Targeted improvement of Steinernema carpocapsae for control of the pecan weevil, Curculio caryae (Horn) (Coleoptera: Curculionidae) through hybridization and bacterial transfer

The entomopathogenic nematode, Steinernema carpocapsae, has shown promise for control of adult pecan weevil, Curculio caryae, a key pest of pecan. In previous studies, the Italian strain of S. carpocapsae was found to be among the most virulent to C. caryae adults, but possessed poor heat and desiccation tolerance. In contrast, the DD-136 strain exhibited high levels of heat and desiccation tolerance but low virulence to the weevil. Our objective was to determine the feasibility of developing improved strains of S. carpocapsae by transferring the nematode’s bacterial symbiont (Xenorhabdus nematophila) from the Italian strain to the DD-136 strain, and through hybridization between the two wild-type nematodes. Three modified strains were created: one through bacterial transfer alone and two hybrids through controlled crosses. We hypothesized that the improvement approaches would result in strains possessing high levels of heat and desiccation tolerance similar to DD-136 and virulence similar to the Italian strain. The hypothesis was supported in two out of three modified strains. Heat and desiccation tolerance in all modified strains was more than 2.5-fold greater than the Italian strain and not different from the DD-136 strain, except one hybrid had lower heat tolerance than DD-136. Mortality of adult C. caryae from the modified strains at 2 or 3 days post-treatment was greater than from DD-136 and similar to the Italian strain. Overall, the results indicate that bacterial transfer and hybridization could be a valuable tool in improving the biocontrol efficacy of S. carpocapsae.