British-born American entomologist who contributed much to the advancement of the systematic study of insects of economic significance in the United States and helped to establish the Division of Entomology (later called Entomology Research Division) of the U.S. Department of Agriculture. In addition, his well-documented and vividly illustrated studies were instrumental in making farmers aware for the first time of the importance of insect pest control in crop production.

In 1868, eight years after having immigrated to the United States, Riley was appointed state entomologist of Missouri. In 1876 he became chairman of the U.S. Entomological Commission, formed to study the Rocky Mountain locust that had devastated vegetation in the Midwestern and Western states since 1874. The success of the commission in dealing with that plague resulted in Riley's appointment as entomologist of the U.S. Department of Agriculture in 1878, and he served in that capacity, with one brief interruption, until 1894. Under his direction, considerable progress was made in the development of arsenic-based insecticides, kerosene emulsions, and other means of controlling insects injurious to agriculture.