Influence of drought on interactions between Rhopalosiphum padi and ground dwelling predators – A mesocosm study

Climate change will lead to extreme droughts, but it is difficult to predict how this will affect crop pests. In particular, it is unclear how interactions between natural en- emies and pests will be influenced. In the field, bird cherry- oat aphids (Rhopalosiphum padi (L.)) have been observed to reside close to, or below the ground surface during dry conditions. We hypothesized that this will increase the niche overlap between R. padi and ground- dwelling predators such as carabid beetles and wolf spiders and that aphid numbers will therefore decline during dry conditions. A fully factorial mesocosm experiment was conducted testing the combined effects of drought and predator presence on aphid position and abundance on barley (Hordeum vulgare) plants. In

support of our hypothesis, we found that (a) aphids moved below ground during dry

conditions, (b) predators reduced aphid numbers, but only during dry conditions, and

(c) predators reduced the proportion of aphids below ground in dry conditions. This

increased predation effect during dry conditions was, however, compensated for by

a corresponding increase in aphid performance on the plants and so the net effect

of drought on aphid numbers ended up being neutral. Thus, pests can be affected by

drought in complex ways via a combination top- down and bottom- up mechanisms.

Predicting how pest populations will be affected by droughts in the future is thus a formidable research challenge.