Subsequent chemical and foraging ecology preferences of Theocolax elegans (Westwood) (Hymenoptera: Pteromalidae) reared on two alternate stored product host insects

Abstract

Theocolax elegans (Hymenoptera: Pteromalidae) is a potential postharvest biocontrol agent whose host range includes Sitophilus oryzae (Coleoptera: Curculionidae) and Rhyzopertha dominica (Coleoptera: Bostrichidae). Both host species are cosmopolitan and destructive pests of bulk wheat. In addition, either species could be used when mass rearing T. elegans. The Hopkins-Host Selection principle suggests the natal host environment (e.g., the habitat in which a wasp emerges from a pupa) may influence the semiochemicals an organism utilizes when foraging for oviposition sites. Thus, later efficacy may be impaired if important semiochemicals are lost from the foraging repertoire of T. elegans. In order to investigate the impact of natal host environment on the behavioral response of T. elegans to potential hosts, we reared T. elegans on either S. oryzae or R. dominica for multiple generations. We then evaluated the orientation and taxis of T. elegans to six treatments: S. oryzae, R. dominica, damaged grain + S. oryzae, damaged grain + R. dominica, damaged grain + insects from the natal environment, or an undamaged control. We found T. elegans reared on R. dominica most preferred damaged grain from R. dominica in a four-way olfactometer, which was 4.2-fold more often chosen than S. oryzae individuals alone. Treatments containing R. dominica were differentiated from others based on headspace volatiles while S. oryzae-infested grain generally overlapped with uninfested grain. Both rearing host and subsequent foraging host affected efficacy of T. elegans released in a pilot-scale elevator. Wasps appeared most effective in suppressing grain damage by 35–38 % when reared on R. dominica and foraging for R. dominica compared to S. oryzae-reared wasps that only reduced damage by 1–18 %. Sitophilus oryzae-reared wasps only effectively foraged on hosts up to 0.5 m, while R. dominica-reared wasps foraged successfully up to 4 m. Overall, we found that the natal host and chemical cues significantly affected taxis and foraging by T. elegans, suggesting that careful attention should be paid to the mass rearing procedure for this parasitoid.

Keywords

• Lesser grain borer
• Rice weevil
• Chemotaxis
• Biological control
• Stored products