Single cell organisms, such as bacteria, fungi and protozoa, and viruses, have been mass produced and formulated for use in a manner similar to insecticides. Products containing these organisms are regulated by the Environmental Protection Agency and use is governed by the Federal Insecticide, Fungicide and Rodenticide Act (FIFRA).
The bacterium, Bacillus thuringiensis (B.t.), reproduces by spores. The spores are produced in the bacterium cell along with a crystalline protein called an endotoxin. The endotoxin, with or without the spores must be ingested by the target insect in order to be effective. Once ingested, the endotoxin is activated by the alkaline conditions in the insect’s stomach. The toxin attaches to specific receptors on the gut wall, causing the gut lining to break down. This method normally allows the spores to enter the hosts blood (hemolymph) where the bacterium can proliferate.
Different species and strains and Bacillus bacteria are known to affect different groups of insect pests, primarily due to differences in endotoxin receptor sites on the gut wall:
- Bacillus thuringiensis var. kurstaki (Dipel®, Javelin® and others) – caterpillars of moths and butterflies
- Bacillus thuringiensis var. israelensis (Vectobac®, Gnatrol®) – larvae of flies such as fungus gnats
- Bacillus thuringiensis var. san diego (M-One®) – larvae of beetles such as elm leaf beetles and Colorado potato beetles
- Another species, Bacillus popollia, or milky spore disease has been marketed for controlling Japanese beetle larvae. Occasionally, this product is marketed in Texas for white grub control. Research conducted in Texas does not indicate that this species affects the species of white grubs found in the state.
For most effective use, B.t. products must be applied when insects are in their early larval stages (first or second instars) and are actively feeding. Several days may be required for larvae to die, although feeding usually stops soon after ingestion. For foliar applications, additives such as feeding stimulants and stickers are often added to the spray mixture to ensure that target pests rapidly ingest the treated leaves and that rain does not wash treated surfaces. On foliage, B.t. treatments degrade rapidly. Applications are most effective when made in the evening or on cloudy days.
B.t. is formulated in liquid concentrates, wettable powders, dusts and granules. One product, MPV®, has been developed by inserting the genes that code for the B.t. endotoxin into another hard-bodied microorganism. The micro-organism is then killed and used as a capsule in which the endotoxin is protected. These endotoxin genes have also been genetically engineered into several plants, including tobacco, tomatoes and cotton. These plants have been shown to be resistant to caterpillars. Unfortunately, there have been several documented cases of insect pests becoming resistant to B.t. endotoxins.
Toxicology of Bacillus thuringiensis. The varieties of Bacillus thuringiensis used commercially survive when injected into mice, and at least one of the purified insecticidal toxins is toxic to mice. Infections of humans have been extremely rare (two recognized cases) and no occurrences of human toxicosis have been reported. From studies involving deliberate ingestion by human subjects, it appears possible, but not likely, that the organism can cause gastroenteritis. B.t. products are exempt from tolerance on raw agricultural commodities in the United States. Neither irritative nor sensitizing effects have been reported in workers preparing and applying commercial products. A single case of corneal ulcer caused by a splash of B.t. into the eye was successfully treated (Morgan, D.P. 1989. Recognition and management of pesticide poisonings. EPA-540/9-88-001. U.S. Environmental Protection Agency, Washington, D.C. 205 pp.).
Example of registered product: Green Light® Plus Bio-Worm Killer® Concentrate – For tomato hormworms on tomato, cabbage looper on broccoli, cauliflower, collards, kale, mustard greens, turnip greens, cabbage, celery, lettuce, melons and tomatoes. Imported cabbage worm on broccoli, cabbage, cauliflower, collards, kale, mustard greens and turnip greens. On shade trees and ornamentals for leaf-feeding worms: spring cankerworm, fall cankerworm and tent caterpillar. Apply at first sign of infestation and repeat at weekly intervals when needed to maintain control. Apply thoroughly; cover all foliage surfaces.
Several fungi have been studied as potential microbial insecticides. Beauveria bassiana can affect a wide variety of arthropods. However, no products containing fungi are currently registered for use in Texas. Environmental conditions, particularly temperature and humidity are important factors effecting the success of fungal treatments, particularly when using preparations of fungal spores.
The protozoan Nosema locustae is available in a few products such as Grasshopper Attach®, and Hopper Stopper. These products are sold for the control of grasshoppers. Effectiveness of these products for small-scale use, such as gardens and yards has not been demonstrated. The disease must be ingested to be effective and it is very slow acting. Grasshoppers are strong fliers and can easily move long distances, making the effectiveness of these treatments on a small scale questionable.
Weinzieri, R. and T. Henn. 1989. Microbial insecticides. Circular 1295. Cooperative Extension Service. University of Illinois, Urbana-Champaign. 24 pp.