The effects of soil nutrients and water on the suitability of silky willow for the imported willow leaf beetle (Salix sericea, Plagiodera versicolora)

Soil nutrient and water availability are two essential resources that influence plant quality to herbivorous insects. Still, relatively little is known about how these resources interact to influence plant traits and herbivore preference and performance. I used a combination of lab, common garden and field experiments to investigate how variation in nutrients and water influence the growth and chemistry of the silky willow, Salix sericea Marshall, and the subsequent response of the imported leaf beetle, Plagiodera versicolora Laichartig.; I first measured the growth response of willows in gradients of water ranging from dry to flooded. Shoot biomass was marginally lower in flooded and significantly lower in very dry conditions compared with moist conditions (well-drained). Leaf water content was low in both flooded and very dry conditions compared with well-watered conditions (moist and wet). In contrast to the concave response of growth and water content, leaf nitrogen increased from flooded to dry conditions in a roughly linear manner.; To test the effects of simultaneously manipulating nutrients and water on larval performance and adult preference, I conducted two experiments that consisted of a 2 x 3 and a 2 x 2 design, with high and low nutrient treatments, and dry, field capacity and flooded water treatments (dry was omitted in the second experiment). In general, nutrient addition increased plant growth and leaf nitrogen, and decreased the phenolic glycoside 2′-cinnamoylsalicortin. Nutrient and water availability interacted to influence these plant traits. I found two important relationships. First, leaf nitrogen was positively correlated in larval preference and performance. Second, adult beetles settled and oviposited preferentially on willows growing in high-nutrient, flooded conditions.; In the field, soil nutrients and water were not predictive of beetle abundance on S. sericea and S. sericea × S. eriocephala hybrids. Instead, larvae were more abundant on willows growing on high pH soils. This result is difficult to interpret, since soil pH could influence host chemistry, host distribution or, alternatively, host chemistry could influence soil pH. What is clear is that leaf beetles preferred hybrid willows, and beetles were more common on willows high in protein and low in the phenolic glycoside salicortin. The controlled experiments suggest that nutrients, water and their interaction influence plant quality to leaf beetles, but whether these effects are relevant in the field requires further work.