The following is information on the use of Bt biological insecticide for caterpillar control in ornamental plants.
Bacillus thuringiensis (Bt)
Bt is one of the oldest and very safe materials used for control of caterpillars. The insecticide produced from this rod-shaped bacterium does not harm animals, birds, beneficial insects or humans. This material is not systemic and must be ingested by a caterpillar to cause death. When eaten by a caterpillar a crystal dissolves in the insect’s stomach and breaks down the stomach wall. The crystal can only dissolve in those insects species whose stomach contain the correct combination of pH, salts, and enzymes. It cannot dissolve the highly acidic stomach of humans. Because of the selectivity, the US EPA permits food crops sprayed with Bt to be eaten right after spraying. Bt is effective in controlling early stages of caterpillars but is rather ineffective against late instar stages of caterpillars. It can provide good control of early instars of bagworms, eastern tent caterpillars, fall webworms, yellowneck caterpillars, orange stripped oak caterpillar and several other lepidopterous species of caterpillars. Note: Bt will not control sawfly caterpillars which are in the order Hymentoptera.
Information from Stanton Gill, Extension Specialist, Central Maryland Research and Education Center, University of Maryland Cooperative Extension