Arizona farmers recieve same yield of crop, using fewer insecticides yet maintaining the insects biodiversity. BT crop is one of the genetically modified crops that produces BT toxin to kill a naturally occuring insectides, Bascilus Thuringienesis, a pink ballworm which is a major pest of cotton. One of the reason for success is that the BT plants only kill the targeted pests, and not kill the non target pests, not harming the insects biodiversity and useful insects. 

BT plants only controls one of the three major pests of cotton. In order to control the other pests , broad spectrum insecticides called  insecticides growth regulator are used. But when BT plants were grown, researcher found that the BT plants yielded in 9% more cotton than non-BT plants, yet in BT plant the insecticide growth regulator was not used in broad spectrum.

However, researchers believe that just because BT plants are a success in Arizona does not mean it is a success elsewhere. Therefore, more systematic study of BT plants need to be done before making general conclusion.

Personal Opinion: I think the success of BT crop in Arizona is a plus point to biotech plants. If most of the GM plants are a success like this leading to high plant yield,good impact on environment, human and ecosystem, then may more people will be open to the GM plants, and feel safe to use it.

source: science daily

weblink: http//www.sciencedaily.com/releases/2006/05/060501232015.htm