The Insect Pathogen Bacillus thuringiensis

Bacterial pathogens used for insect control are spore-forming, rod-shaped bacteria in the genus Bacillus. They occur commonly in soils, and most insecticidal strains have been isolated from soil samples. Bacterial insecticides must be eaten by target insects to be effective; they are not contact poisons. Insecticidal products composed of a single Bacillus species or subspecies may be active against an entire order of insects, or they may be effective against only one or a few species. For example, products containing Bacillus thuringiensis var. kurstaki kill the caterpillar stage of a wide array of butterflies and moths. In contrast, Bacillus popilliae var. popilliae (milky disease) kills Japanese beetle larvae but is not effective against the closely related annual white grubs (masked chafers) that infest lawns in much of the Midwest. The microbial insecticides most widely used in the United States since the 1960s are preparations of the bacterium Bacillus thuringiensis (Bt). Bt products are produced commercially in large industrial fermentation tanks. The bacterial cells usually produce a spore and a crystalline protein toxin - called an endotoxin - as they develop. Most commercial Bt products contain the protein toxin and spores, but some contain only the toxin component. When Bt is ingested by a susceptible insect, the protein toxin is activated by alkaline conditions and enzyme activity in the insect's gut. If the activated toxin attaches to specific receptor sites, it paralyzes and destroys the cells of the gut wall, allowing the gut contents to enter the insect's body cavity. Poisoned insects may die quickly from the activity of the toxin or may stop feeding and die within 2 or 3 days from the effects of septicemia (blood-poisoning). Bt does not reproduce and persist in the environment in sufficient quantities to provide continuing control of target pests. The bacteria may multiply in the infected host, but because few spores or crystalline toxins are produced, few infective units are released when a poisoned insect dies. Consequently, Bt products are applied much like synthetic insecticides. Bt treatments are inactivated within one to a few days in many outdoor situations, and repeated applications may be necessary for some crops and pests.

Until the early 1980s, commercial Bt products were effective only against caterpillars. In recent years, however, additional isolates that kill other types of pests have been identified and developed. The nature of the endotoxin differs among Bt subspecies and isolates, and the characteristics of these specific endotoxins determine what insects will be poisoned by each Bt product. Bt formulations that are now commercially available fall into the following broad categories.

Bt Formulations That Kill Caterpillars. The best-known and most widely used Bt insecticides are formulated from Bacillus thuringiensis var. kurstaki (Btk) isolates that are pathogenic and toxic only to larvae of the butterflies and moths (Lepidoptera). Many such Bt products have been registered by the U.S. Environmental Protection Agency (EPA). The most common trade names for these commercial products include Biobit, Condor, Cutlass, Dipel, Full-Bac, Javelin, M-Peril, and MVP, but many companies sell similar products under a variety of trade names. These products are commercially successful and widely available as liquid concentrates, wettable powders, and ready-to-use dusts and granules. They are used to control many common leaf-feeding caterpillars, including pests on vegetables, especially the "worms" that attack cabbage, broccoli, cauliflower, and Brussels sprouts; bag worms and tent caterpillars on trees and shrubs; larvae of the gypsy moth and other forest caterpillars; and European corn borer larvae in field corn. Some products are used to control Indian meal moth larvae in stored grain. Bacillus thuringiensis var. aizawai is another Bt that kills caterpillars. It produces slightly different toxins and is the active ingredient in the products Certan, Agree and Xentari.

Bt products that kill caterpillars are not effective against other types of pests. Even certain caterpillars are not controlled by Bt, especially those that live in the soil or bore into plant tissues without consuming a significant amount of the Bt applied to plant surfaces. The peach tree borer in stone fruits, corn earworm in corn, and cutworms that clip off field crops or garden plants are examples of caterpillars seldom controlled by Bt treatments. Most Bt products are not labeled for the control of codling moth larvae that attack apples and pears because these larvae do not feed on fruit surfaces.

Bt Formulations That Kill Mosquito, Black Fly, and Fungus Gnat Larvae. Bacillus thuringiensis var. israelensis (Bti) kills the larvae of certain flies and mosquitoes. The main targets for this Bt are the larval stages of mosquitoes, black flies, and fungus gnats; it does not kill larval stages of "higher" flies such as the house fly, stable fly, or blow flies. Aedes and Psorophora are the most susceptible mosquito genera; Anopheles and Culex species require higher than normal rates of Bti. Bti products that are available commercially include Vectobac, Teknar, Bactimos, Skeetal, and Mosquito Attack. Bti is most effective for mosquito or black fly control when it is used on a community-wide basis by mosquito abatement district personnel. For most homeowners or farmers, eliminating sites that periodically serve as sources of standing water (such as tires, birdbaths and empty containers) and controlling weeds around stagnant ponds or drainage lagoons is more effective than applying Bti. Bti products are formulated for spray or granular applications. Bti formulated in corn cob granules, for example, is effective against mosquito larvae developing in tires and other artificial containers where the "Asian tiger mosquito," Aedes albopictus, develops. Bti is not very effective for the control of mosquito larvae in turbid water or waters containing high levels of organic pollutants.

