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Using Bacillus thuringiensis Effectively

May 12, 1999

Bacillus thuringiensis (Bt) is a soilborne bacterium that is widely used in landscapes, nurseries, and greenhouses to control a variety of ornamental insect pests. Different insect pests are controlled by different strains of the bacteria. The common Bt strains and the insect groups that they work on include Bt kurstaki (butterflies and moths), Bt israelensis (mosquitoes and flies), and Bt tenebrionis (beetles). Unlike fungi, bacteria must be ingested to be active. This means that thorough coverage is essential when using Bt products so that the target organism will eat the bacteria. Once the bacteria is consumed, it produces an endotoxin crystal that attacks the gut membrane and creates pores, causing leakage and swelling. The swelling continues until cells burst, which allows the gut contents to leak into the insectís blood, disrupting the blood pH and resulting in paralysis and death within 24 to 72 hours.

It is important to understand the following characteristics of Bt products in order to maximize their effectiveness when they are used to manage ornamental pests.

Selectivity. In contrast to conventional pesticides, Bt products donít have broad-spectrum activity. Because they generally have little impact on non-target organisms, including natural enemies, we see fewer problems, such as target pest resurgence and secondary pest outbreaks, which may occur with the use of broad-spectrum insecticides. Currently registered Bt products have no activity on sucking insects and mites such as aphids, whiteflies, and spider mites. An alternative material must be used if sucking insects or mites are the primary pest.

Timing of application. Bt products must be applied when young insect stages are present. Because of their small size, insects at these stages donít have to consume as much plant material before the bacteria is effective. They are also killed before they cause severe plant injury and before they reach the reproductive phase. If Bt is applied too late, insects have to consume much more plant material containing the bacteria in order for the product to be effective. This results in greater plant injury because it takes longer to kill the insect. In addition, the insect may switch from the growing phase to the reproductive phase before it has consumed enough of the bacteria. If this occurs, less material will be consumed and there is a higher probability of adult survival, which means a new generation of insects will be produced in the future.

Residual activity. Bt products donít remain in the environment long because they are subject to breakdown by sunlight (ultraviolet light degradation) and removal by rainfall. Repeat applications may be necessary.

Speed of activity. Because Bt products are slower-acting than conventional insecticides, they must be applied before pest populations reach damaging levels.

Safety. The mode of action of Bt products is specific to insects. There is no effect on mammals and humans.

Storage life. Bt products must be stored under cool conditions (50įF to 60įF) to prolong its shelf life. Avoid exposing the product to extremes in cold and hot temperatures, which may cause breakdown of the bacteria.

Water quality. Alkaline water (pH greater than 8) can reduce the effectiveness of the Bt toxin, so the water solution must be adjusted to a pH of 7 or lower.

Bacillus thuringiensis products can be used as part of an integrated pest management program to manage ornamental insect pests; however, it is essential that their advantages and limitations be understood in order to use these materials effectively.

Author: Raymond A. Cloyd


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