Technical Abstract: Diapause is an adaptive trait that delays development or reproduction under unfavorable circumstances. The northern corn rootworm, Diabrotica barberi Smith & Lawrence, an important maize, Zea mays L., pest in the Diabroticite species complex, overwinters in diapause during the egg stage. Some NCR populations are adapted to crop rotation by expressing an extended diapause (ED) trait that delays embryonic development for two years. This ED trait has increased in frequency and geographic distribution since first reported in Illinois in 1932. Reciprocal single pair crosses among beetles from a laboratory colony with the ancestral one-year diapause trait and field collected beetles with the two-year ED trait indicated that ED females laid significantly more ED eggs than did females with the one-year diapause trait regardless of male genotype. The ED trait was highly heritable [realized heritability (h2) = 0.698 ± 0.314], with genetic dominance (D) of the trait strongly influenced by female genotype. Selection of the ED trait and maintenance of polymorphic diapause phenotypes within maize-soybean cropping systems is discussed in relation to response to a fluctuating environment and as a potentially advantageous life history adaptation.