Summer walnuts infested with the codling moth furnish breeding locations for the navel orangeworm and—as a result—a population of moths is present in the orchard when the husks begin to crack as the harvest period approaches. The caterpillars of the navel orangeworm are unable to penetrate the green husks of sound walnuts so they are not subject to infestation until maturity is reached and the husks split. After this stage of nut development is reached, the crop is open to attack until it is harvested. As a result the amount of infestation tends to increase as harvest is delayed.