The insecticidal activity of Bt was first discovered in 1911. However, it was not commercially available until the 1950’s. In recent years, there has been tremendous renewed interest in Bt and several new products are available.
How is Bacillus thuringiensis used for control of insects?
Bt acts as a stomach poison. It must be eaten to affect the susceptible insect. It has no contact activity, so it cannot act by being absorbed through the cuticle of the insect.
Most Bt products are designed to be mixed with water. It is then sprayed on leaves to be eaten by pest insects, released into water, or used as a soil drench, depending on the intended use.
Some special formulations occur. Bt granules (typically on corm cob) are used to funnel down the whorl leaves of corn to control European corn borer. Granules or slow release formulations such as mosquito rings are sometimes used for release of Bt to control mosquitoes.
How does Bacillus thuringiensis kill insects?
The insecticidally active ingredients of Bacillus thurgingiensis are various proteins produced by the bacteria. In particular, compounds known as the delta-endotoxins, which form crystals inside the bacteria, kill the insect.
In the gut of susceptible insects, the delta-endotoxin becomes activated. (Chemistry of the insect gut is very important in this stage.) If activated, it disrupts the cell along the lining of the insect gut within minutes after contacting them. Usually within hours, cells lining the gut are killed. The gut is paralyzed and the insect stops feeding by this time.
Death of the insect, however, often takes several days. Ultimately, the insect usually dies from starvation. Alternatively they may succumb to blood poisoning if the bacteria enters the blood in the insect body cavity. Some strains of Bacillus thuringiensis also produce other toxins that are insecticidal. One of these strains is known as thuringiensis (alpha-exotoxin), which currently is being researched as a potential insecticide/miticide.
What are the advantages of Bacillus thuringiensis for insect control?
The primary advantage of Bt products is their safety resulting from their selectivity. Each strain of Bt is capable of affecting only a specific group of insects, for example, caterpillars. Non-susceptible species are not affected. This includes desirable species such as wildlife, pets and beneficial insects. Because Bt does not directly affect natural enemies of insects (e.g. lady beetles, parasitic wasps) as do many other insecticides, it conserves and integrates with natural controls.
Bt also is considered non-toxic to humans. Most common formulations of Bt are registered on essentially all food crops and do not even require an interval lapse between application and harvest. (Preharvest intervals may be several days or even weeks for other insecticides.)
What are the limitations of Bacillus thuringiensis for insect control?
One important limitation is that Bt acts strictly as a stomach poison. In order to be effective, the insect must eat it. Because of this, insects that tunnel into plants are not well controlled, even though they may be susceptible to the Bt toxin. For example, codling moth, the worm in a wormy apple, can be killed by Bt in the laboratory. In field situations, however, the larva avoids it and is not killed since it burrows into the unprotected interior of the fruit. Similarly, corn earworm is a susceptible species, but not well controlled by Bt since it rapidly tunnels into the corn ear tip.
Bt also shows great differences in effectiveness with varying ages of most insects. Young stages often are quite susceptible, whereas older instars may not be easily controlled with Bt applications used at field rates. The short persistence of Bt can be a limitation. Developing insects often stop feeding for 24 hours or more during periods when they molt. It the Bt is applied at this time, it will largely be broken down before insects resume feeding. Also, insects that have not yet hatched will not be controlled so timing becomes more critical. Application during times when ultraviolet light is less intense (e.g. late in the day) can improve persistence.
Bt formulations used for mosquito control are also adversely affected by the turbidity of water. Furthermore, the slow kill of Bt is sometimes seen as a problem. Since the insect typically does not die within two to three days, an application can appear ineffective. However, this is purely a perceptual problem, since affected insects stop feeding and causing plant injury almost immediately.
Finally, the very selectivity of Bt can sometimes by a limitation. More broad spectrum insecticides are effective against several insects. On the other hand, separate formulations of Bt must be used against leaf feeding beetles versus leaf feeding caterpillars (although one company is creating a genetically engineered product containing two strains of Bt). No Bt products kill aphids, plant bugs, or some of the other garden pests. However, the conservation of natural enemies often more than makes up for this latter limitation.
How persistent is Bacillus thuringiensis after it is applied?
Bacillus thuringiensis is a soil bacteria and poorly adapts to leaves or other surfaces exposed to sunlight, particularly ultraviolet light. One study shows a 50 percent reduction in activity after exposure to sunlight for 30 minutes. However, Bacillus thuringiensis often is formulated in ways that extend persistence. Most applications of Bt applied to leaves should be effective for at least 24 to 48 hours. Bt applied to more sheltered sites, such as water (e.g. Bacillus thuringiensis "israelensis" strain for mosquitoes) or shaded lower leaves will persist for longer periods.
Various techniques have been used to extend Bt persistence. Sometimes compounds that absorb ultraviolet light are added. One company has used genetic engineering to cause a more light-tolerant bacteria to produce Bt toxin. The bacteria is then killed, but the thicker coat provides greater protection against the effects of sunlight.
Are there any other microbial insecticides?
Although insect diseases are common and important in natural control, relatively few have been developed as insecticides. Among the few that do exist are Bacillus popillae and Bacillus lentimorbus, bacterial diseases affecting grubs of the Japanese beetle, and Nosema locustae, a protozoan disease that affects some species of grasshoppers. Insect parasitic nematodes also are sometimes considered to be a type of microbial insecticide. These are species of roundworms that actually kill susceptible insects by injecting a bacteria into the body of the insect. Insect parasitic nematodes have become widely available in recent years.
Several microbial diseases of insects are currently in active research and development, including bacteria, viruses and fungi.