Host suitability studies of Douglas-fir and white alder to the gypsy moth Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2227mr95k

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  • Douglas-fir, Pseudotsuga menziesii and white alder, Alnus rhombifolia, species that are prevalent in the Pacific Northwest, were fed to larvae of the gypsy moth, Lymantria dispar L. Gypsy moth larvae from different familial lines (egg masses) from a single geographic population were evaluated on these hosts. The larvae were fed leaves from the two tree species characterized by different foliar levels of either nitrogen, moisture, phenols, or terpenes. Of the two tree species evaluated as hosts of the gypsy moth, white alder appeared to be a highly suitable host, whereas Douglas-fir was a sub-optimal host. Nitrogen and the allelochemics in the foliage affected larval development. On white alder, foliage from trees that had the higher phenolic levels resulted in increased larval survival and higher fecundity. On Douglas-fir, foliage high in nitrogen had lower phenolics and this resulted in increased larval survival and heavier pupae when compared to foliage with low nitrogen and high phenols. Performance of the gypsy moth on these hosts was affected by unknown intrinsic variation among familial lines. Larval survival and duration varied more than other developmental variables among larval familial lines. The variation in larval survival among familial lines was more pronounced in larvae fed Douglas-fir than those fed white alder. A developmental index involving developmental rate and pupal weight differed more for females among familial lines than for males, suggesting a differential sex response. Experiments, manipulating foliar nitrogen, terpenes and phenols of Douglas-fir were conducted to assess the effects on the development of gypsy moth larvae. Foliar chemicals were manipulated by fertilizing three-year old potted trees with two levels of nitrogen (0 ppm and 200 ppm). Gypsy moth pupal weights were significantly greater at the higher foliar N level (1.69%) than at the lower foliar N level (0.40%). The pupal weights were positively correlated with foliar nitrogen, negatively correlated with foliar phenols, and showed no correlation with foliar terpenes. The number of days to pupation and the number of days from pupation to adult emergence were not significantly different between treatments. The effects of terpenes and phenols on larval development of the gypsy moth were studied using artificial diet. The influence of dietary nitrogen on the response of the gypsy moth to the Douglas-fir allelochemics was also studied. The effects were assayed using neonate and later (third and fourth) instar larvae of the gypsy moth. The terpenes in Douglas-fir foliar extracts had little effect on neonate fitness indices. The phenols, however suppressed larval growth and survival. When terpenes and phenols were combined, larval growth and survival was suppressed suggesting a synergistic effect. This synergism depended on levels of dietary nitrogen. Higher levels of dietary nitrogen alleviated the detrimental effects of Douglas-fir plant extracts on neonate and third instar larvae. However, fourth instars performed better on diets with lower nitrogen (2.5 to 2.7% N) than on diets with higher nitrogen (4.1 to 4.5% N) as demonstrated by higher growth rates, consumption rates, higher assimilation efficiencies and higher efficiencies of conversion of digested food.
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