Thank you for your question. First, the insecticidal protein the crop produces is very specific to particular insect orders. For example, the Bt protein Cry1Ab affects only specific caterpillar (the order Lepidoptera) pests, while the Bt protein Cry3Bb1 affects only specific beetle (the order Coleoptera) pests. This specificity is one of the reasons Bt is such a popular insecticide in both GM crops and non-GM crops, including organic agriculture. Thus, the Bt proteins currently being used in crops do not affect bees (the order Hymenoptera).
Secondly, all commercial Bt insecticidal proteins are tested by specifically feeding the Bt protein to honey bee larvae (immatures) and honey bee adults. Scientists compared the results of bees fed the Bt insecticidal protein with bees fed a diet without the protein. In some studies, the Bt protein is plant derived (i.e., the pollen or leaf tissue is part of the diet), and in other studies, the protein (Cry1Ac, for example) is used in the diet. Three papers in scientific journals (Apidologie, the Annual Review of Entomology and PLoS ONE) reviewed the published data from 32 different trials. All three papers concluded that bees are not affected by Bt. A review by Malone and Pham-Delègue (2001) looked at seven studies. Their conclusion was that “Bt transgene products are very likely to be safe for honey bees and bumblebees.” One large study by, Duan et al. (2008), looked at 25 different studies and concluded that “the Bt Cry proteins used in genetically modified crops for control of caterpillar and beetle pests do not negatively affect the survival of honey bee larvae or adults in the laboratory.” The third paper, by O’Callaghan et al. (2005), stated, “Other studies with bees fed purified Bt proteins, or pollen from Bt plants, or bees allowed to forage on Bt plants in the field, have confirmed the lack of effects noted by the U.S. Environmental Protection Agency (EPA).”
Finally, there are studies that, upon first glance, seem to indicate that GM crops affect bees, when actually it’s not the crops at all that are responsible, but other production factors. For a good example of this, read the Morandin and Winston study (2005). This study shows that fewer wild bees were found in the GM canola studied, but the authors concluded the decline was due to fewer weeds in the GM crop, compared with a conventional or organic canola crop.