Submitted to: Journal of Entomological Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: November 21, 2006
Publication Date: July 15, 2007
Citation: Suh, C.P., Spurgeon, D.W. 2007. Supercooling in the adult boll weevil (Coleoptera: Curculionidae) relative to physiological condition and diet. Journal of Entomological Science. 42:320-328. Interpretive Summary: Efforts to plan and conduct boll weevil eradication or suppression programs could be greatly improved by the ability to accurately predict winter mortality of adult boll weevils. Severe winter temperatures are known to reduce overwintering survival of the boll weevil. However, factors influencing the temperatures at which weevils freeze are poorly understood. We examined the effects of adult physiological status, diet, and time since feeding on weevil supercooling. We found no relationship between the physiological status of weevils and their supercooling points. However, the diet weevils were reared upon appeared to have a small, but significant influence on the supercooling points of weevils. We found that those reared on cotton fruit tended to have the lowest supercooling points. We also found that both the type of food given to weevils immediately prior to testing and the time elapsed between feeding and exposure to freezing temperatures influenced weevil supercooling. Because the temperatures at which weevils froze in our study were much cooler than what overwintering weevils would normally encounter in the field, our results suggest freezing of weevils is not a major mortality factor of boll weevils. Nevertheless, the recent feeding history of weevils needs to be considered or documented in future supercooling and overwintering survival studies, particularly in cases where weevils may be exposed to temperatures cool enough to freeze weevils.
Technical Abstract: Severe winter temperatures are known to reduce overwintering survival of the boll weevil (Anthonomus grandis Boheman). However, factors influencing the temperatures at which weevils freeze 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 weevil supercooling. 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. Further, 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.