Effects of rearing conditions on reproduction of Spathius agrili (Hymenoptera: Braconidae), a parasitoid of the emerald ash borer (Coleoptera: Buprestidae)

Abstract

Spathius agrili Yang (Hymenoptera: Braconidae) can be successfully reared on emerald ash borer, Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), larvae feeding in chambers drilled in small ash twigs that are wrapped with floral tape. Females maintained in groups with males for one week can receive enough sperm for production of female progeny throughout their lives. Volatiles released by emerald ash borer adults feeding on ash foliage increased parasitoid fecundity over ash foliage alone or no stimulus. The temperature at which the parasitoids were reared ranged from 20 to 25°C in a daily cycle; however, raising the daily maximum temperature to 28°C did not affect parasitoid longevity or fecundity. Adult females lived between 12 and 127 d, with an average of 60.8 ± 4.5 d. Males lived slightly longer, with an average of 66 ± 4.5 d. The first clutch of eggs was laid when the female was between 2 and 42 d old, with the average preoviposition period lasting 11.4 ± 1.4 or 19.5 ± 2.0 d in 2007 and 2009 trials, respectively. A higher proportion of the emerald ash borer larvae were feeding and thus attractive to parasitoids in the 2009 trial, and female S. agrili laid an average of 9.5 ± 1.0 clutches containing 5.4 ± 0.2 eggs, for an average of 51.2 eggs per female. Approximately three quarters of the progeny were female. The number of eggs per clutch was significantly greater when deposited on larger emerald ash borer larvae, further highlighting the need for quality larvae in rearing. Chilling S. agrili pupae at 10°C to stockpile them for summer release was not successful; chilling resulted in lower survival and lower fecundity of emerging progeny. Female S. agrili proved capable of attacking emerald ash borer larvae through even the thickest bark of an ash tree that was 30-cm diameter at breast height. Even emerald ash borer larvae that were creating overwintering chambers in the outer sapwood of the tree were successfully attacked, suggesting that S. agrili could be reared on field collected logs infested with emerald ash borer. © 2011 Entomological Society of America.