Inverse density-dependent diapause and its influence on population dynamics in Monochamus alternatus alternatus (Coleoptera: Cerambycidae)

Monochamus alternatus alternatus Hope (Coleoptera: Cerambycidae) is the primary vector of pine wilt disease in China and Taiwan. It has a partially bivoltine life cycle with facultative diapause at the final larval instar that is mediated by the initial larval density. To evaluate the effect of density-related induction of diapause on population fluctuation, one, two or four newly hatched larvae were inoculated on individual Pinus thunbergii bolts and the life-history traits including diapause were investigated. This experiment indicated that as initial population size/bolt (crowding) increased, the diapause incidence, larval stage survival, adult body size, development time of nondiapause individuals and pupal period of diapause individuals decreased. The analysis indicated that larval interference as well as food shortage suppressed diapause induction. Increasing larval density decreased the probability of diapausing, survival, adult body mass and other traits. Using the data obtained from this study, a simple mathematical model indicated that inverse density-dependent induction of diapause in the first generation had a destabilizing effect on the population size of overwintering generation, which was composed of diapause larvae of the first and second generations; whereas, direct density-dependent induction of diapause exerted a stabilizing effect on it.