Effects of temperature variation on host-parasitoid synchrony: implications for biological control of emerald ash borer
Climate change (such as increased temperature variation) has frequently been linked with changes in the distribution and/or phenologies of species, potentially causing phenological mismatches between predators and prey. Such mismatches could have implications for many biological control programs, which often rely on parasitoids to suppress populations of pests. Here we investigate the effects of temperature variation on interactions between the invasive emerald ash borer (EAB), Agrilus planipennis, and the EAB egg parasitoid Oobius agrili. We hypothesized that increased daily and seasonal (life-long) temperature variation would disrupt the host-parasitoid interactions and lead to asynchrony between these species. To test this hypothesis we conducted laboratory experiments exposing EAB and O. agrili to one of four different temperature variation treatments (temperature SD: 0, 3, 3.6, and 5.1°C: chambers 1-4, respectively), while maintaining the same mean temperature (25°C) among them all. For EAB, 20 replicate male-female pairs were placed in 1 L containers for each of the four temperature variation treatments, and we quantified EAB adult longevity and fecundity. For O. agrili, an individual parasitoid was placed in a vial with one of three EAB egg densities (6, 12, or 24 eggs) in each of the four temperature variation treatments (with 6 replicates per treatment combination), and we quantified parasitoid longevity and rates of parasitism.
Results of our study showed that EAB and O. agrili longevity were both significantly negatively affected by increased temperature variation. EAB oviposition behavior was also influenced by temperature variation, with higher variation (chambers 3 and 4) causing females to lay more eggs early in the experiment before oviposition abruptly ceased, compared with lower variation treatments (chambers 1 and 2) where females generally laid eggs consistently for a longer duration. Percent parasitism by O. agrili appeared to be negatively related to EAB egg density among all temperature variation treatments. However, because of the increased longevity of the parasitoids when temperature variation was low (chambers 1 and 2), the total number of eggs parasitized was higher than when temperature variation was high (chambers 3 and 4). Diapause rate of O. agrili was not significantly affected by either temperature variation or egg density. Our findings suggest that increased temperature variation could alter EAB phenology, potentially causing asynchrony with one of the main biological control agents released for the management of this pest in North America.