Issues in the Use of
Microsporidia for Biological
Control of European Corn Borer
Timothy J. Kurtti and Ulrike G. Munderloh
Microsporidia are important natural regulators of many insect populations and as such are suitable microbial agents for the biological control of pest insects. Their effects on pest insects, however, are often sublethal and less obvious than those induced by other entomopathogens. Instead, microsporidia exert their adverse effects by reducing longevity and fecundity. They generally do not cause epizootics, when large numbers of insects die in a short time. Generally, epizootics occur when host density is high and after the economic threshold is exceeded. To overcome this drawback, researchers have used inundative inoculation of spores into the habitat of pest insects. This method has yielded some success when spores were used alone or were integrated with other chemical or biological control strategies. Nevertheless, microsporidia are not considered pathogenic enough to be used as biological insecticides. Rather, their biological control potential should be exploited in programs aimed at long-term, not short-term, control of pest insects.
The characteristics of microsporidia important to biological control include the production of large numbers of infective units (spores) per host insect that can persist in the environment, a high degree of infectivity, efficient transmission mechanisms, and moderate pathogenicity. Some pests are susceptible to more than one microsporidian species that can be categorized on the basis of speed of action and impact on the host: those that cause chronic debilitating diseases, and others that are quite virulent and can kill the host quickly, even within twenty-four hours. The majority, however, including the genus Nosema, require time to multiply and develop to debilitate the host. In the species that cause chronic diseases, multiplication of the parasite within the tissues of the host insect is extensive. Meanwhile, the insect continues to eat and grow, and the microsporidia are often transmitted trans