Photoperiodic modulation of diapause induction and termination in Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae), an egg parasitoid of the invasive emerald ash borer
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Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae) is a solitary and parthenogenetic egg parasitoid from China being introduced into North America (NA) as a biological control agent of 2 the emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), an extremely invasive and destructive pest of ash trees (Fraxinus spp.). Oobius agrili is being released over a broad geographical range in NA where photoperiod varies considerably during the season of parasitoid-host activity. We conducted laboratory studies to determine 1) if photoperiod-induced diapause is modulated maternally, grand-maternally, and/or directly in immature parasitoids; 2) interactions of maternal adult age and photoperiod exposure on diapause induction; 3) the critical day length for diapause induction; 4) the critical age at which photoperiod-induced diapause is modulated in developing larvae; 5) the effects of photoperiod and length of chill on diapause termination; and, 6) the effects of photoperiod on O. agrili biology across a latitudinal gradient. We found that photoperiod exposure of O. agrili larvae developing inside host eggs directly induced diapause, and maternal or grand-maternal photoperiod treatments did not affect diapause induction in their progeny. However, older adults that experienced diapause as larvae produced more progeny that entered diapause when their progeny were exposed to short-day photoperiods. All progeny produced by adults that developed from nondiapaused larvae entered diapause when exposed to short-day photoperiods. Diapause response to photoperiod declined dramatically after larvae were 6–7 days old. The critical day length for diapause induction was between 14.25 and 14.5 hours of daylight (at 25°C). Photoperiod and duration of chill affected diapause termination of diapausing O. agrili larvae. The cumulative number of degree days (base 10°C) required for adult emergence was highest for the combination of 12 hours light:12 hours dark (12L:12D) photoperiod combined with the shortest chill period, and lowest for 14.5L:9.5D and 16L:8D combined with the longest chill period. We discuss the effects of photoperiod on parasitoid3 host synchrony, population dynamics, and fitness of O. agrili across the geographical area where it is being released.
Keywordscritical day length; photoperiod; parasitoid-host synchrony; classical biological control; invasive pest
Petrice, Toby R.; Miller, Deborah L.; Bauer, Leah S.; Poland, Therese M.; Ravlin, Forrest W. 2019. Photoperiodic modulation of diapause induction and termination in Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae), an egg parasitoid of the invasive emerald ash borer. Biological Control. 138: 104047-. https://doi.org/10.1016/j.biocontrol.2019.104047.