Susceptibility of adult Phlyctinus (Coleoptera: Curculionidae) to entomopathogens: A first look at potential differences in a newly revised species complex

Phlyctinus callosus and P. xerophilus (Coleoptera: Curculionidae) are two cryptic species of native entimine weevils, previously grouped together under the P. callosus sensu lato concept, that are pests of economic importance to the deciduous fruit and vine industry in the Western Cape province of South Africa. Laboratory bioassays were conducted using entomopathogenic fungi (EPF) isolates of Beauveria and entomopathogenic nematodes (EPNs), Heterorhabditis indica and Steinernema yirgalemense, to determine differences in susceptibility of adult P. callosus and P. xerophilus to potential biological control agents. The test arena used was 24-well bioassay plates with an inoculation rate of 200 infective juveniles (IJs)/insect for EPNs and 5 x 105 conidia/insect for EPF. Insects were inoculated using a 12.7 mm filter paper impregnated with 50 mu L of entomopathogen suspension. Infection was determined after 96 h incubation for EPNs through dissection of cadavers. Insects inoculated with EPF were incubated in the wells for 18 days and mortality recorded daily. Cadavers were surface sterilized and observed for overt mycosis. Differential susceptibility between P. callosus and P. xerophilus was observed in EPF but not in EPN bioassays. Differential susceptibility to EPF could be due to methodology. Low adult weevil pathogenicity was found for all entomopathogens, with variable results obtained from different bioassay batches.

Automated summary

Phlyctinus callosus and P. xerophilus are important pests in the deciduous fruit and vine industry of the Western Cape province in South Africa. In laboratory experiments, the susceptibility of these two species to entomopathogenic fungi (EPF) and entomopathogenic nematodes (EPNs) was tested. Differential susceptibility was observed in EPF bioassays but not in EPN bioassays. The pathogenicity of all entomopathogens was low in adult weevils, with variable results obtained from different bioassay batches.