Aphids in a changing world: testing the plant stress, plant vigour and pulsed stress hypotheses

1 The plant stress, plant vigour and pulsed stress hypotheses describe the relationships between drought stress, plant quality and herbivore performance. We used an aphid-Brassica system to test these hypotheses under different drought treatments. 2 The quantity of water added per plant/week was 75%, 50% and 25% of the control (unstressed) water regime for low, medium and high drought stress, respectively, and 50% applied fortnightly for pulsed drought stress. The performance of a senescence (generalist) and a flush feeder (specialist) aphid species and host plant quality were assessed. 3 Drought treatments had a similar effect on the fecundity and intrinsic rate of increase of both aphid species. Aphid performance on unstressed and highly drought-stressed plants was significantly lower compared with medium drought stress. On average, 20% greater fecundity and 40% greater intrinsic rates of increase were recorded for both aphid species at medium drought stress compared with unstressed plants. 4 Plant biomass and relative water contents were significantly greater for unstressed plants compared with high and pulsed drought treatments. Foliar nitrogen concentration was significantly greater in the high drought stress and pulsed treatments, and the dominant glucosinolate (glucobrassicin) concentration was significantly greater in drought stress treatments. 5 The present study supports the plant stress hypothesis, although the plant vigour and pulsed stress hypotheses are not supported by our data. The implications of these findings for plantherbivore interactions under changing environmental conditions are discussed.