Insecticide use
In Minnesota, Bt is registered for use in caterpillar control on forest and shade trees to control about two dozen insects. Major insects affected included tent caterpillars, gypsy moth, and jack pine and spruce budworms. In addition, bagworm, spanworm, red-humped caterpillar, cankerworms, and saddled prominent can be controlled with Bt. It does not control sawfly larvae, bees, grubs, maggots, or any adult insects.
Unlike chemical insecticides, the active ingredients in a Bt formulation are measured in BIUs (billion international units). It is measured in this way because Bt is a formulation of living organisms and its potency varies from strain to strain. Application rates vary for different Bt formulations and the target caterpillar, but are normally between 8 and 24 BIUs per acre. In some spray programs, a single application of 24 BIU has been as effective as 2 applications of 12 BIUs. Most formulations also list recommended application rates in ounces or pounds per acre.
Bt formulations can be applied to foliage in much the same way synthetic pesticides are applied. Water-based flowable formulations of Bt have recently gained wide acceptance in forest spray programs. These are mixed with water or applied undiluted with ultra low volume application (ULV) equipment. ULV applications are more efficient since the aircraft spends more time spraying the product and less time spraying water. Spreaders/ stickers are sometimes added to the Bt improve the coverage, rain fastness and resistance to wash-off.
Timing of the application and good coverage of the foliage is essential. Forest tent caterpillars, for example, should be treated when the caterpillars are 3/8-inch to 1/2-inch long or the tree leaves are one half of their full size. Bt should be applied to dry foliage and have a droplet size of 75–150 microns.