Growth, development and consumption by four syrphid species associated with the lettuce aphid, Nasonovia ribisnigri, in California

The lettuce aphid, Nasonovia ribisnigri Mosley, was accidentally introduced into California from Europe during the late 1990s and soon became an economic pest of Romaine lettuce along California’s central coast region. Indigenous syrphid larvae attack the lettuce aphid and are believed to be effective in the management of this invasive pest, although there have been no studies on the capacity of the syrphid larvae to kill and consume lettuce aphids. We focused on four syrphid species commonly found in central coast lettuce fields: Allograpta obliqua (Say), Eupeodes fumipennis (Thomson), Sphaerophoria sulphuripes (Thomson), and Toxomerus marginatus (Say). Laboratory feeding experiments were conducted to estimate the development times of all juvenile stages, the daily growth rate of larvae, the number of third instar aphids killed, the aphid biomass killed, and the aphid biomass consumed as measures of predator performance. Results show that during larval development E. fumipennis killed the most third-instar aphids (507 aphids, 88 mg biomass killed) and reached the largest size, followed by A. obliqua (228 aphids, 39 mg killed), S. sulphuripes (194 aphids, 31 mg killed) and T. marginatus (132 aphids, 20 mg killed). Body size alone did not account for species differences in per-capita larval consumption rates. This information is discussed in relation to the predation potential of syrphids through the short cropping cycle of lettuce, and the choice of plant species to use for floral resource provisioning to enhance the activity of syrphids needed for effective management of lettuce aphids in California’s central coast fields.