Colony Collapse Disorder (CCD) has caused much concern among beekeepers nationwide and it is not clear to date what is causing the die-off.
Genetically modified crops, specifically Bt corn, have been suggested as a potential cause of CCD. While this possibility has not been ruled out, the weight of evidence based on a multitude of studies argues strongly that the current use of Bt corn is not associated with CCD.

The hazard to bees due to any potential toxicant depends on toxicity and exposure. The endotoxins currently expressed in Bt corn (Cry1 types against caterpillars; Cry3 types against beetles) are not biologically active against hymenopteran insects such as the honey bee, nor do the CCD symptoms resemble those expected in Bt intoxicated organisms. Exposure is also very low because the expression of endotoxins in pollen is barely detectable in most Bt corn hybrids and corn does not produce nectar.
For these reasons, bees are not commonly found foraging in corn fields. Some argue that the increase in bee loss has paralleled the increase in Bt corn in the United States; however, severe bee losses have occurred in Europe and in areas of Canada where Bt crops are not grown.
What do the scientific studies say about the issue? Numerous laboratory studies have examined the potential non-target effects of Bt corn on honey bees by feeding high doses of the pollen or purified endotoxin mixed with honey or sugar syrup directly to larvae in brood cells. This approach is a standard protocol for Tier I testing of non-target effects on bees and required by EPA before insect-resistant transgenic crops are approved for registration.
Published studies and other technical reports submitted to EPA have all shown no adverse acute effects. In particular, recent laboratory studies in New Zealand and Switzerland exposed bees by feeding on pollen treated with purified Cry1 endotoxins at doses considered well above the maximum environmental exposure levels encountered in the field. Results showed no negative effects on bee survival.
Laboratory feeding studies at the University of Maryland also showed no effects on the weight and survival of honey bees feeding on Cry1Ab-expressing sweet corn pollen for 35 days.
Potential sublethal effects of Bt corn on honey bees have also been addressed, but not as extensively as the acute effects.
For newly-emerged bees, the presence of Bt proteins in ingested pollen may affect hypopharyngeal gland development and thus the ability of nurse bees to make brood food.
However, the same studies in New Zealand and Switzerland reported no effects of Bt pollen or endotoxin on hypopharyngeal gland development of newly-emerged bees.
Results of another recent study conducted in indoor flight cages showed no effects of Cry1Ab protein exposure on mortality, syrup consumption, or learning capacities of free-flying honey bees, but foraging activity was slightly reduced.
A two-year field study (soon to be published in the European bee journal Apidologie) conducted by this author and his graduate student represents the first attempt to expose functional colonies of honey bees to Bt corn pollen under open field conditions. In this Maryland study, colonies placed in Bt sweet corn plots were allowed to forage on corn pollen and also fed Bt pollen cakes for 28 days. The pollen cake consumption alone represented approximately 44 percent of the expected daily pollen requirements of each exposed hive.
Results showed no adverse effects on bee weight, foraging activity, colony bee strength and brood development.
Although there is no evidence thus far of any lethal or sub-lethal effects of the currently used Bt endotoxins on honey bees, insecticidal products expressed by other transgenes in crops may need extended field testing on a case-by-case basis to assess the longer term consequences of sub-lethal changes in colonies and subtle modifications in bee behavior.