The adults are small (less than 1/8 inch long), black, delicate flies (similar to a fungus gnat) that live for an average of 10 days, feeding on aphid honeydew. They hide beneath the leaves during the day, and are active at night. Females deposit 100-250 tiny (1/64 inch) shiny orange eggs singly or in small groups among aphid colonies. The tiny, bright orange, slug-like larvae that hatch in 2-3 days inject a toxin into aphids' leg joints to paralyze them and then suck out the aphid body contents through a hole bitten in the thorax. Larvae can consume aphids much larger than themselves and may kill many more aphids than they eat when aphid populations are high. A single larva grows up to 1/8 inch long and kills 4-65 aphids per day. After 3-7 days the larvae drop to the ground and burrow 3/4 to 1 and 1/2 inches into the soil to pupate. They are most effective at 68-80 degrees Fahrenheit and high relative humidity.
Aphidoletes aphidimyza has been used successfully for biological control of aphids on cucumber, green pepper, tomato, chrysanthemum, rose, and many kinds of ornamentals in greenhouses on a commercial scale. Unlike other predators, it reproduces during the growing season, so only a single introduction may be necessary, although 2 to 4 introductions are often required. It may be more useful in the biological control of aphids on greenhouse crops than lady beetles or green lacewings. However, many aphids killed by midge larvae remain attached to the leaves which detracts from the value of ornamental plants.
Commercially available aphid midges are shipped as pupae (inside their cocoons) in moist vermiculite that can be sprinkled on the soil under the plants. Adults will not emerge from cocoons that fall on dry areas in direct sunlight, so they need to be placed in moist, shaded areas. The bright orange midge larvae should be visible among the aphids about a week after the adults have emerged. Recommended release rates vary depending on the crop plant, growth stage, aphid species, and infestation level. Lower release rates may be used before aphids are observed for preventative control. Two to four applications are often necessary to achieve control. Additional introductions of the midge will not be necessary if successive generations are produced during the growing season. When some aphids can be tolerated on the lower leaves of the plants the midge populations will build up quickly on the good supply of aphids.
Biological control of aphids with A. aphidimyza is most successful in greenhouses with soil beds or gravel floors, probably because the midge larvae easily find suitable sites for pupation on the floor. Sprinkling a thin layer of sawdust or peat moss between the rows on concrete or plastic covered floors can help provide suitable pupation sites in these greenhouses. Without good pupation sites on the floor fewer midge offspring survive, and weekly releases may be necessary throughout the season to provide good control.
One drawback to using A. aphidimyza in the greenhouse is that it diapauses (goes into "hibernation") under cool, short-day conditions. Diapause can be prevented by leaving on a few incandescent walkway lights all night during winter months (through late February). Larvae are very sensitive to light, so a single 100-W bulb will prevent over half of the midges within a 24 yd diameter circle from diapausing. This will not be effective when plants are large enough that light does not penetrate between the rows. If supplementary lighting to prevent the midges from diapausing is not feasible and aphids are a problem in the fall or winter, other natural enemies, such as the parasitic wasp Aphidius matricariae, can be used for aphid control.
- Susan Mahr, University of Wisconsin- Madison
Illustration by Michele Schwengel
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