Juvenile hormone modulation of insect cold hardening: Ice-nucleating activity

Abstract

Data are presented offering the first evidence for probable endocrine involvement in the control of cold hardening in Eurosta solidaginis. Juvenile hormone (JH) deprivation experiments in which the corpora allata were removed by head ligation resulted in a loss of supercooling (SC) capacity in larvae collected over 2 years. This loss of supercooling capacity is indicative of synthesis of organismal pools of ice-nucleating agents (INA). Larval sensitivity to JH removal (ligation) on SC is seasonally dependent. For example, in 1983, larvae were most sensitive in October, secondarily so in September, and relatively insensitive in December regardless of acclimation temperature. While in 1984, larvae acclimated to +5 °C were most sensitive in November, secondarily so in October, and relatively insensitive in December, and larvae acclimated to +15 °C were most sensitive in October and December and least so in November. Supercooling point elevations as great as 7 °C over controls were observed with maximal responses occurring within 1 day following ligation. In 1983, juvenile hormone replacement following ligation generally resulted in an expansion of supercooling capacity when compared to controls. As with ligation, the sensitivity to JH replacement on SC was seasonally dependent: September and October larvae being the most sensitive with December larvae being insensitive. Larvae collected in 1984 were given a greater dose of JH than those in the previous year and showed no significant change in SC over ligated-acetone controls. Hormone analog potentiation experiments in which unligated larvae collected in 1983 and acclimated to +5 °C were given methoprene resulted in depression of supercooling points for September and October larvae. JH titres appear to play an important role in the regulation of SC capacity in E. solidaginis larvae.