How Does Bt Work?

Some of you may be interested in this information especially in light of the Bt resistance that has been identified: Researchers Identify Insect Resistant to Bt Pesticide.

Thanks to Gene McAvoy in Hendery County Florida for passing along this great info!

Bacillus thuringiensis, the active ingredient in the “Bt” insecticides, is a commonly occurring soil bacterium that was first discovered in 1901. Since their commercial introduction in the 1950’s, Bt products have become the most successful and widely used of the biopesticides.

Insecticidal activity of Bt derives from proteins that are produced and crystallize during bacterial sporulation (thus referred to as “Cry” toxins).

Most Bt products contain a mixture of Bt spores and toxin crystals. When a caterpillar ingests Bt, the crystals are dissolved in the alkaline gut environment. The insect’s own digestive enzymes convert the protein to an activated “delta endotoxin” which binds irreversibly to receptor molecules on the surface of cells lining the insect’s midgut, opening holes in the membrane that destroy the cellular lining of the digestive tract. The larva stops feeding (typically within hours) and may die quickly from septicemia as Bt and other spores in its food germinate, or may linger and die eventually of starvation.

Many different subspecies and strains of Bt have been described, producing different combinations of unique Cry toxins. Bt insecticides used for worm control are based on two of these subspecies which produce a few important Cry toxins.

Bt subspecies kurstaki (Btk) products such as Javelin®, Deliver®, and Dipel® contain primarily Cry1A and Cry2 toxins. These give broad spectrum activity against loopers, diamondback moth, imported cabbage worm, tomato fruitworm/corn earworm, tobacco budworm, hornworms, and other defoliating caterpillars common to vegetable production.

Xentari® and Agree® are based on Bt subspecies aizawai, which produces another toxin (Cry1C) effective against beet armyworm and other Spodoptera species, which are not very sensitive to Cry1A toxins.

Agree® contains a “hybrid” Bta, the offspring of a cross (transconjugation) between parent strains of Bta and Btk. As a result, Agree contains Cry1Ac (from Btk) in addition to Cry1C (from Bta). Other products such as Condor® represent unique combinations of Cry toxins resulting from transconjugation between different Btk strains.

Still other advanced Bt’s are products of genetic engineering to create unique Cry toxin combinations or higher levels of expression. CryMax® is such a recombinant Btk product, producing high levels of Cry1Ac in addition to Cry1C. Lepinox® is engineered for high expression of a combined Cry1Ac/Cry1F toxin particularly effective against fall armyworm (Spodoptera frugiperda), which is not highly susceptible to other Cry toxins.

All Bt products must be ingested by the target caterpillars in order to be effective, and small larvae in earlier stages of development are more susceptible than large, mature larva. This means that proper spray timing and technique are important to ensure that vulnerable plant tissues are uniformly covered by Bt spray deposits when young larvae are actively feeding. Worms that enter fruit or stems soon after egg hatch (such as squash vine borer, melon worm, and European corn borer) or are protected within mined or rolled leaves (such tomato pinworm) are susceptible to Bt if applied before they enter these protected environments.

Bt is not a systemic insecticide and tends to have shorter residual effect compared to many chemical insecticides (one of the reasons they present little risk to the environment, workers, and consumers). Reapplication may be required, especially during periods of rapid crop growth when new, untreated tissues may be exposed to infestation. Be sure to read the Bt product label for specific information regarding crops, target insects, and application instructions (such as rates, timing, and reapplication intervals).

Bt in IPM and Resistance Management

Bt insecticides are an ideal tool for the grower focused on IPM due to their selective control and low environmental risk. Bt also provides a unique mode of action (IRAC group 11) that can complement other insecticides in resistance management strategies. No evidence of cross-resistance has ever been detected between Bt and other insecticides; insects resistant to multiple classes of insecticides are still susceptible to Bt. Btk and Bta products also can be rotated to reduce risk of resistance to specific Bt’s.

At a recent symposium on “Insect Resistance in Vegetable Production” organized by the Entomological Society of America (Southeastern Branch), entomologists and IPM specialists (including Dr. Dakshina Seal of IFAS in Homestead) pointed to Bt products as the best rotational partners for delaying resistance to new insecticide chemistries such as the diamides.

Symposium participants listed the following benefits of Bt insecticides in a rotation program:

• High efficacy on a wide variety of worm pests

• Very low impact on beneficial insects.

• Avoidance of secondary pest problems.

• Not toxic to bees and predatory mites.

• No preharvest interval required after application.

• No restrictions on amount used.

• Resistance to Bt is slower to develop than resistance to chemical insecticides.

Through proper field scouting, correct identification of the worm pest, good timing of applications, correct selection of the Bt used, and judicious application of the Bt’s in rotation with new or older conventional chemistries, growers can gain effective control while reducing the risk of resistance and maintaining beneficial insect populations.

For more information contact Dr. Brett Highland, Certis USA, at bhighland@certisusa.com, or visit www.certisusa.com.