Abstract
Observed changes in mean temperature and increased frequency of extreme climate events have already impacted the distributions and phenologies of various organisms, including insects. Although some research has examined how parasitoids will respond to colder temperatures or experimental warming, we know relatively little about how increased variation in temperature and humidity could affect interactions between parasitoids and their hosts. Using a study system consisting of emerald ash borer (EAB), Agrilus planipennis, and its egg parasitoid Oobius agrili, we conducted environmentally controlled laboratory experiments to investigate how increased seasonal climate variation affected the synchrony of host–parasitoid interactions. We hypothesized that increased climate variation would lead to decreases in host and parasitoid survival, host fecundity, and percent parasitism (independent of host density), while also influencing percent diapause in parasitoids. EAB was reared in environmental chambers under four climate variation treatments (standard deviations in temperature of 1.24, 3.00, 3.60, and 4.79°C), while O. agrili experiments were conducted in the same environmental chambers using a 4 × 3 design (four climate variation treatments × 3 EAB egg densities). We found that EAB fecundity was negatively associated with temperature variation and that temperature variation altered the temporal egg laying distribution of EAB. Additionally, even moderate increases in temperature variation affected parasitoid emergence times, while decreasing percent parasitism and survival. Furthermore, percent diapause in parasitoids was positively associated with humidity variation. Our findings indicate that relatively small changes in the frequency and severity of extreme climate events have the potential to phenologically isolate emerging parasitoids from host eggs, which in the absence of alternative hosts could lead to localized extinctions. More broadly, these results indicate how climate change could affect various life history parameters in insects, and have implications for consumer–resource stability and biological control.
Highlights
Various climate models predict seasonal temperature and rainfall extremes to increase in severity and frequency over the coming century (Kharin, Zwiers, Zhang, & Hegerl, 2007; Yao et al 2013; Wuebbles et al, 2014)
We found that parasitoid diapause was positively associated with humidity variation, with 18.67% of both generations of O. agrili entering diapause under the highest humidity variation treatment compared with 5.35% when humidity variation was lowest (Table 2; Figure 4c)
Oobius agrili percent parasitism declined with emerald ash borer (EAB) egg density from 77.98% with six host eggs to 40.07% with 24 host eggs (Table 2; Figure 4d), most likely reflecting the maximum threshold for parasitoid egg production between exposure events (48 hr)
Summary
Various climate models predict seasonal temperature and rainfall extremes to increase in severity and frequency over the coming century (Kharin, Zwiers, Zhang, & Hegerl, 2007; Yao et al 2013; Wuebbles et al, 2014). Oobius agrili Zhang and Huang (Hymenoptera: Encyrtidae) is a solitary egg parasitoid of emerald ash borer (EAB), Agrilus planipennis Fairmaire (Coleoptera: Buprestidae), a wood-boring beetle that has become invasive in North America and Europe (Herms & McCullough, 2014; Orlova-Bienkowskaja, 2014). Both O. agrili and EAB are native to northeastern Asia (i.e., China, the Korean Peninsula, and the Russian Far East), and O. agrili has been released for biological control of EAB in the United States since 2007 (Bauer, Duan, Gould, & Van Driesche, 2015). We hypothesized that increased temperature and humidity variability would disrupt larval development and lead to greater variation in O. agrili F1 emergence times
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