Adaptations to water gradient in three Rorippa plant species correspond with plant resistance against insect herbivory under drought and waterlogged conditions

1. Plants live in environments where they are constantly, and often simultaneously, exposed to different types of biotic and abiotic stress, such as insect herbivory and water availability. How plants are adapted to abiotic conditions may determine how a surplus or shortage of water affects plant resistance to insect herbivory. Moreover, this effect may vary depending on the feeding mode of the herbivore.
2. We explored how three closely related Rorippa plant species that vary in adaptations to different water levels, resist herbivory by four different insects (aphids: Myzus persicae, Lipaphis erysimi, and caterpillars: Pieris brassicae, Plutella xylostella) under waterlogging or drought conditions. We hypothesized that plants that are differently adapted to water availability will be disparately affected by water availability in their resistance to insect herbivory.
3. On the semi-aquatic plant species Rorippa amphibia, both aphid species reached a larger colony size under drought conditions. This indicates that R. amphibia was compromised in resistance to aphid feeding when under drought conditions, to which it is less well adapted. Water conditions did not affect aphid performance on the flood-plain species Rorippa palustris. On the terrestrial plant species Rorippa sylvestris, aphids performed worse on waterlogged than drought-treated plants. Neither caterpillar species was significantly affected by the water availability of their food plant.
4. Our findings suggest that water availability can have distinct effects on plant–insect interactions. We propose that plant adaptations to water conditions can be a major predictor towards explaining the variation of effects that water availability can have on plant–insect interactions.