According to the published studies, the drop is mainly due to systemic herbicides—the kind that penetrate the plant. When it comes to genetically-engineered crops, herbicides are already in the plant. However, no connection has yet been established with the bee, except in the case of a very specific systemic herbicide whose name I cannot recall.
Be that as it may, there is an impact. In 2000, German zoologist Hans Hinrich Kaatz conducted a study, which concluded that the transgene crossed the bee's intestinal barrier. In 2008 or 2009, other similar studies were conducted.