Much has been learned in recent years regarding the influence of environmental conditions on top-down and bottom-up effects acting on insect herbivores. Temporal variation in environmental conditions, however, has gone largely unstudied in spite of undoubtedly strong influences in most systems. In this study we conducted a two year experiment to examine the legacy effects of previous manipulations of host plant quality and parasitism pressure on the top-down and bottom-up effects influencing population densities of the salt marsh planthopper Pissonotus quadripustulatus (Homoptera: Delphacidae). The effects of increased plant quality persisted for at least five months after treatments, while the effects of reduced parasitism attenuated much more quickly. Increases in plant quality had similar effects in both years, but reduction of parasitism in the second year, on plots that had been previously fertilized, resulted in greater increases in planthopper density than in the first year. The results of this study suggest that both current and historical variation in environmental conditions can significantly influence the density of phytophagous insects.
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