A method for determining the resistance of a pest to a plant that produces insecticidal toxins. The method involves sampling pests in fields with known pest problems, and in control fields with no known pest problems. Eggs are obtained from the sample pest populations, and larvae hatched from the eggs are evaluated in laboratory bioassays for their survival on two transgenic crop hybrids, each of which contain a unique toxin targeting the pest, and two near isogenic hybrids that lack a gene for the toxin. Mortality rates are determined by counting the larvae recovered, and measuring the larvae instars based on head capsule width to calculate the resistance levels.
