Reproductive isolation between Diabrotica barberi Smith and Lawrence and Diabrotica longicornis (Say) (Coleoptera: Chrysomelidae)

Diabrotica barberi Smith and Lawrence and Diabrotica longicornis (Say) (Coleoptera: Chrysomelidae) are sister species. D. barberi, the northern corn rootworm, is an important pest of corn in the U.S. Corn Belt; D. longicornis is a nonpest species that occurs in grassland habitats. The species’ ranges overlap in eastern and central Nebraska and Kansas, and observational data suggest that some habitat overlap occurs as well. Previous work performed under laboratory conditions has shown that hybrids of female D. longicornis and male D. barberi have life history traits comparable to individuals of the parental species, but that the reciprocal cross (female D. barberi, male D. longicornis) is rarely viable. This study tested the hypothesis that the pairings of female D. barberi and male D. longicornis have low viability as a result of unsuccessful mating. Individuals of both species were paired for different lengths of time to form conspecific and interspecific crosses. Pairs were frozen, and females were examined for the presence of a spermatophore. Beetle head capsule and spermatophore size were also measured. D. longicornis male mating frequency was low with all females. However, in matings that did occur, spermatophores from D. longicornis were significantly larger than those of D. barberi. Other variables, including duration of pairing and head capsule size, were not significantly different. These data suggest that the two species may differ in certain reproductive characteristics, which may limit their ability to hybridize under field conditions.