Research by Chiang (1965) and Krysan et al. (1984) has demonstrated that eggs of the northern corn rootworm, Djabrotica barberi, are capable of remaining in diapause for longer than a single winter chill period. Subsequently, researchers in several northern Corn Belt states have used controlled environmental and field experiments to confirm the presence of what has come to be known as "extended diapause." In 1986 Krysan et al. used empirical evidence provided fortuitously by the Payment-In-Kind program to attribute larval damage in rotated field corn to northern corn rootworms with the extended diapause trait. During 1987 the incidence in Iowa of corn rootworm larval damage in corn grown in an annual rotation with another crop, usually soybeans, made its third consecutive, dramatic increase. The probability of farmers responding by applying a prophylactic soil-insecticide treatment to rotated corn stimulated the Leopold Center for Sustainable Agriculture to support a three-year survey of the seriousness of extended diapause in Northwest Iowa.