Insect Natural Enemies as Regulating Factors

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Abstract
The view that natural enemies are important in the regulation of temperate Lepidoptera populations has recently been challenged by Dempster (1983). This conclusion was based on examining 24 sets of life tablets of Lepidoptera populations with more-or-less discrete generations, all but 3 of which failed to reveal natural enemies as density-dependent factors when analyzed using the conventional technique of plotting some measure of proportionate mortality against population density per generation. This conclusion is challenged here on the basis of how difficult it is to detect natural enemies as density-dependent factors, especially when they act together with other stochastic processes. The point is illustrated by analyzing model populations in which the parasitoids are the only regulating factor. The problem lies in attempting to detect density dependence that arises within a generation (e.g. by non-random parasitism between patches of different host density) from estimates of mean population size per generation. The only real solution is to seek much more detailed information from within each generation, and thus to design life table sampling programs that are stratified in 2 dimensions of space and time.