Do Parasitoids Regulate Pea Aphid Populations in Alfalfa?   A Model-based Approach to Analyzing Ecological Time Series.

ABSTRACT
In southern Wisconsin, pea aphid (Acyrthosiphon pisum) populations are regularly held in check below economic threshold.  Cage experiments and ecological theory suggest that this regulation is most likely biological control by the parasitoid wasp Aphidius ervi.  However, it is difficult to draw inferences about dynamics at the scale of a field on the basis of small-scale experiments, and additional mechanisms may be more important at larger scales (e.g., predation, host-plant interactions).  Here, I present a model-based method for analyzing monitoring data from four alfalfa fields that attempts to separate the effect of parasitoids from the other determinants of aphid dynamics at the scale of the field.  The model is constructed in a way that allows the data to determine the functional relationship between parasitism rate and aphid density, a relationship is key to understanding host-parasitoid dynamics.  Although the model is developed explicitly in the context of aphids and parasitoids, the general framework could be applied to a number of ecological systems where investigators seek to analyze time series data from monitoring studies.
 
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College of Natural Sciences