Classifying Instars of the Navel Orangeworm (Lepidoptera: Pyralidae) Based on Observed Head Capsule Widths

A novel statistical procedure is applied to head capsule width measurements of larval navel orangeworm, Amyelois transitella (Walker), to determine classification boundaries of head capsule width when the head capsule widths of adjacent instars overlap. The two-step procedure involves estimating the mean and standard deviation of each instar size class by means of the estimation-maximization algorithm, and determining the head capsule width boundaries for each instar with a generalized discriminant function used to discriminate among several instar sub populations. The procedure also provides a statistical means to determine the number of instars represented in the frequency distribution of head capsule width.