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.