Tree Fruit Leader, Vol 1(1) February 1992

What is Bacillus thuringiensis (Alias Bt)?

Hugh G. Philip, P.Ag. Entomologist, Kelowna

Introduction In nature, insect population changes are regulated by a number of factors such as weather, competition for food and space, and by parasites, predators and diseases, to name just a few. Until the widespread reliance on synthetic pesticides to control insect pests, food producers had to rely on the presence of natural control agents such as diseases to help protect their crops from insect feeding. Since the Second World War, protection of food crops has involved development and application of various insect control products to ensure maximum production of high quality produce demanded by consumers.

However, the consequences of using these products on the health of not only consumers but also on the environment and its biological components has become a matter of serious, often very emotional, public debate. This debate has fostered greater interest by the agri-food industry, chemical manufacturers and regulatory agencies in alternative approaches to insect pest control that lessen or eliminate risks to human and environmental health.

Insect diseases caused by bacteria, viruses and fungi offer one such alternative approach to insect control, and their value in such a role has been recognized for many years. This article will describe the role played by the first insect pathogen commercially developed - Bacillus thuringiensis Berliner, or Bt - to protect crops biologically from certain insect pests.

What is Bt?

Bacillus thuringiensis is the name given to a spore-forming bacterium isolated in 1915 in Germany from diseased larvae of the Mediterranean flour moth. The potential role of this bacterium for insect control was studied, however it was not until the early 1950's that serious attention was devoted to developing Bt as a commercial biological control product. Since that time, several subspecies or varieties of Bt have been isolated from insects around the world. The variety in the two products commonly used on tree fruits - Dipel and Thuricide - is B. thuringiensis variety kurstaki (referred to as Btk in this article).

Btk is a rod-shaped, gram-positive crystalliferous, spore-forming bacterium. During the sporulation process, a crystalline protein parasporal body, called the delta-endotoxin, is formed. When sporulation is complete and the bacteria is placed in an alkaline environment, the bacterial cell breaks open and releases the spore and crystal. The spores germinate and the bacteria multiply.

What is the mode of action of Btk?

To be effective, that is, toxic to susceptible insects, Btk must be ingested by susceptible insects such as lepidopterous larvae (for example leafroller and green fruitworm caterpillars). It does not kill by contact or systemic activity. Once the crystal or spore-crystal complex is consumed by the larvae, it enters the gut where, if the pH is high enough (9+), the intact bacterial cells break apart releasing the crystals and spores.

The crystals or delta-endotoxin release a substance or substances that destroy the cells lining the gut walls. In some lepidoptera, this is followed in a few hours by a general paralysis and death. In most cases, however, larvae stop feeding within a few minutes of eating the toxin due to paralysis of the gut. Some larvae may die immediately, but the majority die from a septicemia two to three days later. Septicemia is caused by the escape of the Btk spores through the damaged gut walls into the body cavity where the spores germinate and the vegetative cells multiply.

Therefore the pattern for Btk-induced mortality following application is cessation of feeding within hours and death within two to three days. Although death does not occur as rapidly as with synthetic insecticides, no further feeding damage occurs once the larvae have ingested a lethal dose.

What determines susceptibility to Btk?

As previously mentioned, Btk is "activated" in an environment of pH 9 or more (alkaline). This condition is found in most lepidopterous larvae in order for them to digest plant tissue. Other insects with lower gut pH's, such as parasitic and predaceous insects, honey bees and other pollinators, are not affected. On the basis of laboratory studies, about 110 species of lepidoptera are susceptible to Btk. Thus Btk has a wide host range but is specific for certain lepidoptera and has little effect on other life forms.

Can insects develop resistance to Btk?

Until recently it was assumed that resistance to Btk in field populations of insects was very unlikely due to its novel mode of action. However, a population of diamondback moths in Hawaii has been confirmed to be 20 to 40 times more tolerant of Btk than that of less exposed populations of moths. This resistant population had been receiving up to 15 treatments per year! It is very unlikely leafrollers and green fruitworms will develop resistance to Btk in B.C. because these pests have only one to two generations a year, therefore Btk is only applied once or twice a season. This treatment exposure is not enough to cause resistance development.

How should you use Btk against fruit pests?

