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Biology Fungus gnats (Bradysia impatiens) are small (2 - 5 mm long) black flies found on wet soil. They are distinguished from shore flies by their long segmented antennae, long legs and a prominent Y-shaped vein on their wings. Their larvae are thin white threadlike maggots with black heads and are found in the soil. They feed on decaying organic matter, roots and the bases of young plants and cuttings. Damage can be considerable for young plants and diseases from fungal, bacterial or viral pathogens can be spread. Females can lay up to 200 eggs that hatch into larvae within 4 - 6 days. Maggots remain active for approximately two weeks and then pupate in the soil 4-6 days at which time the adult emerges. Shore flies (Scatella stagnalis) are strong flyers in contrast to fungus gnats. They are larger, have short antennae, red eyes and dark wings with five clear spots. Their larvae are up to 6 mm in length and are yellowish brown in color with no identifiable head. The life cycle is similar to that of the fungus gnat. Both adults and larvae eat algae on the soil surface and surrounding areas. Although they do not damage plants directly, they can be vectors for soil pathogens. Control is dependent on reduction of wetness and subsequent elimination of algae. Cultural Controls The first step in cultural control should be to reduce organic matter. This can be accomplished by reducing the amount of peat or humus in the soil mixture and removing all decaying leaves and other plant parts from the soil surface of the pot and underneath benches. The next step should be to reduce wetness by improving soil drainage and watering only when the soil surface is dry. Fans should be installed to improve ventilation. Physical Controls Physical techniques such as covering the soil surface with light colored sand act as a repellent and barrier preventing adults from mating (Doug Walker, UC Davis, personal communication). Yellow sticky traps can help reduce population numbers by catching flying adults. Biological Controls Biological control agents include Hypoaspis miles, Steinernema species and Bacillus thuringiensis. Hypoaspis is a soil dwelling mite that attacks fungus gnat larvae. It is small (less than 1mm in length) and light brown in color. Reproduction is sexual with eggs hatching within 2 - 3 days. The mite eats 1 - 5 larvae per day. Control is best when Hypoaspis is introduced prior to fungus gnat population establishment (Gillespie and Quiring,1990). Steinernema carpocapsae and Steinernema feltiae are two nematode species that parasitize fungus gnat larvae. They are applied directly to the soil through irrigation. Reproduction is sexual with third stage larvae infecting the fungus gnat larvae. The nematode enters the host larva through openings in its body cavity and releases bacteria that digest it. The available food products are then metabolized by the nematodes that subsequently reproduce and seek out new prey. Bacillus thuringiensis (B.t.) is bacterium that is applied to the soil through irrigation. Different strains of the bacterium affect a variety of different insects. Insects eat the bacterium that then releases toxic cystaline proteins in their guts. As the intestinal lining is paralyzed, the insect stops eating, becomes less active and eventually dies from starvation and tissue damage. Chemical Controls Adult populations can be reduced by spraying with insecticidal soap. Keys to success:
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