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

Authors: Duan, Jian; LARSON, KRISTI - University Of Massachusetts

Submitted to: Biological Control
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/15/2019
Publication Date: 4/16/2019
Citation: Duan, J.J., Larson, K. 2019. Effects of chilling on diapause development and reproductive fitness of two congeneric species of encyrtid parasitoids (Hymenoptera: Encyrtidae) attacking the invasive emerald ash borer. Biological Control. 134: 163-169. https://doi.org/10.1016/j.biocontrol.2019.04.010.
DOI: https://doi.org/10.1016/j.biocontrol.2019.04.010

Interpretive Summary: Two Asian parasitic wasps (Oobius agrili and Oobius primorskyensis) are important natural enemies of the invasive emerald ash borer that has killed tens of millions of North American ash trees since it was discovered in 2002 in the U.S. Both natural enemies attack ash borer eggs and survive winter in a state of dormancy inside the EAB eggs. To develop methods for continuous mass production of these two natural enemies, researchers at the ARS Beneficial Insects Introduction Research Unit and the University of Massachusetts determined the effects of different chill regimes to simulate overwintering on their dormancy. Our study showed that both species require periods of winter chilling in order to complete their development to adults under normal (warmer) rearing conditions. For purposes of biological control release programs, both natural enemies may be safely stored in the dormant stage for up to 6 months at cool temperatures.

Technical 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 1.7oC or 12.8oC 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 (25oC, 16:8 h L:D). Results show that both species require periods of chill at either 1.7oC or 12.8oC 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 1.7oC or 12.8oC for 1-9 months resulted in the development of the 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 progeny were chilled at 1.7oC for = 3 months. In addition, the mortality rate of diapausing O. agrili larvae increased significantly with length of time chilled at 1.7°C, but not at 12.8oC. In contrast, = 3 months of chilling at 1.7oC or = 1 month at 12.8oC did not result in the development of diapausing O. primorskyensis larvae to adults, whereas 6 months of chilling at 1.7oC or 3 - 6 months at 12.8oC did. Furthermore, chilling for 9 months at 1.7oC and 12.8oC caused 100% and 75% mortality of the diapausing O. primorskyensis larvae, respectively. These results have relevance to laboratory rearing and release strategies of these parasitoids for biocontrol of EAB.