Spider predation onColeophora parthenicaMeyrick (Lepidoptera: Coleophoridae), an insect introduced into the western United States for control of Russian thistle,Salsola australisR. Brown (Chenopodiaceae), was studied at Indio, Calif., during April through October 1980. The two most common of seven species of spiders observed feeding onC. parthenicaadults were studied in detail.Dictyna reticulataGertsch and Ivie (Dictynidae) reached mean densities of 93 per plant, and each spider captured up to five moths in each of three emergence periods. There was a positive correlation between the numbers ofD. reticulatawebs and moths captured per plant.Dictyna reticulataalso captured moreC. parthenicaper web as the density of the insect increased.Diguetia mojaveaGertsch (Diguetidae) reached densities of 170/ha, and mature females captured up to 221 moths during the summer.Coleophora parthenicacomprised a mean of 71% of the arthropods in the diet ofD. mojavea. Web site location on Russian thistles influenced the number of moths captured byD. mojavea, unlikeD. reticulata. The total numbers of prey captured byD. mojaveafemales, but not the numbers ofC. parthenicaalone captured, affected the number of egg sacs this spider produced. Field data suggested that these spiders were responding functionally and numerically to their prey densities augmented by the introduction ofC. parthenica.