The well-known weedkiller glyphosate (marketed as Roundup) is supposed to break down quickly and pose little threat to humans and wildlife. This is the weedkiller farmers must use when they grow Roundup Ready soybeans and other weedkiller-resistant GE crops manufactured by the Monsanto corporation. It is widely used around the world because it is supposed to be almost harmless to everything except the weeds. But a European Union review of the herbicide says that it can harm predatory mites and parasitic 
insects. It is also highly toxic to earthworms, useful bacteria and fungi as well as predatory insects such as lacewings and ladybugs (ladybirds). It can also greatly slow the activity of nitrogen-fixing bacteria (found among the roots of clovers, for example, and which boost soil fertility naturally). 
So glyphosate and GE crops could turn out to be a lethal cocktail for wildlife, not necessarily because of direct poisoning but because of the elimination of weeds that support insect prey. Worryingly, glyphosate has been linked in a Swedish study to non-Hodgkins lymphoma, a type of cancer already associated with other pesticides. [1]
Studies show that GE crops do not, on average, much improve yield or reduce pesticide costs. A U.S. Department of Agriculture study of a comparison of costs and yields did not show a clear economic advantage for herbicide-tolerant soy and cotton, nor for Bt [toxin-containing] cotton. [2][1] The Organic Way (Henry Doubleday Research Association journal), Winter 1999, 3