|Abstract or Summary
- The bionomics of Medetera aldrichii Wheeler (Diptera:
Dolichopodidae), a predator of the Douglas-fir beetle (Dendroctonus
pseudotsugae Hopkins (Coleoptera:Scolytidae)), was investigated
in western Oregon.
The predator pupated in host galleries proximal to bark beetle
ventilation holes. The tendency of prepupal larvae to form pupal
cells near illuminated areas in a photo-response apparatus suggested
that light entering the bark of infested trees through ventilation
holes may enable the larvae to locate potential exit sites.
Larvae maintained under a 16:8 LD, 200 FC regime for 11 to 14
days pupated with significantly higher frequency than larvae maintained
in darkness, indicating that light may also stimulate pupation.
The photo-positive response of the pupa in a laboratory apparatus
simulating the bark habitat suggested that it also orients to light
when moving from the pupation site to the bark surface.
Red pigmented mites and Collembola inhabiting the bark of
host-infested trees are common components of the diet of the
predaceous adults. Flies fed Onchiurus sp. (Collembola:
Poduridae) consumed approximately 40 per day over a 13-15 day
Mating, observed in the laboratory, is by superposition and
lasts 20 to 30 minutes. Both sexes mate more than once and may
copulate several times in a single day. Females maintained in the
laboratory in small cages containing host-infested bark oviposited
up to 630 eggs. The potential egg production of an individual, based
on the maximum observed rate of oocyte turnover, and a longevity
of 36 days, approximates 750 eggs.
Field collected gravid females, isolated, from host-infested
bark for five hours, deposited eggs in the pleats of paper cups during
brief exposures to small quantities of volatilized commercial alphapinene.
The response to the pinene, a common fraction of the
oleoresins of various tree species which habor bark-beetle hosts of
the fly, suggests that olfactory stimuli, released as the beetles mine
the phloem, may guide in the selection of predator oviposition sites.
The newly eclosed larva, provided with well developed
pseudopodia, moves over the bark surface and enters the host gallery
through a beetle entrance hole. Olfactometric studies indicated that the larva is strongly attracted to such holes, apparently orienting
klinotactically to volatile materials, including alpha and beta-pinene,
escaping from them.
The attack and feeding behavior was observed in Lucite-bark
sandwiches infested with developing broods of the Douglas-fir
beetle. The larvae cannot penetrate un-mined phloem and gallery
penetration is restricted until drying creates a bark-wood interspace.
Predators feeding at the terminus of extended mines are
usually isolated from other prey and completely consume each host
before initiating new attacks. At high prey densities, however, the
hosts galleries may be contiguous, enabling the predator to attack
several prey in quick succession.
Larvae maintained individually in plastic arenas in the laboratory
consumed an average of 15 Douglas-fir beetle larvae during
their development. Since the number of prey consumed decreased
with the size of the prey, predators feeding on the largest prey instar
acceptable, during each of the three stadia, required an average of
6.2 prey to achieve maturity.
The impact of the predator on field populations of the Douglas-fir
beetle was estimated by comparing bark beetle mortality in control
and predator infested samples of five trees. At a mean predator
density of 5.7 third-instar larvae per square-foot, mortality in
the infested samples was 81.8 percent; significantly different from
the 62.5 percent mortality occurring in the controls. Each predator
larvae recovered at the end of the study killed an average of 3.7
hosts that would have been expected to survive in the absence of