Aphid–hoverfly interactions under elevated CO 2 concentrations: oviposition and larval development

The performance of predators of plant pests is mainly driven by their ability to find prey. Recent studies suggest that rising atmospheric carbon dioxide (CO 2 ) concentrations will affect the semiochemistry of plant–insect relationships, possibly altering prey‐finding behaviour. In the present study, we test the hypothesis that higher atmospheric CO 2 concentrations affect the oviposition behaviour of an aphidophagous hoverfly and alter the development of its larvae. We also test the hypothesis that volatile compounds released by the plant–aphid association are modified under elevated CO 2. Broad bean plants infested with pea aphids are grown under ambient (450 ppm) or elevated CO 2 (800 ppm) concentrations. Plants raised under each treatment are then presented to gravid hoverfly females in a dual‐choice bioassay. In addition, emerging Episyrphus balteatus larvae are directly fed with aphids reared under ambient or elevated CO 2 conditions and then measured and weighed daily until pupation. Odours emitted by the plant–aphid association are sampled. A larger number of eggs is laid on plants grown under ambient CO 2 conditions. However, no significant difference is observed between the two groups of predatory larvae grown under different CO 2 concentrations, indicating that the CO 2 concentration does not affect the quality of their aphid diet. Although plant volatiles do not differ between the ambient and elevated CO 2 ‐treated plants, we find that the quantity of aphid alarm pheromone is lower on the plant–aphid association raised under the elevated CO 2 condition. This suggests that an alteration of semiochemical emissions by elevated CO 2 concentrations impacts the oviposition behaviour of aphid predators.