Aphid–hoverfly interactions under elevated CO2 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 (CO2)
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 CO2 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 CO2.
Broad bean plants infested with pea aphids are grown under ambient (450 ppm) or
elevated CO2 (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 CO2 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 CO2 conditions. However, no significant difference
is observed between the two groups of predatory larvae grown under different CO2
concentrations, indicating that the CO2 concentration does not affect the quality of their
aphid diet. Although plant volatiles do not differ between the ambient and elevated
CO2-treated plants, we find that the quantity of aphid alarm pheromone is lower on the
plant–aphid association raised under the elevated CO2 condition. This suggests that
an alteration of semiochemical emissions by elevated CO2 concentrations impacts the
oviposition behaviour of aphid predators.