HOST-PLANT QUALITY INFLUENCES DIAPAUSE AND VOLTINISM IN A POLYPHAGOUSINSECT HERBIVORE

Facultative diapause is a strategy that allows insects to initiate an additional generation when conditions are favorable, or to enter diapa use when they are not. The occurrence of additional generations within one year can have a profound effect on the ecology and evolution of a species. Most previous studies of the cues that induce facultative di apause in insects have concentrated on photoperiod and temperature. In contrast, we studied the effects of host-plant quality on the inducti on of larval diapause and subsequent voltinism in a poryphagous insect herbivore, Choristoneura rosaceana (Lepidoptera: Tortricidae). Field experiments demonstrated that the proportion of larvae entering diapau se differed among four host-plant species. Larvae feeding on chokecher ry were more likely to continue development and produce a second gener ation than larvae feeding on red maple or black ash. Paper birch was i ntermediate in diapause induction. Laboratory experiments with excised foliage supported the results of the field experiments and demonstrat ed that the induction of diapause depended on host species under const ant conditions of photoperiod and temperature. A further study with lo w- and high-quality artificial diet demonstrated that the nutritional quality of larval diet has a direct effect on diapause induction: low- quality diet favored diapause induction. Dispersal of neonate larvae f rom host foliage did not necessarily reflect host quality and the pote ntial for bivoltinism. We argue that the effect of plant quality on di apause induction in natural populations of insect herbivores merits mo re attention.