a given infestation was determined. Density of M. aldrichii was found to be positively correlated with tree diameter. Analysis of predator and bark beetle abundance in relation to subcortical moisture of host trees revealed that M. aldrichii was more susceptible to low moisture extremes than either the prey or common coleopteran predators. Larvae of M. aldrichii were not encountered in trees, sampled just prior to beetle emergence, in which subcortical tissues had dried below the fiber saturation point. As a result, survival of N. aldrichii and its effectiveness as a predator would be reduced in individual trees that dry rapidly and excessivly after bark beetle attack. Although mature larvae of E. sphegeus, E. lecontei and T. virescens appeared able to withstand low subcortical moisture extremes, early instar larvae may be susceptible. In epizootic bark beetle infestations M. aldrchii occurred in greatest numbers, shortly after bark beetle attack, in trees with high prey densities. At the relatively low predator to prey ratios present in study areas, predation in trees of high prey density could have done little more than reduce intraspecific competition of bark beetle brood.