The Mediterranean Fruit Fly Preventive Release Program (Medfly PRP) located in Los Alamitos, California, is a joint program of the United States Department of Agriculture (USDA) and the California Department of Food and Agriculture (CDFA). The objective of the PRP is to prevent establishment of Medfly colonies in California utilizing a scientific process known as sterile insect technique (SIT). SIT is a biologically-based birth control method involving sustained releases of large numbers of sterile males into a target area to reduce the reproductive potential of wild introductions. Matings between these sterile males and wild females result in the production of infertile eggs. This "birth-control" approach can be used to prevent, and eradicate Medfly populations, with no side-effects on the environment. The PRP also maintains an operational infrastructure to be utilized as needed to respond to Medfly infestations both within and outside the PRP area.

Prior to the establishment of the PRP, California employed a reactive approach to repeated Medfly introductions throughout the state. These eradication efforts were costly and sometimes involved controversial aerial applications of pesticide. In 1996, the Medfly PRP was formed to proactively address the escalating threat of introduction present in the Los Angeles Basin as a result of increased trade and travel. Several changes have been made to the coverage area since program inception, but the Medfly PRP currently encompasses 1,750 square miles and includes portions of Los Angeles, Orange, San Bernardino, and Riverside counties. When infestations do occur, the strategic location of the Medfly PRP facility in Los Alamitos allows for a swift and economical response anywhere in California.

Map depicting the Medfly PRP release area

The cost to fund the PRP is approximately 16 million dollars annually with each Department contributing equivalent funding through a cost share agreement. This cost is minimal when compared to the costs of eradication efforts and also serves to limit quarantines imposed upon U.S. growers and industry, thus enhancing global trade efforts. Additionally, no pesticides are utilized in the Medfly PRP, making the program environmentally-friendly.

The Medfly PRP consists of four major components that operate year round. These components involve the release of sterile Medflies at the weekly rate of 62,500 to 125,000 flies per square mile within the coverage area, trapping for the detection of wild Medfly introductions at the rate of five Jackson traps and five McPhail traps per square mile with inspections at weekly intervals, identification of trapped flies by entomologists, and data management and review to monitor the quality and effectiveness of the PRP.

The Medfly PRP has proven to be an effective tool in the battle to keep California free of Medfly. Prior to inception of the PRP, California averaged 7.5 infestations per year. Since preventive releases began, the number of infestations has dropped by over 90%, averaging less than one annually.