AN ALDER (ALNUS) HYBRID ZONE1

Population sizes of insect herbivores on natural plant hybrids may be higher, lower, or intermediate to those on the parent species. I studied the biology and population dynamics of a species of free-living aphid (Pterocallis alni) in a hybrid zone of alder (Alnus sp.). The parent trees were Alnus glutinosa and A. incana and their hybrid was A. pubescens. Hybrid trees differed from parents in phenological and morphological characters. Leaf trichome density on hybrid trees was intermediate to trichome densities on A. glutinosa (high density) and A. incana (low density), and thus appeared to be inherited in an additive manner. High trichome density on A. incana resulted in very high mortality and eventual elimination of aphids colonizing this tree. Aphid populations on the hybrid A. pubescens were higher than on A. incana, but lower than on A. glutinosa. However, aphid susceptibility did not appear to be inherited in a strictly additive fashion, as there was a dominance deviation in favor of the susceptible parent. This lack of additivity in insect resistance was most likely due to a threshold effect of trichome density, with the density on the hybrid species below the level for the tree to be aphid resistant. Population differences on A. glutinosa and A. pubescens were not caused by differences in growth or reproduction of aphids between hosts. Cage-reared aphids on these two hosts had identical performance in five successive generations. The effect of caging probably reduced wind movement of trichomes and thereby reduced the probability of aphids being dislodged from their feeding site. Predator and parasitoid species responded in a similar manner to the aphids; their numbers were intermediate on the hybrid hosts.