June 21

Bacillus thuringiensis (Bt) is a soil-borne bacterium that is widely used in landscapes and nurseries to control a variety of ornamental insect pests. Different insect pests are controlled by different strains, sub-species, or variants of the bacteria. The common strains and the insect groups that they are active on include Bt var. kurstaki (caterpillars), Bt var. israelensis (larvae of mosquitoes and certain flies), and Bt var. tenebrionis (leaffeeding beetles). Unlike fungi, bacteria have to be ingested in order to be active. This means that thorough coverage of all plant parts is essential when using Bt products so that the target insect pest will consume the bacteria. Once the bacteria are ingested, they produce endotoxin crystals that bind to the gut membrane creating pores, which results in leakage and swelling. The swelling continues until cells burst, allowing the gut contents to enter the insect’s blood (hemolymph). This disrupts the blood pH, and results in paralysis and death within 24 to 72 hours after consumption of the bacteria. It should be noted that Bt var. kurstaki is not effective against sawfly larvae since most of them are in the order Hymenoptera (bees and wasps), not Lepidoptera (butterflies and moths).