A biologically based phenology model is described for use in integrated pest management. The model predicts the proportion of insects in a population in various stages of development as a stochastic function of accumulated degree-days. The model is based on a logistic probability distribution with mean and variance changing through time. Maximum likelihood parameters of the estimates are easily computed with nonlinear regression packages. Appropriate statistical tests are presented for comparing models. Results should be useful to researchers and biological modelers who describe insect or plant phenology.