The 1992 Tree Fruit Production Guide recommends Btk be applied at either pink, bloom or petal fall (depending on crop, pest and weather) for the control of leafrollers and fruitworms at 2.25 to 3.35 kg Dipel per ha (0.9 to 1.4 kg per ac). Other lepidopterous larvae present at those times such as eye-spotted budmoth, apple ermine moth and Bruce spanworm, are also susceptible to Btk.

Because Btk products contain both non-living (delta- endotoxin crystals) and living (bacterial spores) components, their efficacy is subject to not only application method but also to environmental conditions. Btk is considered a microbial insecticide because of its biological action and application. For best results, Btk products should:

1. be applied at the correct dosage in at least 600 to 800 L/ha (240 to 320 gal/ac) using an air-blast sprayer to deliver fine spray droplets to ensure thorough and uniform coverage. Research has shown that smaller, more uniformly deposited droplets significantly improve efficacy of Btk compared to coarse, unevenly distributed droplets;

    

2. be applied in combination with a spreader/sticker to improve coverage and resistance to wash off by rain;

    

3. be applied in the evening under warm, calm conditions. Because Btk begins breaking down within three hours of exposure to ultraviolet light, application in the evening will ensure the maximum dosage is available for larvae to ingest. It is important that the larvae ingest a lethal dose of Btk in the 24 hours following application because of the short residual activity of Btk. Larvae will recover from sub-lethal doses;

    

4. be applied when larvae are about 1/4 to 1/2 full grown. If applied when larvae are just hatched, they may not eat enough to get poisoned before the Btk breaks down. Waiting until most of the larvae have grown somewhat ensures that most of the eggs have hatched and a second spray will not be needed to control later hatched larvae;

    

5. not be mixed and applied with any compounds that raise the pH of the mixture to an alkaline condition and cause a breakdown of the Bt and subsequent reduction in efficacy.

What is being done to improve Btk?

Because of the sensitivity of Btk to ultraviolet (UV) light and wash-off, inconsistent performance has been the rule rather than the exception. Research is ongoing to overcome some of these problems through modifications to the formulations. The insecticidal half-life of Btk is between 1.5 to 2 days (50 percent of insecticidal activity is lost) due in part to UV irradiation. Early research focused on protecting the spores from ultraviolet radiation damage using compounds applied to or mixed with the spores that either screened out or absorbed UV light.

With the improvement in formulation technology, researchers have been able to bind UV-absorbing compounds to the spores and crystals to prolong their half-life. In addition, certain starches have been found to be effective encapsulating agents as well as to improve resistance to wash-off. Research has extended the residual insecticidal activity of Btk from four days up to 12. By adding feeding stimulants to the encapsulation process, larvae feed longer on treated foliage thus ensuring ingestion of a lethal dosage. This in turn has allowed the dosage to be decreased because of the additional feeding activity of the larvae.

As previously mentioned, new strains and subspecies of Bt are being discovered and tested that are specific for other groups of insects such as black flies, mosquitoes, and Colorado potato beetle. Biotechnology has allowed transfer of the gene responsible for the delta-endotoxin to a bacteria that can be cultured to produce large quantities of the endotoxin for commercial formulation. This gene has also been successfully transferred into the genetic make-up of certain plants such that they become toxic to some insects. This latter development is somewhat controversial because of the possibility of resistance development due to the continual exposure of the target pest species to the delta-endotoxin during feeding.

Btk has also been discovered to produce a heat-stable beta-endotoxin during the vegetative growth stage. This endotoxin has been named thuringiensin. It has been found to be toxic to many economically important insects and mites but safe for beneficial insects. Thuringiensin is not registered for use in Canada.

Summary

Bacillus thuringiensis var. kurstaki Berliner (Btk) is a naturally-occurring bacterium that is toxic to many species of plant-feeding lepidopterous larvae. It is non-toxic to animals and beneficial insects, and has no known adverse effects on the environment. Btk products such as Dipel, applied to fruit trees for leafroller and fruitworm control, are as effective as chemical insecticides when used properly. However, sensitivity of Btk to certain environmental conditions (UV light, rainfall) reduces its residual activity compared to chemical pesticides, making proper timing and application critical to maximizing the efficacy of this microbial insecticide. Fortunately research is ongoing to improve the potency and residual activity of Btk products with the hope of their wider acceptance in lepidopterous pest management programs.

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