Studies On Succession, Distribution, and Phenology of Imported Parasites of Therioaphis Trifolii (Monell) In Southern California

The three introduced hymenopterous parasites of Therioaphis trifolii (Monell) were intensively studied for six years in four major climatic regions of southern California. These regions were the Colorado Desert the south coastal plain (two subdivisions), the western Mojave Desert, and the southern San Joaquin Valley. The studies revealed a succession in predominance of Trioxys utilis Muesebeck over Praon palitans Muesebeck, which dominated in the years immediately following parasite introduction. The status of the three wasps varies from one climate area to another. Trioxys is essentially exclusive in the Colorado Desert valleys and strongly dominant elsewhere. Praon attains its greatest importance in the western Mojave Desert and southern San Joaquin Valley. The third wasp, Aphelinus, reaches its most important status on the south coastal plain and is also relatively important in the western Mojave Desert. Parasite phenologies vary. Trioxys has the ability to be intensively active during the several seasons of the year. Praon is most active in the spring and fall. Aphelinus is relatively most active during the cooler, more humid times of the year. The general importance of these several phenomena to biological control programs is discussed.