Seasonal Changes in the Cold Tolerance of Eggs of the Migratory Locust, <I>Locusta migratoria</I> L. (Orthoptera: Acrididae)

Abstract

The migratory locust, Locusta migratoria L., has one to four generations per year in China, overwintering as eggs in the soil. In temperate Hebei province, this species has two generations per year, known as the summer and fall generations. In this study, locust populations occurring in different seasons were compared to investigate the adaptation of eggs to a cold environment. Large seasonal variations were observed in the supercooling point (SCP) and lower lethal temperature (Ltemp50, the temperature that causes 50% mortality in a population) of locust eggs. Mean SCP values of the fall population were −26.0 ± 0.1°C, while those of the summer and postwinter populations were −22.0 ± 0.8 and −23.4 ± 0.7°C, respectively. The fall population also had significantly lower Ltemp50 values (−14.2°) than the summer (−4.8°) and postwinter populations (−6.7°C). The survival ability of eggs at different development stages was also compared; 10-d-old eggs incubated at 25°C were more cold hardy than other stages. Prechilling at 5°C was effective in increasing the cold hardiness of eggs. These results confirm that eggs of the fall population are better conditioned for overwintering. The low mortality of overwintering eggs often leads to high population density and serious crop damage in the next season or following year.