If you think you have a fungus gnat problem, you may want to take a second look. A 'new' insect is showing up in Texas greenhouses and nurseries. Some growers have seen the fly and made the assumption that it must be bad. For once this is not an insect you will need to control. Coenosia attenuate, an old world hunter fly, was first identified in 2004 at Nortex Greenhouses in Collin County. Since that time, this beneficial fly has also been identified in Cherokee and Smith County. I am sure if I looked I would be able to find it in many additional counties. In North America, this hunter fly was previously known to occur in New York, California and Ontario, Canada. In California, C. attenuate is associated with gerbera, roses, poinsettia and chrysanthemum production.
Coenosia attenuate resembles a small housefly. They range in size from 2.5 mm to 4 mm. The female has a dark grey body and black legs while the male has pale yellow legs. You can observe the adult flies perched on plants, pipes, or other objects in the greenhouse. These predators actually wait to fly out and grab any flying insect of suitable size. The prey is then subdued by stabbing them with specialized mouthparts. Hunter flies prey on insects such as fungus gnats, shore flies, whiteflies, winged aphids and leafminers. Once prey is stabbed, they return to their perch to feed. Each adult occupies a defined territory and sometimes territorial “dogfights” can be observed when boundaries are compromised.
Adult flies lay their eggs in the soil and the developing larvae are predators of fungus gnats and shore flies. Research conducted at Cornell University indicates that this is an effective predator of fungus gnat larvae.
We sampled fly populations at Nortex Greenhouse for a year. It appears from these results that this predator is capable of managing fungus gnat populations, especially in pot plants.
If you see a new fly showing up in your greenhouse do not automatically reach for the nearest bottle of insecticide. Have the fly identified. Coenosia may prove to be a valuable biological control agent. Even more importantly, one you do not need to purchase!