Repeated Cold Exposure Effects on Mortality and Feeding Activity of the Salvinia Weevil, Cyrtobagous salviniae (Coleoptera: Curculionidae).

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.

The influence of host species and location in the host detection ability of tiphiid (Hymenoptera: Tiphiidae) parasitoids.

Tiphia vernalis Rohwer and Tiphia popilliavora Rohwer are ectoparasitoids of root-feeding larvae of the Japanese beetle, Popillia japonica Newman, and oriental beetles, Anomala orientalis Waterhouse (Coleoptera: Scarabaeidae). Little is known about the influence of host species and location in the host detection ability of tiphiid wasps. In this study, we examined the response of female T. popilliavora wasps, an understudied Tiphia species, to potential host stimuli using dual choice tests in an observation chamber filled with soil. T. popilliavora wasps were able to successfully discriminate the trails containing body odor or frass of P. japonica grubs from trails without cues. Frass trails of P. japonica grubs elicited stronger responses than body odor trails. We also examined the preference of host cues by tiphiid wasps using dual choice behavioral assays. Both T. vernalis and T. popilliavora wasps did not show preference toward trails that either contained P. japonica or A. orientalis cues. In addition, we also determined the detection of host cues by tiphiid wasps in a dual-choice test for cues presented at varying soil depths. Wasps were able to successfully discriminate between the Y-tube arms with and without cues when the cues of P. japonica were buried at a depth of 2 cm. In contrast, both Tiphia species were unable to distinguish between the Y-tube arms with and without cues when the cues were buried at a depth of 5 cm. Thus, our findings suggest that once Tiphia wasps land on the ground, they can detect the presence of their specific hosts, just below the soil surface by exploiting the kairomones present in grub body odor trails and frass and once the wasps are in the soil, they use the same cues to direct themselves to the host grubs.