Hybridization in Diabrotica barberi Smith and Lawrence and Diabrotica longicornis (Say) (Coleoptera: Chrysomelidae): biology, behavior, field introgression, and a reevaluation of taxonomic status

Abstract

Diabrotica barberi Smith and Lawrence, the northern corn rootworm, and D. longicornis (Say) are currently recognized as closely related chrysomelid species. Multiple studies of morphological and genetic data have strongly supported that the two species are sister taxa, and hybridization has been proposed to occur between them. The objectives of this work were to: (1) assess life history parameters of the two taxa and their hybrids, (2) document and compare mating behavior, pheromone response, and spermatophore transfer of both taxa and their hybrids, and (3) examine genetic (amplified fragment length polymorphism, AFLP), and morphological (color and head capsule width) data of both taxa for evidence of introgression. Using lab-reared D. barberi and D. longicornis, fitness parameters and potential for population growth were assessed. D. barberi and D. longicornis were very similar for most life history traits but differed in longevity and fecundity. Hybrids of a D. longicornis mother and D. barberi father were as viable as individuals of either parental species, but hybrids of a D. barberi mother and D. longicornis father demonstrated consistently poor fitness. Studies of pheromone response, mating behavior, and spermatophore size demonstrated that F₁ hybrids (D. longicornis mother, D. barberi father) are reproductively fit and also that the two parental taxa exhibit numerous small differences in reproduction. The genetic and morphological data provided evidence for field introgression between D. barberi and D. longicornis. Gene flow between the two taxa was relatively high, which prevents strong differentiation of populations. In total, this research demonstrates that hybridization most likely occurs under field conditions, and that the relative fitness of hybrids of a D. longicornis mother and D. barberi father is comparable to the parental taxa. Currently available data support a reevaluation of the current taxonomic status of the taxa. Redesignation of the taxa as subspecies may be most appropriate. Greater work on the ecology of both taxa is needed to assess the degree of ecological separation and how gene flow occurs.