Estimating the ends of the sensitive period for photoperiodically induced diapause in arthropods

Abstract

A new method is proposed and tested for estimating the beginning and end of the sensitive period for the photoperiodic induction of diapause (a type of dormancy) in arthropods. The method incorporates three experiments already commonly used to study photoperiodism: (1) an experiment in which samples of individuals are transferred at successively older ages from a diapause inducing short day length to a diapause averting long day length, (2) an experiment in which samples are transferred at successively older ages in the opposite direction from long to short day conditions, and (3) an experiment to determine the number of short days required to induce diapause, the required day number. Using the Mexican bean beetle ( Epilachna varivestis Mulsant), which has an adult diapause, the new method estimates that this beetle's sensitive period at 25°C begins approximately on day 9 for males and on day 10 for females corresponding to the beginning and early second instar, respectively, and ends on day 21 for both sexes corresponding to the start of the pupal stage. The observation that the sensitive period is 1 day longer for males resolves the otherwise inconsistent findings that the mean required day number is 1 day greater for males but the critical photoperiod is the same for both sexes. To assess the responses of the beetles to the experiments, they were dissected on day 4 after adult eclosion and classified according to their level of gonadal development. Four classes were recognized in females and 3 in males yielding 3 and 2 thresholds, respectively, instead of the usual single threshold separating the diapause and nondiapause classes. Estimates of the ends of the sensitive period using the different thresholds were consistent. Obtaining reliable estimates of the ends of the sensitive period is an important step toward the goal of using the physiological model of diapause induction based on the required day number-sensitive period interaction to address ecological and evolutionary questions concerning the timing of diapause induction.