Transgenic crops were first introduced into the United States in 1996 and have become a major component of American agriculture. In a transgenic organism (also known as a genetically modified organism) some genes from one species are spliced into the chromosomes of another species. This is quite different from traditional plant or animal breeding in which individuals with desirable characteristics are crossed with other individuals having desirable characteristics.
By 2007 three transgenic crops—soybeans, cotton, and corn—were planted on 280 million acres worldwide, mostly in the United States. Many of these plants are registered as pesticides with the Environmental Protection Agency. (That’s right, your morning cornflakes may be made with a registered pesticide, but I digress.) In Canada, a large portion of the 17 million acres of oilseed rape (canola) is transgenic and the percentage is increasing every year.
There are two major types of genetic modification, both of which have implications for honey bees. One type of transgenic crop is resistant to certain herbicides, and one type is resistant to insects. Some crops, such as cotton, have been modified to resist both. Honey bees are regular pollinators of oilseed rape, frequently visit cotton and corn, and occasionally visit soybeans.
The insect-resistant genes have been transferred from Bacillus thuringensis (Bt), a bacterium that lives in the soil. The introduced genes produce an endotoxin throughout the plant that causes damage to the walls of the gut in susceptible insects. The damaged walls leak their contents into the lumen (interior space) of the gut, causing death of the insect. Researchers fear that Bt toxin in the pollen could damage the adult nurse bee gut or the larval honey bee gut.
Although many scientists have looked at the possible effects of Bt plant pollen on bees, so far there is no evidence of injury. However, because the toxins in the plants are able to kill the larval stages of other insects such as moths, butterflies, beetles, and weevils, many people believe that trouble may lie ahead as other plants—which may produce slightly different toxins or different quantities of toxin—become available. Currently only corn, cotton, potatoes, and tomatoes are commercially available with Bt genes. But in 2008 field trials were conducted on 30 additional crops, including apples, cranberries, grapes, peanuts, rice, soybeans, poplar, sunflowers, and walnuts.
The herbicide-resistant crops have a gene that resists glyphosate (RoundUp). This gene, too, was isolated from a bacterium. While the gene itself seems to have no adverse effect on insects, the application of glyphosate eliminates all the plant life except the resistant crops. Flowering weeds within the crops, as well as those in ditches, borders, paths, and irrigation canals are all killed, leaving the bees a very poor diet of only one flowering species.
A large number of researchers believe that these monoculture diets are a major factor in honey bee decline. Poor diet leads to loss of vigor and a depressed immune system which makes bees more susceptible to pathogens, parasites, and other stressors.