Insect Management

Insect Pathogens

Microbial pesticides are becoming cornerstones to IPM systems, particularly in some vegetables in California and Mexico. Insect pathogens are safe to use around workers, beneficial insects, and other non-target species and can often be tank mixed with commercial insecticides. Most do not become established in the field, however, and must be re-applied each time control is necessary. Since these products are living organisms, they should not be stored at hot temperatures or below freezing. The only widely used pathogen at this time is Bacillus thuringiensis (Bt), a microbial insecticide.

Bacillus thuringiensis (Bt). Bt is an effective control for many types of butterfly and moth larvae including some, such as cabbage loopers, which are hard to control by other means. Bt products are generally considered safe for people, bees, predatory and parasitic insects, predatory mites and spiders, and the environment in general. These products are usually exempt from residue tolerance restrictions, days-to-harvest restrictions, and re-entry restrictions.

Commercial Bt products are prepared from either Bacillus thuringiensis var.

or Bacillus thuringiensis var. azawa, but various genetically manipulated strains of Bt are also available. Below is a summary of some of the available Bt products. All these bacteria produce crystalline spores and proteins. Caterpillars that eat these die within 2 to 5 days because of toxins released when the spores and protein crystals are dissolved by digestion in the insect midgut. Caterpillars stop eati ng within minutes after ingesting the Bt so they must receive a lethal dose within a few bites.

Summary of Bt products generally available

Caterpillars controlled by Bt include European corn borer (on corn), cabbage looper, imported cabbageworm, diamondback moth, and tomato/tobacco hornworm. Corn earworm on tomato and pepper is controlled with Bt, but corn earworm on corn and the squash vine borer are not controlled since larvae do not feed long enough on the plant surface to be reached by Bt sprays. Due to increased industry emphasis, improved Bt products are being developed which work on insects other than butterfly and moth larvae. Bacillus thuringiensis var. israelensis kills larvae of mosquito, black flies, and fungus gnats but requires community-wide application for greatest effectiveness. Bacillus thuringiensis var. san diego controls young Colorado potato beetle larvae. M-Trak® is a type of Bt var. san diego that is encapsulated in killed bacteria cell (Pseudomonas fluorescens). It is only active against first and second instar larvae of the Colorado potato beetle and some beetle pests of trees. Applications need to be repeated and M-Trak® is washed off by rain.

Bt works best when applied to young caterpillars less than ˝ inch long, and it is only effective for one or two days after application. Since it is deactivated by sunlight, spraying in late afternoon or evening or on cloudy days is most effective. Alkaline water (pH of 7.8) or acid water will also deactivate the crystals. Repeat applications every three to five days are necessary, especially when there are lots of caterpillars present. The toxin is not spread by infected caterpillars, and the bacteria do not multiply on their own. Good coverage of leaf tissue is essential. Bt should not be applied through irrigation systems, nor is it compatible with oils, sulfur, copper fungicides, or malathion. In fact, it is recommended that Bt not be mixed with any other insecticides since this encourages the development of multiple insecticide resistant insects. If plants are stressed by high temperature, too much or too little water, or high light following overcast conditions, application of Bt, like most sprays, can damage the foliage.

Insects may develop resistance to Bt as they have with so many other insecticides. Resistance to Bt has been found in Colorado potato beetle in the lab but not yet in the field. Resistance to Bt has been found in diamondback moth in Hawaii, Texas, Florida and on transplants raised in Florida. Newer Bt products, contain a different type of crystals produced by another Bt strain, Bt var. azawa, rather than the commonly used var. kurstaki. At this point, the diamondback moth is not resistant to the azawa strain. There are over 2000 other strains of Bt available, plus genetically altered strains to be used in future formulations. Development of resistances to particular strains, or even cross-resistances, are hard to predict, however, especially if the ability to synthesize Bt toxins is widely incorporated into crop plants, as is being proposed. Some argue, however, argues that transgenic plants will not only be more effective than sprays in protecting plants, but will delay resistance buildup. Dependence on Bt or any other material should supplement rather than replace other aspects of an IPM program. Improved Bts and transgenic plants have the potential advantages of improved efficacy, speed of kill, broader host range and increased residuals.

Nosema locustae. Nosema locustae is a protozoan that will control crickets and the Melanoplus group of grasshoppers. Most effective if applied early and over large areas, Nosema is commercially available in a wheat bran formulation. Grasshoppers die after feeding on Nosema-laced wheat bran spread as bait or after feeding on their own dead. There is some Nosema carryover the next year.

Paecilomyces fumorosoroseus. The fungi Paecilomyces fumorosoroseus (PFR) has been used to control the silverleaf whitefly.

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