Hunter Flies May Be Small, but They Have a Big Appetite

Features - Views from the Buglady

Most growers do not really think of flies as their friends because they are dealing with pests like fungus gnats, shore flies and leafminers.

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October 13, 2009
Suzanne Wainwright-Evans

Suzanne Wainwright-EvansMost growers do not really think of flies as their friends because they are dealing with pests like fungus gnats, shore flies and leafminers. It is time to re-think at least some of your fly feelings. There is a fly out there trying to lend you a helping hand if you will let it. It’s called “The Hunter Fly.” This fly is in the same family as the common housefly, but looks like a miniature version. They range in size from 2.5-4 mm. If you look very closely you will notice they have either black legs or pale yellow legs. This is how you can tell males from females — males having pale yellow legs, while females have black legs. The body is dark green for both the sexes.

New bug on the block
This mini eating machine has not been in North America for a long time. It only showed up about 10 years ago. An observant greenhouse scout in New York, noticing that this fly was something she had not seen before, sent it for identification. The results? It was identified as the hunter fly, Coenosia attenuata. This fly is native to southern Europe but has recently been popping up all over North and South America.
 
More and more growers are finding these flies showing up on their sticky cards, especially in greenhouses. These beneficials are not being released intentionally, but researchers believe they are moving around in growing media.
 
One hypothesis for the recent appearance has to do with the development of modern pesticides. Growers are using more targeted insecticides, so several non-target insects are not being knocked out with broad-spectrum spray applications. And with the widespread development of scouting programs as a standard growing practice, fewer sprays are being applied, thus allowing more of these good guys to work for the grower.

Don’t shoo this fly
Just how exactly is the hunter fly beneficial to growers? The adults feed on fungus gnats, shore flies, some leafhoppers, winged aphids and whiteflies.
 
They do this by sitting or perching on a leaf and waiting until a prospective meal flies past, grabbing it right in the air. Hunter flies only attack flying prey. While holding the prey in its front legs, it punctures the insect with specialized mouth parts, its proboscis, and eats the insides of the prey, leaving the dead carcass behind. In cases where food supplies are low, they may turn on each other, becoming cannibalistic.
 
How many pests will the hunter fly feed on in a day? It depends on the pest and the density. One example that has been researched is its feeding habits on fungus gnats (Sciarid flies). One hunter fly can feed on three to nine fungus gnats a day.
 
Adult hunter flies and those in the larval stage help control pests. These guys are on patrol right from the start in the soil. The adult females lay eggs in the soil, where the fly larvae are predatory on fungus gnat larvae and other immature insects for about two weeks before pupating. They remain in the pupal stage for about two weeks before emerging as an adult killing machine. Some of the adults can live for up to seven weeks, although most live about three or four weeks.