A pest insect known as bollworm, or Helicoverpa zea, is the first to evolve resistance in the field to plants modified to produce an insecticide called Bt, according to a new research report. Entomologists discovered the existence of Bt-resistant populations of bollworm in Mississippi and Arkansas by analyzing published data from monitoring studies of six major caterpillar pests of Bt crops in Australia, China, Spain and the US.