Protozoa and Microsporidia
Protozoans are one-celled life forms. Some species are responsible for serious human diseases, such as malaria, vectored by mosquitoes. However, there are about 1200 species, out of 15,000 described, specific to and causing diseases in insects.
One group, the Microsporidia, contains many species that have promise for biological control. Microsporidian infections in insects are thought to be common and responsible for naturally occurring low to moderate insect mortality. But these are relatively slow acting organisms, taking days or weeks to debilitate their host. Frequently they reduce host reproduction or feeding rather than killing the pest outright. Microsporidia often infect a wide range of insects. Some microsporidia are being investigated as microbial insecticides, and at least one is available commercially, but the technology is new and work is needed to perfect the use of these organisms.
Mode of Action
Most microsporidia must be eaten to infect an insect, but there may also be some natural transmission within a pest population, for example by predators and parasitoids. The pathogen enters the insect body via the gut wall, spreads to various tissues and organs, and multiplies, sometimes causing tissue breakdown and septicemia.
Infected insects may be sluggish and smaller than normal, sometimes with reduced feeding and reproduction, and difficulty molting. Death may follow if the level of infection is high. One advantage of this type of infection is that the weakened insects are more likely to be susceptible to adverse weather and other mortality factors.
Some Beneficial Microsporidia
Nosema pyrausta (=Perezia pyraustae) is a microsporidium that infects several insect species, including European corn borer, for which it can be an important natural control. This disease was widespread in the midwestern United States during the 1950s and '60s, causing considerable natural mortality, but its commercial use is still in the developmental phase. Infection can spread from diseased to healthy larvae via contaminated frass, and by migration of infected larvae between plants.
Nosema locustae is the only commercially available species of microsporidium, marketed under several labels for the control of grasshoppers and crickets. It is applied with an insect-attractant bait. Because of its slow mode of action, this product is better suited to long-term management of rangeland pests than to the more intensive demands of commercial crop or even home garden production. Other Nosema species have been shown to infect spider mites and webworms, but have yet to be developed sufficiently for commercial use.
Vairimorpha necatrix is another microsporidium with commercial potential. It has a wide host range among caterpillar pests, including corn earworm and European corn borer, various armyworms, fall webworm, and cabbage looper. It can be more virulent than other species and infected insects may die within six days of infection.
Hoffmann, M.P. and Frodsham, A.C. (1993) Natural Enemies of Vegetable Insect Pests. Cooperative Extension, Cornell University, Ithaca, NY. 63 pp.
Tanada, Y., and Kaya, H.K. (1993) Insect Pathology. Academic Press, Inc., San Diego. 666 pp.
Weinzierl, R., and Henn, Tess. (1989) Alternatives in insect management: Microbial insecticides. Cooperative Extension, University of Illinois, Circular 1295. 12 pp.