Studies on the olfactory behavior of the Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins Public Deposited

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  • Behavioral responses exhibited by the Douglas-fir beetle, Dendroctonus pseudotsugae Hopkins, to various attractant sources were investigated under both laboratory and field conditions. Tests were designed to observe olfactory responses. Results were correlated and compared with collections from sampling nets and with emergence patterns. Field data were collected during the flight seasons of 1962, 1963, and 1964. Laboratory tests revealed that the attractant produced by females boring in both host and non-host logs arrested adult beetles. Crawling beetles did not exhibit a directed response to attractive materials without the presence of an air stream passing over the material toward the insect. Beetles crawling near to an attractant source were passively arrested. A histological study of both male and female digestive tracts indicated that the midgut may be the point of origin of the attractant material. Rapid changes in the appearance of the ventricular epithelium that occurred shortly after feeding indicated a disintegration of nucleated cells into the lumen. Since this phenomenon was much less pronounced in the male, it was thought to include more than simple digestive processes, and therefore be responsible for release of the attractant compound by the female. Differences in hindgut epithelium associated with feeding were not apparent. The occurrence of seasonal activities depends upon the time of spring emergence and is directly regulated by environmental conditions. The exact time of emergence depends upon climatic conditions during the period of development and maturation. The beetle is stimulated to flight by temperature and light, but later olfactory stimuli predominate in regulation of its flight. Field attraction by logs of various tree species, attack and subsequent brood development were studied. The finding that the unmated female produces an attractant while feeding on phloem of various tree species was confirmed. Attack and development varied between the different tree species. Response of field populations to attraction produced by reemerged females was less than response to virgin females. Significant responses were obtained using low concentrations of Douglas-fir oleoresin and pine oleoresin fractions. Largest responses occurred with 2-1/2 percent Douglas-fir oleoresin. Tests with fractions of pine oleoresin showed that alpha pinene was the most attractive constituent, followed by limonene. Tests confirmed that Douglas-fir beetles respond to fresh uninfested Douglas-fir logs. The freshness of cut definitely regulated the responses obtained. The sex ratio of two females to one male responding to oleoresin compounds and to fresh cut logs indicated the importance of oleoresin vapors in host selection, since the female initiates the gallery. Oleoresin functions as an attractant for the beetles in flight but at high concentrations repels crawling beetles. Investigations confirmed that the number of beetles attracted decreases immediately after mating. Those beetles mated early in the morning before flight had started remained nearly nonattractive all day, while check samples continued to show the expected diurnal pattern of response. Experiments with boring dust extracts collected in the laboratory and later tested in the field suggested the volatility of the attractive substance. Limited attraction to dust extracts collected in water as opposed to those collected in ethyl alcohol indicated that the attractant is too volatile to be retained in water.
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