Plant water stress intensity mediates aphid host choice and feeding behaviour

The effects of drought-induced changes in plant quality on aphid performance and population growth is well-studied. The response of aphid behaviour to plant water limitation has received less attention. Water limitation may affect host-plant colonization by altering the attractiveness of plants. Additionally, plant water limitation may inhibit feeding site establishment and phloem ingestion. Our goal was to examine bird cherry-oat aphid (Rhopalosiphum padiL.) host selection and feeding behaviour under water limitation. We assessed aphid response to well-watered, mildly-stressed, and highly-stressed wheat (Triticum aestivumL.) by evaluating (i) host-plant selection through two-choice assays, (ii) feeding behaviour using the electrical penetration graph technique, and (iii) phloem ingestion by quantifying honeydew production. Aphids were less likely to select highly stressed plants than a mildly stressed or well-watered alternative. Aphids did not distinguish between mildly stressed and well-watered plants. Aphid feeding behaviours, including duration of phloem ingestion, were not affected by water availability. However, honeydew production was reduced under both levels of water limitation. These results suggest that the volume of phloem ingested by aphids per unit time declined on stressed plants. The combination of lower colonization and diminished access to food on stressed plants may lead to a reduction in aphid abundance, independent of the direct effects of nutrition on individual aphid performance. This study highlights the potential contribution of herbivore behaviour to documented changes in aphid abundance on stressed plants and underscores the important role of plant water stress intensity in mediating plant-herbivore interactions.