Some insect species, including many pests, are particularly susceptible to infection by naturally occurring, insect-pathogenic fungi. These fungi are very specific to insects, often to particular species, and do not infect animals or plants. Fungal growth is favored by moist conditions but fungi also have resistant stages that maintain infection potential under dry conditions. Fungi have considerable epizootic potential and can spread quickly through an insect population and cause its collapse. Because fungi penetrate the insect body, they can infect sucking insects such as aphids and whiteflies that are not susceptible to bacteria and viruses.
Several fungal species have potential as microbial insecticides and, in some countries, are commercially available in formulations that can be applied using conventional spray equipment.
Most crops including soybeans, greenhouse crops, vegetables, cotton, citrus, and ornamentals; also interior plantscapes and forests. An aquatic fungus infects mosquito larvae of some genera.
Most insect pests are susceptible to fungal pathogens. Some fungi, such as the Entomophthora and related species, are fairly specific with regard to the groups of insects affected; others, such as Beauveria, have a wider host range.
Mode of Action
Fungi invade insects by penetrating their cuticle or "skin." Once inside the insect, the fungus rapidly multiplies throughout the body. Death is caused by tissue destruction and, occasionally, by toxins produced by the fungus. The fungus frequently emerges from the insect's body to produce spores that, when spread by wind, rain, or contact with other insects, can spread infection.
Infected insects stop feeding and become lethargic. They may die relatively rapidly, sometimes in an upright position still attached to a leaf or stem, perhaps in an elevated location or concentrated near crop borders. The dead insect's body may be firm and "cheese-like" or an empty shell, often but not always with cream, green, red, or brown fungal growth, either enveloping the body or emerging from joints and body segments. Infected aphids can be swollen and discolored; infected root maggot flies may be clustered on shoot tips, tall grasses, or other prominent features.
Insect-pathogenic fungi usually need moisture to enable infection, and natural epizootics are most common during wet or humid conditions. The effectiveness of these fungi against pest insects depends on having the correct fungal species and strain with the susceptible insect life stage, at the appropriate humidity, soil texture (to reach ground-dwelling pest species), and temperature. The fungal spores, which can be carried by wind or water, must contact the pest insect to cause infection. Naturally occurring fungal epizootics may decimate aphid, root maggot fly, caterpillar, leafhopper, and thrips populations. They can be an important natural control of aphids in potatoes and other crops.
Many insect-pathogenic fungi occur in the soil. There is evidence that application of some soil insecticides, fungicides, and herbicides can inhibit or kill these fungi. For example, even quite low concentrations of some herbicides can severely limit the germination and growth of Beauveria bassiana fungal spores in soil samples.
Metarhizium anisopliae is registered in the U.S. for control of household cockroaches. Beauveria bassiana Strain GHA (trade names Mycotrol GH-OF and Mycotrol GH-ES) is registered to control grasshoppers, locusts, and Mormon crickets on rangeland, improved pastures, alfalfa, corn cotton potatoes, rapeseed, safflower, small grain crops, soybeans, sugarbeets, and sunflowers. Paecilomyces fumosoroseus Apopka Strain 97 has been approved for use on ornamentals, non-food crops in greenhouses, and interiorscapes to manage whiteflies, aphids, thrips, and spider mites and is sold by Thermo Trilogy Corp. In addition, there are fungal antagonists available to combat various fungal pests.
Some Common Insect-Pathogenic Fungi
Entomophthora muscae infects flies. Susceptible pest species include the adults of the onion maggot, cabbage maggot, and seedcorn maggot. The fungus multiplies within the body of the adult fly which becomes enlarged; yellowish bands of fungal spores stripe the abdomen.
Related species include Pandora neoaphidis, a common, naturally occurring pathogen of aphids that can be particularly effective during moist periods. Zoophthora radicans is another common fungus, with a broad host range. Strains have been isolated from several caterpillars, including diamondback moth, leafhoppers, aphids, and some weevils. Other related species infect thrips.
Beauveria bassiana is an insect-pathogenic fungus found naturally on some plants and in the soil. Epizootics are favored by warm, humid weather. It is known as the white muscardine fungus because infected insect larvae eventually turn white or gray. Beauveria is used as a fungal microbial insecticide in some countries. It has an extensive host list that includes such important pests as whiteflies, aphids, grasshoppers, termites, Colorado potato beetle, Mexican bean beetle, Japanese beetle, boll weevil, cereal leaf beetle, bark beetles, lygus bugs, chinch bug, fire ants, European corn borer, codling moth, and Douglas fir tussock moth. It has even been found infecting the lungs of wild rodents, and the nasal passages of humans. Unfortunately, natural enemies, such as lady beetles, are susceptible too. One possible application method that may avoid harming beneficial insects is the use of fungus-contaminated insect baits that are attractive to pest species only. There are many different strains of the fungus that exhibit considerable variation in virulence, pathogenicity and host range. It occurs in the soil as a saprophyte.
Metarhizium spp. is being tested as a natural enemy of corn rootworm, white grubs (scarabs), and some root weevils. It has a very broad host range and is extensively used in Brazil against spittle bugs in sugar cane and alfalfa.
Other species of insect-pathogenic fungi have been tested as microbial insecticides for the control of pests. Verticillium lecanii is used in Europe against greenhouse whitefly and thrips and aphids, especially in greenhouse crops. Neozygites floridana has been successfully tested against spider mites, and Nomuraea rileyi has activity against green cloverworm, cabbage looper, imported cabbageworm, armyworms, corn earworm, and tobacco budworm. Hirsutella thompsonii infects mites and was previously registered for use in the United States.
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. 666pp.
Vandenberg, J.D. (1995) Personal communication.
Weinzierl, R., and Henn, Tess. (1989) Alternatives in insect management: Microbial insecticides. Cooperative Extension, University of Illinois, Circular 1295. 12 pp.
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