A contribution to the biology of the Pseudohylesinus nebulosus (LeConte) (Coleoptera:Scolytidae), especially in relation to the moisture stress of its host, Douglas-fir Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/8g84mq65w

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  • The study (1) describes the life cycle of P. nebulosus, (2) examines stimuli that may cause the beetles to locate brood material, and (3) establishes the relationship between moisture stress in Douglas-fir and colonization by P. nebuZosus.and other meristem insects. (1) Development of P. nebulosus goes through the egg stage, three larval instars, and the pupal and callow adult stages. Teneral adults emerge from late spring through fall, disperse, and feed in tissues of live Douglas-fir twigs before attaining sexual maturity. Progeny initiated in early spring may become capable of reproduction and initiate colonization of susceptible host material in fall. P. nebulosus overwinters in all stages except the egg and pupal stages, hibernating in feeding tunnels or galleries of newly colonized breeding sites. The main breeding period is the early spring. Females begin gallery construction. (2) The flight of immature beetles is governed by temperature, but is induced by light and appears primarily oriented toward light. Positive photic response appears to overpower response to vegetative stimuli. Temperature induces a reversal in the beetle's photic response at two thresholds (15.5°C and 34°C). Decrease of light intensity to 17 f.c. induces a light negative and thigmotactic response. Two temperatures (19°C and 34°C) limit the range for flight take-off, which occurs only under conditions inducing positive phototropism in the beetle. Field tests failed to attract flying immature beetles to materials known as highly attractive to mature beetles. In laboratory tests, walking beetles responded to ethanol vapors in darkness but were unresponsive in the light. Circumstantial evidence suggests that the beetles alight into Douglas-fir crowns by chance and respond to feeding stimuli under condition-favoring negative phototaxis and thigmotaxis. The flight of mature P. nebulosus differs from the flight of immature beetles primarily in orientation. Field tests showed strong response by mature P. nebulosus to fresh Douglas-fir twigs and stem sections as well as to certain monoterpene solutions and ethanol. Ethanol was also attractive to Pseudohylesinus grandis Sw.; Dendroctonus pseudotsugae Hopk.; Hy1astes nigrinus (Mann); Gnathotriohus spp.; Trypodondron linoatum Oliver; and Anisandrus pyri (Peck). This suggests the possibility that ethanol is a constituent of primary host-attraction also to other scolytids besides timber beetles as suggested by others. The bimodal flight pattern of mature P. nebulosus and its possible causes are discussed. It is possible that trees under midday moisture deficits may become a temporary attraction source and 'absorb' part of the airborne population. (3) Diurnal and seasonal changes in moisture relations of Douglas-fir were determined by Scholander's pressure chamber technique of measuring plant moisture stress (PMS) in vascular plants. The ecological interpretation of PMS, determined at dawn and at midday, is discussed. The relationship between PMS in Douglas-fir and colonization by meristem insects was established by correlating the infestation status of a tree with its PMS (diurnal minimum) condition. PMS thresholds were found which indicate whether a host tree is in a condition (a) inducive; (b) tolerant, or (c) abortive to initial colonization by P. nebulosus, Scolytus unispinosus Lec., D. pseudotsugae (Scolytidae), and Melanophila drummondi Kirby (Buprestidae). These PMS thresholds appear specific for each insect species. The relation between the cardinal PMS condition in the insect-host relationship and other physiological parameters of the tree, such as photosynthetic activity, cell division, etc., are discussed. Only trees under PMS levels indicative of physiological condition with curtailed integrative processes become inducive to attacks by meristem insects. Ecological, silvical and entomological conclusions based on the obtained results may be important in forest management considerations regarding reduction of damage caused by bark beetles.
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