Pathogenicity, prevalence and intensity of a microsporidian infection by Nosema fumiferanae postvittana in the light brown apple moth, Epiphyas postvittana, in California.
While biological invasions are increasing, in some cases exotic species exhibit an initial phase of population growth and spread, followed by a subsequent phase of natural decline. The light brown apple moth, Epiphyas postvittana (Walker) (Lepidoptera: Tortricidae), provides a unique opportunity to examine potential mechanisms for the natural suppression of an exotic insect species that has become established in coastal California. We recently discovered a microsporidian pathogen, Nosema fumiferanae postvittana, from E. postvittana in its novel range. In the laboratory, we examined the pathogenicity and latent period of this microsporidium, and in the field we determined its prevalence and intensity in five locations using quantitative real-time PCR (qPCR). In the laboratory, when comparing healthy larvae to larvae infected with up to 105 spores, we found a reduction in juvenile survivorship (from 100% to 26%), a prolongation of juvenile development time (of up to 9-10 days), a reduction in viable lifetime fecundity (from 788 to 1) and a reduction in the intrinsic rate of increase (from 0.18 to 0.008). The median lethal dose (LD50) was estimated to be 1.8×104 spores, and the mean latent period for infections with 103 spores was 12.67 days. Our field sampling revealed that E. postvittana populations have further declined from previously reported densities in San Francisco and Santa Cruz. We detected N. fumiferanae postvittana in all five locations with an overall prevalence of 5%, which did not appear to be influenced consistently by either host density or season. Mean microsporidian intensity in field-infected individuals was 226 spores. Although the laboratory results demonstrated the potential for host suppression, the field sampling indicated that the prevalence and intensity of microsporidian infection were too low to account for the continued decline in population densities of E. postvittana in coastal California.