Influence of Natural and Synthetic Compounds on the Infectivity of a Vairimorpha (Microspora) sp. in Trichoplusia ni (Lepidoptera: Noctuidae) Larvae

In this study, we examined the influence of phytochemicals and antiprotozoan drugs on the time to death in Trichoplusia ni (Hübner) larvae infected with a microsporidian pathogen belonging to the genus Vairimorpha. Of the 15 compounds tested, only xanthotoxin, a linear furanocoumarin with phototoxic properties, delayed mortality in infected insects. In the absence of UV light, larvae fed an artificial diet containing 0.02% fresh weight xanthotoxin survived ≈2 d longer than did their drug-free counterparts. With UV exposure, survival time increased to ≈3 d longer than that for larvae under drug-free conditions. The alkaloid camptothecin caused developmental delays at concentrations of 0.02% and this delay resulted in enhanced mortality in infected larvae due to their slower growth and therefore smaller size. Other compounds (onion extract containing alkyl sulfides, metronidazole, and albendazole) that have been shown previously to influence microsporidian infection under different conditions had no effect on Vairimorpha (T. ni isolate) pathogenicity in T. ni. The lack of response of Vairimorpha (T. ni isolate) to a wide range of biologically active compounds may reflect evolutionary exposure of the phytophagous host to a wide range of toxins.