Repeated Cold Exposure Effects on Mortality and Feeding Activity of the Salvinia Weevil, Cyrtobagous salviniae...

Biological control of the aquatic fern, Salvinia molesta D. S. Mitchell, by Cyrtobagous salviniae Calder and Sands (Coleoptera: Curculionidae), in temperate regions has been less reliable relative to tropical and subtropical regions. High mortality of overwintering adults is presumed to be an important factor. At temperate sites, overwintering adult C. salviniae may experience multiple exposures to low temperatures that could compound to lethal levels. We examined the effects of repeated cold exposure (RCE) on the overwintering mortality and feeding rate of adult C. salviniae. Mortality rate of adults following exposure to 2, 4, and 6 cycles of RCE was 22, 70 and 82%, respectively. Mortality in the RCE treatment was similar to that in the matched-cold treatment for 2 and 6 cycles but was significantly less than in the matched-cold treatment for the 4 cycles. The relationship between the number of RCE and adult mortality rate was modeled and model results were compared to adult survival of caged weevils during four winters on two lakes in northeast Texas. Also, the feeding activity of adult weevils surviving RCE declined following exposure to 4 and 6 cycles of RCE. These results provide insight into how cold impacts overwintering survival of C. salviniae and should assist in anticipating the need for inoculative releases of C. salviniae the following spring to achieve consistent biological control of S. molesta at temperate locales.