The Role of Imported Parasites in the Biological Control of the Spotted Alfalfa Aphid in Southern California in 19571

Abstract

The spotted alfalfa aphid, Therioaphis maculate (Buckton), is attacked by three imported parasites in southern California. These parasites, Praon palitans Muesebeck, Trioxys utilis Muesebeck, and Aphelinus semiflavus Howard, were studied in three alfalfa fields in three distinct climatic areas in 1957. The roles of native coccinellids and fungus diseases (principally Entomophthora exitialis Hall & Dunn) in the biological control of the aphid were also evaluated. T. maculate was under heavy biotic pressure in all three fields over most of the year. In two of the fields, located at Cawelo and Lancaster, California, the aphid reached economic abundance on only one occasion. In the third field, located at Calexico, California, it did not reach economic status at any time during the year. Coccinellids appeared to play the most important role in the biological control of the aphid in all three fields. Disease of major importance at Calexico. Parasitization reached high levels in each of the plots at certain times and contributed significantly to the biological control of the aphid at each locality. The alfalfa harvesting process appeared to have an adverse effect on the parasites, particularly in midsummer. Periodic invasion of the study fields by large numbers of alate T. maculate had all unfavorable effect on the parasite- host ratio. The parasites seemed to show varying climatic adaptation. P. palitans was dominant at Cawelo and Lancaster, while T. utilis was dominant at Calexico. A. semiflvus was encountered in fair abundance at Cawelo only.