This publication summarizes 25 years of heretofore unreported efforts to effect the biological control of prickly pear cacti infesting rangeland on Santa Cruz Island, situated off the coast of southern California. To date, partial to substantial biological control of the pest cacti, Opuntia littoralis (Engelmann) Cockerell, O. oricola Philbrick, and their hybrids, has been achieved. The principal biological control agent is a cochineal insect, Dactylopius sp., which is native to the southern California mainland. It was introduced to the island free of its principal natural enemies—Hyperaspis taeniata significanis Casey and Laetilia coccidivora (Comstock)—and has multiplied markedly, at the same time destroying numerous clumps of cacti throughout the island.

Effective range management practices, i.e., eradication of wild sheep and restrained cattle grazing, have aided the biological control efforts, and both have resulted in the return and persistence of annual grasses on formerly overgrazed and cacti-infested grazing lands.

The coreid bugs, Chelinidea tabulata (Burmeister) and C. vittiger Uhler, were also successfully introduced to Santa Cruz Island from Texas and mainland California, respectively, but neither species is thought significant as a biological control agent. The cactus-feeding phycitids, Olycella junctolineella (Hulst), Melitara dentata (Grote), and M. prodenialis Walker, were not successfully colonized on Santa Cruz Island.