Vairimorpha sp. and V. necatrix were assayed in combination with one another, and independently, with a nuclear polyhedrosis virus (RoMNPV) from a mint looper, R. ou, against neonate and 3rd-stage black cutworm larvae, Agrotis psilon. Initially the effect of Vairimorpha sp. was subadditive, additive, or slightly inhibiting to the V. necatrix in the dual microsporidian assays; later, V. necatrix antagonized the effect of the Vairimorpha sp. In combination with Vairimorpha sp. gradients, V. necatrix significantly (P < 0.05) reduced, in most instances, the LT50 values in both neonate and 3rd-stage larval assays. RoMNPV assayed against neonate and third instars, in combination with either Vairimorpha sp. or V. necatrix gradients, usually significantly reduced the LT50 values (P < 0.05). RoMNPV, when combined with either Vairimorpha species, had varying effects on its pathology. In all assays, the particular relationship that was expressed seemed to be a function of the concentration of each pathogen, which may indicate that the two microorganisms compete for entry and/or infective sites within the larval host. Larval size, which was significantly reduced (P < 0.05) by both microsporidia, would be involved in such competition because it would limit the tissue mass available for infection. Histological examinations of larvae with dual infections revealed pathogens in the tissues that they normally infect. Vairimorpha sp. primarily infected epithelial, fat body, and Malpighian tubule tissue and, occasionally, muscle tissue. Viral polyhedral inclusion bodies were found in the same fat body tissues as the Vairimorpha spores.
