Bacillus thuringiensis (Bt) is a bacterium that must be eaten by lepidopteran larvae (caterpillars) to be effective. Bt is a great material for leafroller control because it is specific and has little effect on natural enemies. However, it must be applied 2-3 times to be effective when leafroller populations are high. Experience has also shown that in the spring, the high temperatures need to be above 65°F for 3 or more days so that larvae have a chance to feed on it before sunlight breaks it down. DAS provides forecast temperatures for all stations in the "Weather Forecast" and in the "Show Data Grid" table so that you can decide whether or not to use Bt or other recommended chemicals.
How does Bt work?
Bacillus thuringiensis (Bt) is an insect pathogen that produces one or more toxins that are utilized as bacterial insecticides. Bt is effective against lepidopteran larvae such as leafrollers, peach twig borer and cutworms. Bt is not a contact insecticide and must be consumed by the larvae to be effective. When ingested, the Bt toxins are activated and cause holes in the insect's gut membrane. Gut bacteria then get into the insect's blood stream and poison it. Once it has consumed a toxic dose, the larva stops feeding but may remain alive for several days. Bt is most effective against young larvae, as it takes a smaller dose to kill them than it takes for more mature larvae.
Bt has a short effective life of 3 to 7 days. It breaks down in sunlight and high temperatures and must be applied more frequently than traditional insecticides to achieve adequate control. Because larvae must consume Bt products, thorough coverage is essential.
Bt to control leafrollers
Bt products have their best fit in apple and pear IPM as leafroller controls. Several Bt products have been tested over the last decade and few differences have been detected in their relative efficacy assuming that appropriate rates are used.
In the spring, Bt products are best used between bloom and 10-14 days after petal fall. Because the leafroller larva must consume the Bt it is important to have good coverage of foliage and to apply sprays when temperatures are predicted to be 65°F or higher for 3 or more days. Leafroller larvae are not active feeders when temperatures are below 65°F, and long periods of cool wet weather following a Bt application will greatly reduce the efficacy of the treatment.
It is usually necessary to apply more than one Bt treatment to obtain adequate leafroller suppression. We have consistently observed 50-60% control with one application and 80-95% control with two applications 7 to 10 days apart. In the summer Bt treatments last only 5 to 7 days; however, with good coverage, it is possible to obtain good control.
When dealing with a high leafroller population three applications may be necessary to achieve adequate control. Leafroller larvae that consume a sublethal dose of Bt will continue to develop at a normal rate after a 7-10 day recovery period. This delayed development has consequences in implementing a leafroller degree-day model. In orchards that have received a Bt application, model predictions may be delayed by more than 100 DD.
Bt effects against other pests
Bt will kill codling moth larvae in the laboratory, but because CM larvae are exposed for such a short time before entering the fruit (probably not long enough to consume a lethal dose) it is not a highly effective field control.
Bt products have been evaluated against Lacanobia fruitworm but have not proven effective.
Bt is not active against (sucking) indirect/secondary pests.
Advantages over traditional pesticides
Because all Bt products must be consumed to have activity against insects, they do not affect most natural enemies of insect pests making Bt more selective than other pesticides. Due to its fast degradation it has a much less harmful effect on the environment.