A comparison of three sampling techniques to estimate the population size of caterpillars in trees.

Three sampling techniques commonly used to estimate the population size of caterpillars and sawfly larvae in trees (branch samples, frass production, water basins), were compared with respect to sampling error and economic costs. At the level of tree populations (e.g. forests), on an arbitrary date, the mean caterpillar intensity per tree (expressed in numbers of larvae or their biomass per 100 shoots) was predicted from the mean frass production per tree (expressed in mg frass per m2 forest floor per day). At the level of the single tree, the frass production on an arbitrary date was related to the population intensity, but, due to the large sampling error, did not provide an accurate prediction. Summing the frass produced over the whole season reduced this error and predicted the seasonal abundance of larvae in single trees, estimated as their maximum intensity or their density (numbers of larvae or their biomass per m2 forest floor). The maximum population intensity was not related to the population density. The sampling techniques suffer from large errors unrelated to larval abundance. The main sources of error (i.e. weather or predation of the larvae) usually cause an underestimation of population size. Labour, the main cause of high costs, was low in the basin technique and high in the frass production technique. Possible ways of reducing errors and applications of the three techniques are discussed.