Overwintering ecology of Chrysophtharta agricola: mechanisms of reproductive diapause induction and termination

Abstract

Reproductive diapause is common in the Chrysomelidae, and allows 'escape in time' of conditions unfavourable for growth and development. Chrysophtharta agricola (Chapuis) (Coleoptera : Chrysomelidae), which undergoes one or two generations per year, spends 7–8 months in reproductive diapause, emerging in spring to feed and oviposit. We manipulated photoperiod and temperature to test their effects on induction and termination of diapause in the laboratory; we also conducted field studies for validation of results, and to examine patterns of voltinism as determined by the onset of diapause. A critical photoperiod of between 12 and 16 h of light was required to initiate diapause in the laboratory and field, although 55% of beetles entered diapause in response to low temperature (9°C) alone. At short photoperiods (8L : 16D), high temperature (21°C) subverted diapause in 20% of adults. Therefore, voltinism in C. agricola is a seasonally plastic trait dependent on emergence time of teneral adults. Diapause termination was primarily influenced by temperature, although photoperiod had an effect at low temperatures. Beetles accumulated day-degrees (DD) during overwintering, with beetles collected later terminating diapause sooner than those collected earlier (total DD was similar). Diapause termination is, therefore, dependent on DD accumulation above the threshold temperature (6.7°C), rather than a specific environmental cue. Results are discussed in ecological and applied contexts, and provide the ability to predict the population phenology of C. agricola. Seasonal plasticity, as demonstrated here, allows insects to balance resource availability, reproductive strategy and climatic tolerance.