Effect of insecticides on Tiphia vernalis (Hymenoptera: Tiphiidae) oviposition and survival of progeny to cocoon stage when parasitizing Popillia japonica (Coleoptera: Scarabaeidae) larvae.

The effect of insecticides on oviposition of Tiphia vernalis Rohwer and subsequent survival of parasitoid progeny to the cocoon stage was determined in the laboratory by using larval Japanese beetle, Popillia japonica Newman, as the host. Insecticides tested were imidacloprid, thiamethoxam, halofenozide, chlorpyrifos, and carbaryl at labeled rates. Female T. vernalis were allowed 2 d to parasitize P. japonica larvae after the parasitoids had received a 4-d exposure to insecticide-treated soil. Another group of female T. vernalis were allowed 2 d to parasitize P. japonica larvae that had been exposed to insecticide-treated soil for 3-4 d. Percentage of parasitism of P. japonica larvae in these trials after exposure of adult parasitoids to carbaryl, chlorpyrifos, halofenozide, or imidacloprid-treated soil (23.3-50.0%) or adult parasitoids to chlorpyrifos, halofenozide, or imidacloprid-treated grubs (33.0-56.7%) was not negatively affected relative to the control treatment (21.7-54.2%). A third group of adult T. vernalis and P. japonica larvae were simultaneously exposed to chlorpyrifos or carbaryl treatments. Percentage parasitism in these trials was lower for T. vernalis adults exposed to the chlorpyrifos and carbaryl (15.0-25.0%) relative to the control (57.5-62.5%) with the exception of one trial with carbaryl (40.0%). However, exposure of the parasitoid and P. japonica to chlorpyrifos 0.5X, carbaryl 0.5X, imidacloprid, halofenozide, or thiamethoxam in several trials resulted in parasitism that was equivalent or greater than (45.0-80.0%) the untreated control (57.5-62.5%). Japanese beetle larval mortality in these trials was greater in the insecticide and parasitoid combination (97.5-100.0%) than with insecticides alone (45.0-100.0%). Percentage of survival of T. vernalis progeny to the cocoon stage was not negatively affected by a 4-d adult parasitoid exposure to carbaryl and chlorpyrifos treated soil (11.7-16.7% versus 18.3% control) or a 2-d exposure to P. japonica-treated larvae (16.7-18.3% versus 28.3% control). However, simultaneous exposure of T. vernalis progeny and P. japonica larvae to chlorpyrifos- and carbaryl-treated soil resulted in no parasitoids surviving to the cocoon stage. Between neonicotinoids, thiamethoxam had more adverse impact on percentage parasitism (52.5%) and survival to the cocoon stage (10.0%) than imidacloprid (80.0 and 32.5%, respectively). Results of this study indicate soil incorporation of imidacloprid and halofenozide had minimal effect on the number of P. japonica larvae parasitized by T. vernalis or survival of T. vernalis progeny to the cocoon stage; therefore, they are more suitable for use with T. vernalis. In contrast, chlorpyrifos, carbaryl, and thiamethoxam lowered the number of T. vernalis progeny surviving to the cocoon stage, and carbaryl and chlorpyrifos reduced the number of P. japonica larvae parasitized. The soil incorporation of insecticides is discussed as one explanation for the minimal effects of some insecticides on T. vernalis.