Graduate Thesis Or Dissertation

The bionomics of Medetera aldrichii Wheeler (Diptera: Dolichopodidae) in western Oregon

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  • 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 period. 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 predation.
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