DIAPAUSE IN RELATION TO THE ECOLOGY OF INSECTS

A New Role for Temperature in Insect Dormancy: Cold Maintains Diapause in Temperate Zone Diptera

In early autumn, high temperatures terminate diapause in the alfalfa blotch leafminer Agromyza frontella; low temperatures maintain diapause. These responses subserve a thermally malleable dormancy and allow flexibility in the annual number of generations. The view that favorable conditions cannot reverse the course of diapause are contradicted by the data on A. frontella. A better understanding to life history studies and phenological models in insect pest management.

Action of photoperiod at cold in diapausing Chrysopa carnea females (Chrysopidae, Neuroptera)

To reveal the effect of photoperiod on insects under low temperatures, dormant females of Chrysopa carnea were exposed for 3 months (November-January) to cold (7°±2.5°C) and to long- or short-day conditions (18L:6D or 12L:12D). They were thereafter activated under short-day conditions at 25°C.The samples differed considerably in the incidence of ovipositing females, duration of pre- and post-oviposition period, and fecundity. All parameters evidenced a considerably higher reproductive activity of insects chilled under short-day conditions. It was thus confirmed that adults of C. carnea perceive photoperiod at cold. The action of short photoperiod at cold does not impair the subsequent oviposition; by contrast it promotes future reproduction, apparently also due to photoperiod changes. Although short-day photoperiod inhibits the activation at warm temperatures, it enables diapause development under cold temperatures.

DIAPAUSE IN THE EGGS OF GRYLLUS PENNSYLVANICUS BURMEISTER (ORTHOPTERA: GRYLLIDAE)

Eggs of Gryllus pennsylvanicus enter diapause after about 100 hours of incubation at 23–26 °C when the embryos are in the postanatrepsis stage. The diapause is obligatory, but the so-called 'intensity' of diapause varies greatly in different eggs. Some hatch 28 days after oviposition while others require 140 days when incubated continuously at 23–26 °C.In eggs exposed to 6–7 °C for varying periods, the effect of cold treatment is to reduce the 'intensity' of diapause. Cold treatment for 90 days or more results in virtually complete termination of diapause. Such eggs require further incubation at 23–26 °C for about 15 days to complete their development.One physiological effect of the cold treatment appears to be that the yolk becomes semifluid, thus possibly allowing its easier assimilation by the embryos so that growth is speeded up, with the result that almost all eggs hatch within a relatively short period of time. It is suggested that non-availability of suitable food for the embryo may be one of the causes of diapause in G. pennsylvanicus.