Effects of chilling on diapause development and reproductive fitness of two congeneric species of encyrtid parasitoids (Hymenoptera: Encyrtidae) of the emerald ash borer

Abstract

Oobius agrili Zhang and Huang and Oobius primorskyensis Yao and Duan (Hymenoptera: Encyrtidae) are egg parasitoids of the emerald ash borer (EAB) Agrilus planipennis (Coleoptera: Buprestidae). Oobius agrili has been introduced for EAB biocontrol and has established populations in several areas of the U.S., whereas O. primorskyensis is currently under evaluation for future biocontrol introduction. Both species overwinter as diapausing larvae inside parasitized host eggs; however, few studies have determined the effects of chill (or overwintering) on their diapause development and reproductive fitness. We exposed diapausing O. agrili and O. primorskyensis larvae to either 1.7 °C or 12.8 °C for 1–9 months, subsequently evaluated their post-chill development to adults, and assessed the longevity and lifetime fecundity of the emerged adult parasitoids under normal rearing conditions (25 °C, 16:8 h L:D). Results show that both species require periods of chill at either 1.7 °C or 12.8 °C to resume their development to adults under normal rearing conditions; however, there are clear differences between species in their responses to chill regimes. Chilling at either 1.7 °C or 12.8 °C for 1–9 months resulted in the development of diapausing O. agrili to adults and no significant reduction in the adults’ longevity and fecundity, although the adult emergence time was significantly longer and less synchronized when the diapausing parasitoids were chilled at 1.7 °C for ≤3 months. In addition, the mortality rate of diapausing O. agrili larvae increased significantly with chilling time at 1.7 °C, but not at 12.8 °C. In contrast, ≤3 months of chilling at 1.7 °C or ≤1 month at 12.8 °C did not result in the development of diapausing O. primorskyensis to adults, whereas 6 months of chilling at 1.7 °C or 3–6 months at 12.8 °C did. Furthermore, chilling for 9 months at 1.7 °C and 12.8 °C caused 100% and 75% mortality of the diapausing O. primorskyensis, respectively. These results have relevance to laboratory rearing and release strategies of these parasitoids for biocontrol of EAB.