Large-scale migration synchrony between parasitoids and their host

1. Parasitoids are a valuable group for conservation biological control. In their role as regulators of aphid pests, it is critical that their lifecycle is synchronised with their hosts in both space and time. This is because a synchronised parasitoid community is more likely to strengthen the overall conservation biological control effect, thus damping aphid numbers and preventing potential outbreaks. One component of this host-parasitoid system was examined, that of migration, and the hypothesis that peak summer parasitoid and host migrations are synchronised in time was tested. 2. Sitobion avenae Fabricius and six associated parasitoids were sampled from 1976 to 2013 using 12.2-m suction-traps from two sites in Southern England. The relationship between peak weekly S. avenae counts and their parasitoids was quantified. 3. Simple regression models showed that the response of the peak parasitoids to the host was positive: generally, more parasitoids migrated with increasing numbers of aphids. Further, when averaged over time, the parasitoid migration peak date corresponded with the aphid migration peak. The co-occurrence of the peaks was between 51% and 64%. However, the summer peak in aphid migration is not steadily shifting forward with time unlike spring first flights of aphids. Cross-correlation analysis showed that there were no between-year lagged effects of aphids on parasitoids. 4. These results demonstrate that the peak in migration phenology between host and parasitoid is broadly synchronised within a season. Because the threshold temperature for flight (>12 degrees C) was almost always exceeded in summer, the synchronising agent is likely to be crop senescence, not temperature. Studies are needed to assess the effects of climate change on the mismatch potential between parasitoids and their hosts.