Modelling the effect of host patch size on Mexican bean beetle emigration

The resource concentration hypothesis states that specialized herbivores are more likely to find and remain in pure, large, and/or dense stands of their host plants. This study examined whether in fact herbivores tended to remain longer in large vs. small patches of host plants and attempts to explain the results by modelling herbivore movement. Both movement within a patch and emigration of marked Mexican bean beetles (MBBs) were studied by releasing beetles into small (19 plants) and large (61 plants) patches. Beetles left small patches at a faster rate than large patches, but the difference was not statistically significant. A stochastic model that was developed translates individual patterns of MBB movement into population patterns of beetle distribution within a patch, and predicts how this distribution changes with time. Estimating and testing a model on two separate sets of data provides a more rigorous test than applying the model to the data set on which it was developed. The parameters of the model, therefore, were estimated from data collected in the large patches, and the model was tested by comparing the predicted with the observed spatial distributions of beetles in small patches (i.e., a novel situation). Predicted distributions were very similar to the observed distributions; the predicted proportion of insects remaining in a patch at each census time differed from the observed by < 10% on the average. The model predicts that MBB emigration rate should decrease with patch size, con- firming the observed trend.