Trichogramma species in Samoa and their potential as biological control agents against the large cabbage moth, Crocidolomia pavonana

Abstract

Crocidolomia pavonana F. (Lepidoptera: Crambidae) is a major pest of Brassica crops. No effective natural enemies of C. pavonana were known until the discovery of a population of the egg parasitoid Trichogramma chilonis Ishii (Hymenoptera: Trichogrammatidae) that was effective in suppressing the pest in Samoa. Records of significantly lower parasitism of C. pavonana by T. chilonis throughout Asia raise questions about whether the Samoan population of T. chilonis is misidentified or represents an unrecognized cryptic species. In this study, egg parasitoids of C. pavonana and three co-occurring Lepidoptera species were surveyed across Samoa. Although T. chilonis was found to be the dominant parasitoid of all host egg species, two other Trichogramma species, T. australicum and T. achaeae, were recorded from Samoa for the first time. We tested for the presence of cryptic species among sequenced global populations of T. chilonis and populations within Samoa using a combination of species delimitation analyses using DNA sequences and population genetic analyses based on newly developed microsatellite markers. Species delimitation analyses indicated that multiple species were present among global populations identified as T. chilonis, suggesting that some records of parasitism attributed to T. chilonis represent misidentifications or undescribed cryptic species. However, the boundaries between putative species did not fully explain the significant parasitism of C. pavonana in Samoa. Microsatellite markers revealed high levels of gene flow across T. chilonis populations developing from different host egg species in Samoa. The potential importance of multiple host use by T. chilonis for suppression of C. pavonana populations is discussed. We recommend that further assessments of the species status of T. chilonis populations in the Asia-Pacific region should be prioritized to inform the development of a biological control program utilizing the Samoan T. chilonis population.