Emerging and Important Insect Pests of Alfalfa in the Western United States

Four insects species have emerged as important pests of alfalfa in the Western United States during the past decade. Cowpea aphid has long been a resident insect of the Western United States, but became a significant alfalfa pest during the past two years. The silverleaf whitefly has been a serious pest of alfalfa since 1991. Silverleaf whitefly reduces hay yield and contaminates alfalfa hay with a sugary excrement, “honeydew”, causing problem with harvest and marketing. Spider mites have long been a pest of alfalfa grown for seed and have become a serious pest of alfalfa grown for hay production since the late 1980's. Spotted alfalfa aphid has been a pest of alfalfa in the United States for nearly a half -century, but has been kept in check by highly resistant alfalfa varieties. During the mid-1990's, spotted alfalfa started showing signs of overcoming the host plant resistance in alfalfa.