Identification And Reproduction
The adult syrphid fly is about ¼-inch long. This species (Allograpta obliqua) may be recognized by the generic yellow thoracic stripes and abdominal cross bands and yellow face lacking a complete median stripe. Adults of A. obliqua are found throughout the year in North Florida but become much more abundant during spring and summer. In South Florida, they often are abundant year round. The life cycle varies from as little as three weeks in summer to nine weeks or more in cooler months.
Females lay their tiny (1 millimeter) whitish-to-gray oblong eggs near aphids or within aphid colonies. Larvae are legless and maggot-shaped and can vary in color and patterning, but most have a yellow longitudinal stripe on the back. They can be distinguished from caterpillar larvae by their tapered head, lack of legs and their opaque skin, through which internal organs can be seen. The larval stage can take from nine to 20 days averaging 10 to 14 days and one larva can consume an average of 17 aphids per day. The larva fastens itself to a leaf or twig when it is ready to pupate. The pupal stage takes eight to ten days in summer and longer in winter.
Many species of aphids have been reported to be preyed on by syrphid flies. Species of major economic importance include the cabbage aphid, green peach aphid, pea aphid, melon aphid, and cotton aphid. Larvae move along plant surfaces, lifting their heads to grope for prey, seizing them and sucking them dry and discarding the skins.
Adults often visit flowers for nectar or may be seen around aphid colonies where they feed on honeydew secreted by the aphids and lay their eggs. Adults feed on pollen and nectar and are effective in the cross pollination of some plants.
The larvae are important predators, feeding primarily on aphids that attack citrus, subtropical fruit trees, grains, corn, alfalfa, cotton, grapes, lettuce, and other vegetables, ornamentals, and many wild host plants of the aphids. When larval populations are high, they may affect 70% to 100% control of aphid populations.
In addition to aphids, whiteflies and other soft-bodied insects have been reported to serve as food for the larvae of the syrphid fly.
Growers can encourage syrphid fly populations by leaving weeds around field margins or planting refugia that will provide adults with pollen and nectar and by using soft insecticides that will minimize impact on this beneficial insect.