Populations in which the age of individuals is distributed with respect to time will have only a portion of their numbers subject to sampling at any point in time by any method which does not sample all age classes. An estimate of the total seasonal incidence of a population can be made from a single population density estimate by considering some of the theoretical factors affecting the proportion of the whole population which is counted under such circumstances. In particular, the width of the sampleable age class, the timing of the sample with respect to the frequency distribution of the sample able stages, and the variance of the sampleable age class with respect to time were considered.As a particular example, one method of using this theory is applied to cereal leaf beetle {Oulema melanopus(L.)} populations.