Bt corn was introduced in 1996. There are four genetic modifications, or transgenic events used for Bt corn production, developed by different biotech companies and having different results in the corn itself. Combinations of various promoter genes in combination with different portions of the Bt genome can result in the gene expressing at different times in the crop, and may or may not be expressed in the grain itself, only the foliage.
Advantages to using Bt corn include minimizing timing issues for pesticide application, no special application equipment, no need for personal protective gear during application, is compatible with biological control. As Bt is an order specific pesticide, it has minimal effects on non-target pests and may control other corn pests of the Lepidoptera order (earworm, fall armyworm, Indianmeal moth, black cutworm, and southwestern corn borer) and reduces the need for pest monitoring. Also, as most corn varieties become increasingly susceptible to secondary fungal infections after being weakened by the corn borer, Bt presence also mitigates fungal infections.
The disadvantages are primarily the seed cost and variable pest populations, development of Bt resistance by pests, impact on non-target organisms, variation in effectiveness, marketing of Bt grain, cross-pollination of Bt corn and non Bt corn. In 1999, Cornell University published the results from a poorly designed trial which suggested that Bt contaminated pollen represented a threat to monarch caterpillars. This was later refuted as pollen contamination rarely reaches lethal levels, there is limited overlap during pollen presence and caterpillar presence and that only a portion of caterpillars will feed on milkweeds adjacent to cornfields.
Recommendations have been made to plant non Bt corn in fields adjacent to Bt corn to reduce the development of Bt resistance in the corn borer. As resistance is believed to be a recessive allele, increasing the chances of a Bt resistant moth mating with a non-resistant moth, with a high chance of producing more non-resistant offspring. However, this planting strategy increases the amount of pollen contamination to non Bt corn nearby.
The National Corn Growers Association (NCGA) lists hybrids which have full food and feed approval for the 2008 planting season in the US. The information includes regulatory information, as well as the trade names, characteristics and genetic events for all of the current GM hybrids currently available.