Compiled by Aloy Gu, Department of Entomology, Cornell University/NYSAES, Geneva, NY 14456 email@example.comSyrphid flies are also called flower flies or hover flies as the adults are usually seen hovering over or feeding on nectar or pollen at flowers. The larvae can be voracious predators of small insects, especially aphids, and all aphid-feeding (aphidophagous) species are in the subfamily Syrphinae. Common aphidophagous syrphid flies in California vegetable crops include Toxomerus marginatus, Allograpta obliqua, Syrphus opinator, Eupeodes americanus, Eupeodes volucris, Paragus tibialis, Platycheirus spp., Scaeva pyrastri, Sphaerophoria spp., Syrphus opinator, among many others. Syrphids are found throughout North America and are common on many crops attacked by aphids and other small soft-bodied insects.
Adult flies vary in length from 3 to 13 mm depending on species. Adult bodies are black or brown marked with bands or dots of white or yellow covering abdomen and/or thorax. Their resemblance to bees and wasps is mimicry to ward off predators. However, they can be distinguished by a single pair of wings, with vestigial hind wings as balancing organs, characteristic of all dipterans. Syrphid fly larvae are slug-like maggots that are wrinkled and tapered anteriorly. Their color can be pink, yellow, green or brown marked with white or black, and they vary in size from 4 to 18 mm in length.
Adult flies visit flowers and feed on nectar and pollen. They are considered important pollinators for some plants. They can also be often found near aphid-infested leaves where they lay eggs and also feed on honeydew secreted by aphids. Maggots of most species are found on aphid-infested plants like citrus, subtropical fruit trees, grains, corn, alfalfa, cotton, grapes, lettuce and other vegetables, ornamentals, and many wild host plants of aphids. Larvae of some species live in various habitats such as the nests of social insects (ants, termites or bees), decaying plant or animal matter, and stagnant water (Eristalis spp.). A few feed on live plants.
Most syrphid fly maggots feed on aphids, thrips, leafhoppers and and other soft-bodied prey like small caterpillars. They move along plant surfaces, lift their heads to grope for prey, seize and suck them dry and then discard the exoskeleton.
The life cycle varies among species and depends on environmental conditions and availability of food. In optimal conditions, single white eggs are laid on leaves infested with prey or other suitable food source. After hatching in about 3 days, larvae develop through several instars (usually 3) over a period of 1 to 3 weeks. The last instar turns into tan-brown teardrop-shaped puparium either on the host plant or in the soil. Unless the pupal stage remains for overwintering, adults emerge in 1 to 2 weeks. Up to seven generations can occur within a year.
Line drawing of Toxomerus spp. life cycle (Brett Blaauw, Department of Entomology, Michigan State University, East Lansing, Mi 48823).
Each larva can consume up to 400 aphids during development. When syrphid larvae are abundant, they may reduce aphid populations by 70 to 100%.
Syrphid flies may be tolerant of some chemical insecticides applied at recommended field rates. But tolerance may be influenced by prior exposure of the natural enemy population to the chemical, and there is considerable variation. For example, Coleomegilla maculata has been shown to be susceptible to carbaryl and methamidophos at doses that might be used to control aphids in potatoes. Overwintering adults may be less susceptible to chemical insecticides than active adults and larvae.
Applications of insecticides, especial broad-spectrum ones, should be avoided when possible. As adult hover flies feed on pollen and nectar, planting flowering plants near the field or between crop plants will attract them and increase the possibility that they lay their eggs on infested plants. Managing hedgerow plants or other vegetation may help provide alternative prey, wind shelter and even overwintering habitat.For general information about conservation of natural enemies, see Conservation in the Tutorial section on this site, and Feature Article on conservation in Volume II, No. 1 of Midwest Biological Control News.
Episyrphus balteatus from Koppert Biological Systems is the only currently available hoverfly for purchase in the United States, although a few other species are considered amenable for commercial production.
R. Bugg et al. Flower flies (Syrphidae) and other biological control agents for aphids in vegetable crops. UC ANR Publication 8285, 2008