Bt proteins are degraded within just a few hours in sunlight. Because degradation occurs so rapidly, the spores and crystals are usually applied in some type of formulation to decrease the degradation rate. But even formulated Bt products lose their insecticidal activity within about 4 days. Degradation time may be even shorter than 4 days, depending on the effects of microbial degradation, dew, and washoff due to rainfall events. (for more background on BT see:http://pmep.cce.cornell.edu/profiles/extoxnet/24d-captan/bt-ext.html)
In plants that have been genetically modified to produce Bt insecticidal proteins, there will be a fairly constant supply of the Bt proteins in the leaf tissues while the plant is alive. These Bt proteins generally interact only with insects that have a specific pH in their digestive system and a binding site in their gut that can attach to a specific site on the Bt protein. The requirement for a specific pH and a unique binding site means that a Bt protein can have an effect on target pests or closely related insects, but at the same time have no effect on other insects or organisms. After the plant dies at the end of the season, the Bt proteins a rapidly degraded in the soil along with all of the other proteins contained within the plant materials, which is fitting considering Bt is naturally found in soil.