Supercooling in the Adult Boll Weevil (Coleoptera: Curculionidae) Relative to Physiological Condition and Diet

Abstract

Severe winter temperatures are known to reduce overwintering survival of the boll weevil (Anthonomus grandis Boheman), but the mechanisms responsible for overwintering mortality are poorly understood. We examined the effects of adult physiological status (reproductive, diapause, intermediate diapause), diet (flower buds or fruit of cotton, Gossypium hirsutum L.) and time since feeding (0 or 2 d) on freezing points of weevils. Differences in supercooling points among weevil physiological states were not detected. However, adults fed cotton fruit (bolls) supercooled to lower temperatures (mean ± SE; −17.4 ± 0.23°C) than those initially fed flower buds (squares) followed by bolls (−15.8 ± 0.40°C). Weevils fed squares exhibited the highest mean supercooling point (−14.4 ± 0.35°C). Additional study confirmed the effects of diet on supercooling when weevils were exposed to freezing temperatures immediately after feeding (bolls, −12.5 ± 0.45°C; squares, −10.2 ± 0.45°C). However, supercooling points were lowered but similar for both diets when exposure to freezing temperatures occurred 2 days after feeding (bolls, −17.1 ± 0.45°C; squares, −17.2 ± 0.46°C). Also, tissues excised from bolls froze at lower temperatures (−10.7 ± 0.41 °C) than pollen sacs excised from squares (−8.2 ± 0.41°C). We found that supercooling ability did not indicate weevil physiological status. Furthermore, the observed supercooling points suggest that acute freezing is an unlikely mortality mechanism in the more southerly U.S. cotton production regions. Regardless, both diet type and the time since feeding are important factors that should be considered in supercooling or survival studies involving temperatures low enough to freeze weevils.