Some Bti products are used effectively for the control of fungus gnat larvae in greenhouses and in mushroom culture beds. Gnatrol is a Bti formulation labeled for fungus gnat control. For these uses, Bti is applied as a drench to potting soils or culture media.

Although not a Bt, another bacterium that is pathogenic to certain mosquitoes is Bacillus sphaericus. B. sphaericus is especially active against larvae of mosquitoes in the genera Culex, Psorophora, and Culiseta. B. sphaericus kills larvae in laboratory tests, but field results have not been promising. It remains effective in stagnant or turbid water, however, and is potentially a valuable insecticide. B. sphaericus is not yet registered by the U.S. EPA.

Bt Formulations That Kill Beetles. Another group of Bt isolates, including those from Bacillus thuringiensis var. san diego and Bacillus thuringiensis var. tenebrionis, are toxic to certain beetles. Within the order Coleoptera (the beetles), species exhibit great differences in susceptibility to these isolates, presumably because of differences in the receptor sites in the gut wall of the insects where the bacterial toxins must attach. Consequently, the range of susceptible hosts for the beetle-targeted Bt formulations does not include all beetles, or even all of the species within a family or subfamily. Bacillus thuringiensis var. san diego, sold under the trade names M-Trak, Foil and Novodor, is registered for use against larvae of the Colorado potato beetle. This product also kills adults and larvae of the elm leaf beetle and willow leaf beetle, but it is not pathogenic or toxic to some other key beetle pests, such as the corn rootworms and other related species. Considerable research effort is now directed to identifying and developing additional Bt isolates that are active against more or different beetle species. Although entomologists and consumers alike will need to consider the specific target insect when judging the potential for these new products, Bt formulations effective against beetles seem to offer great promise.

Recent Research. Advances in biotechnology have produced improved prospects for developing new Bt insecticides and an ability to place Bt toxins within crop plants in a variety of ways. For example, genes directing the production of Bt toxins can be incorporated into certain plant-dwelling bacteria. When these altered bacteria grow and multiply within an inoculated host plant, the Bt toxin is produced within the plant. Genes coding the production of Bt toxins have also been inserted directly into the chromosomes of certain crop plants. Although the development of this technology may seem ideal, the season-long, high-level control it can provide will also pose a great risk for the development of insect resistance to the Bt toxin. Bt products have been used successfully for many years, but resistance in field or laboratory populations of the diamondback moth, Indian meal moth, Colorado potato beetle, and tobacco budworm has been reported. One mechanism for resistance is the reduced binding of the Bt toxin to the midgut receptor sites. As genes for production of insecticidal compounds are added to crop plants, developers must devise methods of preventing or managing insecticide resistance in target pests. Current plans to develop and use "Bt-corn", "Bt-potatoes", and other crops that produce Bt toxins are progressing much more rapidly than plans and actions designed to manage resistance in target pests.

Using Bt Insecticides. Insecticides containing Bt can be very effective for insect control in a variety of situations. Reviewing key facts about these products can help users obtain the best results possible. Each Bt insecticide controls only certain types of insects. It is, therefore, essential to identify the target pest and to confirm that the Bt product label states that the insecticide is effective against that pest. Separate stages of insects differ in their susceptibility to Bt; isolates that are effective against larval stages of butterflies, moths, or mosquitoes generally do not kill adults. Because susceptible insects must consume Bt to be poisoned, treatments must be directed to the plant parts that the target pest will eat. Poisoned insects normally remain on plants for a day or two after treatment, but they do not continue feeding and will soon die. Where Bt applied to plant surfaces or other sites is exposed to sunlight, it is deactivated rapidly by direct ultraviolet radiation. To maximize the effectiveness of Bt treatments, sprays should thoroughly cover all plant surfaces, including the undersides of leaves. Treating in the late afternoon or evening can be helpful because the insecticide remains effective on foliage overnight before being inactivated by exposure to intense sunlight the following day. Treating on cloudy (but not rainy) days provides a similar result. Production processes that encapsulate Bt spores or toxins in a granular matrix (such as starch) or within killed cells of other bacteria also provide protection from ultraviolet radiation. MVP and M-Trak are encapsulated Bt products currently on the market.

Users are advised to handle all microbial insecticides cautiously. Bacterial spores, mold spores, and virus particles become "foreign proteins" if they are inhaled or rubbed into the skin and can cause allergic reactions. The dusts or liquids used to dilute and carry these microorganisms also can act as allergens or irritants. These problems do not prevent the safe use of microbial insecticides, but users should not breathe dusts or mists of microbial insecticides. Users should wear gloves, long sleeves, and long trousers during application and wash thoroughly afterwards. These are common-sense precautions that will help prevent unexpected reactions and minimize any effects from unknown toxicity.

Adapted from

- Rick Weinzerl and Lee Solter, University of Illinois.

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