Significant groups of pests have evolved with genetic resistance to pesticides. The most serious problems of resistance have been encountered in the control of arthropods. The greatest increase and strongest impacts have occurred in the last 30 years following the extensive use of synthetic organic pesticides. Scattered populations of rats have developed high levels of resistance to some rodenticides. Resistance to anticoagulants in the Norway rat has created a serious problem in some U.S. cities.