A Method for Estimating the Effects of Mortality on Aphid Populations

In insects where the generations are distinct, a census of the number of individuals at the beginning and the end of the generation is generally sufficient to estimate the overall population losses. Where, as in aphid populations, the generations overlap so as to become indistinguishable, the population is reproducing the whole time while the depletion is occurring. Because of the continuous additions to the population, the overall losses cannot be defined from the changes in observed numbers alone, a measure of the potential increase of the population being required as well. When a population is growing in a stable environment for a long period its age structure should, under certain circumstances, reflect this potential increase and make possible an estimate of its value. The overall losses are then given by the difference between the potential increase and the observed increase (which may of course be negative). If the losses caused by any particular mortality factor, such as a natural enemy,