Susceptibility of Lepidopterous Pests to Two Types of Mortality Caused by the Microsporidium Vairimorpha necatrix12

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Abstract
Median lethal concentrations (LC50’s) were determined for 2 types of mortality caused by the microsporidium Vairimorpha necatrix (Kramer) in 6 lepidopterous pests, i.e., relatively quick death by gut damage caused by large spore doses, and death at pupation by microsporidiosis caused by small doses. Diatraea saccharalis (F.) was the least susceptible and Trichoplusia ni (Hübner) the most susceptible to both types. Pseudoplusia includens (Walker) was one of the most susceptible insects in the 1st instar and least susceptible in the 3rd. Microsporidiosis LC50’s were similar in Heliothis zea (Boddie) and H. virescens (F.), but the latter was more susceptible to gut damage. Of the larvae that ate spores and survived to the adult stage, 32% of the Spodoptera frugiperda (J. E. Smith) and 0–8% of the other 5 insects were infected. The results indicated that V. necatrix has promise for control of T. ni, H. virescens, P. includens, S. frugiperda, and H. zea and that they should be treated in the earliest possible instar.