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 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.
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.
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 Weinzerl, R. and T. Henn. Alternatives in
insect management: biological and biorational approaches. North Central Regional Extension Publication 401.
Introduction
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.
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 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.
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.
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