Combined effects of drought stress and psyllid herbivory on the invasive weed Scotch broom, Cytisus scoparius

Increases in the frequency and duration of droughts under global climate change could have implications for plant–insect interactions, and could either increase or decrease the effects of weed biological control agents. In this study, we used greenhouse and field experiments to examine the impacts of drought stress on the abundance and impacts of the adventive psyllid Arytainilla spartiophila Forster (Hemiptera: Psyllidae, Arytaininae) on its target weed Scotch broom, Cytisus scoparius (L.) Link (Fabaceae), in California, USA. The psyllid impacted plant growth in both field and greenhouse experiments, whereas drought stress affected plant growth in the greenhouse only, suggesting that other factors besides water availability may be more limiting for plants in the field. Effects on psyllid survival were consistent with the plant vigor hypothesis, which predicts that herbivores will perform better on vigorously growing plants; psyllid numbers were lower on drought‐stressed plants in the greenhouse and were correlated with plant growth in both the greenhouse and the field. In the greenhouse, the combined effects of the psyllid and drought stress were additive, indicating that the effects of the psyllid were consistent across unstressed and drought‐stressed plants. Although the psyllid is unlikely to control Scotch broom on its own, results suggest that it may work in conjunction with drought stress to suppress Scotch broom.