The effects of abiotically induced changes in host plant quality (and morphology) on a salt marsh planthopper and its parasitoid

1. In this study, the effects of shading and fertilisation on the density o f the salt marsh planthopper Pissonotus quadripustulatus (Homoptera: Delpha cidae), which is monophagous on Borrichia frutescens (Asteraceae), were ass essed. The effects of these abiotic factors on the rate at which Pissonotus eggs were parasitised by Anagrus sp. nr armatus (Hymenoptera: Mymaridae) w ere also examined. 2. Field plots were shaded by shade cloth suspended above the plots, and fe rtilised by the application of NH4NO3 fertiliser. Both shading and fertilis er increased plant foliar nitrogen levels significantly. Both shading and f ertiliser also increased the density of Pissonotus, presumably because of t heir effect on plant quality. 3. In addition, both shading and fertiliser increased the percentage of Pis sonotus eggs parasitised by Anagrus. This increase in parasitism is probabl y attributable to changes in host plant morphology induced by the treatment s. 4. The results of this study suggest that in stressful environments such as salt marshes, changes in the abiotic environment may play an important rol e in shaping the community by directly affecting organisms on multiple trop hic levels.