The predatory gall midge Feltiella acarisuga is a spider mite predator with a nearly cosmopolitan distribution. It will feed on all stages of spider mites, but generally prefers eggs or young mites. There are several closely related species that are also good mite predators, including F. occidentalis, a common natural enemy on strawberry in California. Feltiella attacks several species of spider mite, including the twospotted spider mite (Tetranychus urticae), a ubiquitous greenhouse inhabitant, and the less common, but more difficult to control carmine spider mite (T. cinnabarinus).
The adult is a delicate, pink-brown fly, only about 1 mm long, with long legs. They do not feed and only live 3-4 days after emerging from the cocoon. Adults actively search for spider mite colonies. Each female lays an average of 30 shiny yellow eggs near high densities of mites, usually where webbing occurs. The tiny eggs hatch in 5-7 days. The yellow- or orange-brown midge larvae grow to about 2 mm long. Upon hatching they move to a prey, sink their mandibles in, and suck out the contents. They can consume over 300 mite eggs as they complete their development in about a week in the greenhouse. Under cooler conditions the larval stage may take up to a month to complete. They then spin fluffy white cocoons on the underside of leaves, usually along a leaf vein, in which to pupate. The pupal stage lasts approximately one week in the greenhouse, but longer outdoors under cooler conditions.
Feltiella larvae are voracious feeders. One F. occidentalis larva consumed 13 mites in 5 minutes. F. insularis will eat about 46 mite eggs per day. Spider mites that are killed by Feltiella larvae shrivel and become brown or black. Dozens of larvae can ususally be found on a heavily spider mite-infested leaf. In the field, natural populations of Feltiella do not appear until mite populations reach a certain density, and occur only seasonally.
In the greenhouse, however, where temperatures are regulated, and with regular introductions, Feltiella can be an effective year-round predator. F. acarisuga can provide good spider mite control when used in conjunction with the predatory mite Phytoseiulus persimilis. Even though Feltiella larvae eat at least five times as many spider mites per day as does Phytoseiulus, the midge alone is usually not able to control spider mites. The predatory mite has low dispersal ability, so it may fail to find patches of high pest density. Feltiella is good at finding hot spots, so the two predators are complimentary.
Feltiella is particularly useful on hairy leaved plants (such as tomatoes) that P. persimilis does not work well on. The glandular trichomes of tomato trap and kill predatory mites as they move in search of spider mites, but do not affect Feltiella.
Although Feltiella may invade greenhouses naturally, augmentative releases are essential for mite control. F. acarisuga is commercially available from several suppliers. The midges are shipped as pupae in cocoons on leaves or the carton. The carton should be opened and placed on the soil surface of plants in the greenhouse, protected from direct sunlight, for at least one week. Adults emerging from the cocoons in the container will fly to spider mite colonies. High humidity improves midge emergence. Optimal conditions for Feltiella are 68-81°F and relative humidity greater than 60%, although larvae can tolerate a wider range of conditions than can the adults. Weekly releases of one midge per 10 ft2 of plant material are recommended, but because of the high cost of these insects many growers use only one per 40 ft2 and allow the population to build up over time. A minimum of 4 weekly introductions in infested spots are usually necessary.
- Susan Mahr, University of Wisconsin - Madison
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