Soil nutrients and water availability interact to influence willow growth and chemistry but not leaf beetle performance

We investigated the effects of soil nutrient and water availability on the growth and chemistry of the silky willow ( Salix sericea Marshall), and on the performance of the imported willow leaf beetle ( Plagiodera versicolora Laichartig). Our major aims were to determine whether there are nutrient–water interactions on plant traits and whether this leads to parallel interactions for herbivore performance. We used a 2 × 3 fully factorial design, which consisted of high and low nutrient treatments crossed with dry, field capacity, and flooded water treatments. We found that nutrient additions increased plant growth, but only in field capacity and flooded conditions (nutrient–water interaction). Leaf nitrogen content also depended on the interaction between soil nutrients and water: nutrient addition resulted in a larger increase in foliar nitrogen in the field capacity treatment than in the flooded and dry treatments. Of the two phenolic glycosides measured, salicortin and 2 ′ -cinnamoylsalicortin, only one was affected by the treatments. 2 ′ -cinnamoylsalicortin concentration was lower in the high nutrient–dry treatment compared with the other treatments. In contrast to plant responses, there were no interactions found for larval or pupal weight or development time. Nutrient addition led to an increase in female pupal weight, and foliar N was positively correlated with female pupal weight and negatively correlated with female development time. In addition, leaf water was positively correlated with female development time. The lack of interactions for insect performance may stem from the small absolute differences in foliar nitrogen content associated with the interaction between the nutrients and water. Taken together, our results suggest that nutrient–water interactions influence plant traits that are potentially important for insect performance (leaf nitrogen and water), but these interactions do not produce parallel interactions in beetle performance.