Spider-planthopper interactions: Influence of predator aggregation, vegetation structure, and temporal refuges on prey suppression

The key to elucidating the potential of invertebrate predators to suppress prey populations lies in an understanding of those factors which couple or stabilize the interaction. Using Prokelisia planthoppers (Hemiptera: Delphacidae) and wolf spiders (Araneae: Lycosidae)) in an intertidal Spartina marsh, I investigated the ability of predators to suppress prey populations, and examined how several habitat-related factors influenced the dynamics of the interaction.; Two lines of evidence demonstrated that wolf spiders suppressed populations of Prokelisia planthoppers (Chapter 1). First, there was a significant negative relationship between the growth of Prokelisia planthopper populations and wolf spider density. Second, when spiders were removed from the habitat and their aggregative response impeded, planthopper populations increased rapidly. Thus, a strong aggregative response was linked to prey suppression.; Vegetation structure also had a very striking impact on spider-planthopper interactions (Chapter 2). Spiders aggregated more in small experimental plots containing high densities of planthoppers, a response which was exaggerated by the presence of litter. Also, spiders were much more abundant and planthoppers were rare in large experimental plots supplemented with litter compared to litter-free plots. Consequently, litter intensified the numerical response of wolf spiders and enhanced their ability to suppress planthopper populations.; The stage-class composition of the prey population had a strong impact on the predator-prey population dynamic (Chapter 3). Planthopper populations were more vulnerable to wolf spider predation when their stage-class composition was biased toward early-instar nymphs. In contrast, planthopper populations composed of a stage-class mix containing a high fraction of egg-laying adults escaped predation and increased significantly in size. Once in the egg stage, there is a "window of invulnerability" and the planthopper population finds a temporal refuge from predation.; In both natural and managed systems, invertebrate predators inflict much more mortality on planthopper populations than do parasitoids (Chapter 4). The important predators responsible for this mortality are spiders (Lycosidae) and hemipterans (Miridae). The biological control of pest planthoppers with predators, predator removal experiments, and the resurgence of planthopper populations following predator kill with insecticides, suggest that invertebrate predators play an important suppressing role in the population dynamics of planthoppers.