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

Authors: Hogg, Brian; Moran, Patrick

Submitted to: Entomologia Experimentalis et Applicata
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/27/2020
Publication Date: 3/18/2020
Citation: Hogg, B.N., Moran, P.J. 2020. Combined effects of drought stress and psyllid herbivory on the invasive weed Scotch broom, Cytisus scoparius. Entomologia Experimentalis et Applicata. 168(3):209-220. https://doi.org/10.1111/eea.12880.
DOI: https://doi.org/10.1111/eea.12880

Interpretive Summary: Reductions in water availability cause plants to become stressed, which can either increase or decrease the abundance and impacts of herbivores on plants. Herbivores may benefit or suffer when plants are stressed. In this study we examined how drought stress affects the ability of an insect herbivore, a psyllid, to suppress its host plant, the invasive weed Scotch broom, in California. The psyllid was intentionally introduced to New Zealand as a biological agent for Scotch broom, but appeared on its own in North America. The psyllid reduced growth of Scotch broom in field and greenhouse experiments, whereas changing the amount of water in the soil affected plant growth in the greenhouse but not in the field, suggesting that other factors besides water availability limited growth of Scotch broom in the field. However, psyllid numbers increased on unstressed plants in the greenhouse, and on more vigorous plants in both the field and the greenhouse, showing that the psyllid performs better when plants are unstressed. The effects of the psyllid in the greenhouse did not change between stressed and unstressed plants. Thus, our results suggest that the psyllid acts in combination with drought stress to limit plant growth, although more research is needed to examine effects of drought stress on plants in the field.

Technical Abstract: Increases in the frequency and duration of droughts under global climate change could have implications for weed biological control. Drought stress of plants 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 on its target weed Scotch broom, Cytisus scoparius, in California. The psyllid impacted plant growth in both field and greenhouse experiments, although 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.