EVALUATING MODELS FOR SPRUCE BUDWORM FOREST MANAGEMENT - COMPARING OUTPUT WITH REGIONAL FIELD DATA

The evaluation of models used in the management of populations can be complicated by the number of component parts and by the large temporal and spatial scales often required. This is particularly true of models developed for the analysis of management policy in forest pest situations. In this study, two large-scale spruce budworm-forest simulation models were evaluated by comparing their output with data collected annually by the Maine Forest Service survey at 1000 sites from 1975 to 1980. In practice, model evaluation typically involves a comparison of observations, independent of those used to construct the model, with overall model output. We did this, and in addition, separate tests were performed on major components of each large-scale budworm model. These components represent Maine's forest protection policy, the budworm-forest dynamics, and pest control efficacy. Both models produced output that was in some way inconsistent with the Maine survey data. Inconsistencies were most prevalent at low budworm densities, especially after pesticide spraying, when model output predicts budworm populations increase more slowly than the survey data suggest. These inconsistencies pointed to inaccuracies in the models' representation of Maine's forest protection policy, of budworm population growth at low densities, and of the effectiveness of spraying (especially at low budworm densities). Problems translating the results of studies of nonlinear population dynamics from small experimental plots to the larger spatial scale used in the models are implicated. Our results suggest that the optimal threshold density of budworm for insecticide application is probably higher than the upper threshold of 20 egg masses/m2 inferred from the models.