Genetic Variation in the Timing of Larval Mortality and Plant Tissue Responses Associated with Tree Resistance against Galling Adelgids

Abstract

Large genetic variation in resistance to galling adelgids, Adelges japonicus exists in ezo-spruces, Picea jezoensis. To address genetic differences in the mechanisms of tree resistance against A. japonicus, we examined the timing of larval mortality and tissue responses in a number of susceptible and resistant clones of P. jezoensis. We placed a winged adult on a branch in each of 21 clones, and examined the survival of the next generation. Although the number of larvae that settled on buds in autumn did not differ between susceptible and resistant clones, galls were induced almost exclusively on susceptible clones, indicating that resistance was manifested as greater mortality of fundatrices when they were sedentary on buds. However, the mortality was greater only in spring on resistant clones from the Oketo population, while the mortality was greater in both winter and spring on resistant clones from the other populations than on susceptible clones. Brown lesions with necrotic cells were observed in attacked tissue in both resistant and susceptible clones. The proportion of buds with lesions was greater only in resistant clones from non-Oketo populations than in susceptible clones suggesting that hypersensitive responses may have been involved in these resistant clones. Larval size was smaller on resistant clones from non-Oketo populations than that on susceptible clones. These results suggest that resistance exhibited in different timings had different underlying mechanisms.