Executive SummaryU.S. farmers have adopted genetically modified crop varieties rapidly since their introduction. By 2000, roughly one fifth of U.S. corn acreage, over half of the soybean acreage, and almost three-quarters of the cotton acreage was planted to crops genetically modified to be resistant to insects and/or herbicides.This report updates our previous estimates of the benefits associated with the adoption of genetically modified crop varieties. Updated impact estimates have been calculated for corn, cotton and soybeans. A discussion of potential impacts for potatoes is included. Each of these crops delivers a unique set of benefits to growers who adopt them. These benefits largely depend on pest control issues particular to each crop and whether other effective and affordable pest control options are available. Understanding the reasons why growers are adopting these varieties is critical in evaluating the their impact.Insect resistant Bt corn varieties have allowed farmers to control the European Corn Borer, an insect pest that is difficult to control using conventional insecticides because it tunnels into the corn stalk soon after hatching, and is protected from the insecticides. Prior to the introduction of Bt corn, few growers were spraying for the corn borer. Instead, yield losses sometimes reached 300 million bushels of corn per year. With Bt corn, losses to the corn borer are eliminated. The primary benefit of Bt corn varieties has been increased yields. In 1999, it is estimated that 66 million bushels of corn were saved from the corn borer, or the equivalent of production on nearly 500,000 acres. Modest reductions in corn insecticide use have also been observed since the introduction of Bt corn. Cotton growers have adopted genetically modified varieties faster than growers of any other crop. Both insect and herbicide resistant varieties have been adopted widely. Insect protected varieties of Bt cotton provide control of three of the most destructive insect pests in cotton: tobacco budworm, cotton bollworm and pink bollworm. Growers were managing these pests with several insecticide treatments per year. In some areas, the efficacy of these insecticides was diminished by the development of insecticide resistant insect populations. Bt cotton has allowed growers to reduce insecticide use and attain better control of these pests, which has resulted in increased yields. It is estimated that cotton growers reduced insecticide use by 2.7 million lbs and made 15 million fewer insecticide applications per year due to the introduction of Bt cotton. Cotton production has also increased, by 260 million lbs per year. Net revenues are estimated to have increased by $99 million in 1999.