No fly zone

The lives of flies are woven into ours, but in some cases too tightly. For greenhouse crop growers, managing flies can be a serious issue. Fortunately, it’s not a question of who’ll win, but rather of how we fight.

In greenhouse production, the most important “true flies” we encounter are shore flies and fungus gnats (leafminers would be close third), especially during late winter and spring. Shore flies thrive on algae and don’t damage plants directly; however, they are annoying to workers and leave dark spots on flowers and foliage. They also can transport spores of some root rot pathogens. Fungus gnats can be almost as annoying, and the larval stage feeds upon roots and sometimes lower leaves — damage too small plants can be severe. They are also implicated in the spread of some plant pathogens, such as Thielaviopsis basicola.

The good news is good management tools are available. First, accurately identify the two species — the threat and the controls differ. Second, monitor populations to detect them early and watch their trends. Third, decide what controls to use. Information from monitoring also helps gauge how effective controls are.

The Internet can help in identification, such as this UConn page: bit.ly/X4d73l. For monitoring, use a few yellow sticky cards set just above the crop — check periodically to see how numbers of trapped flies change over time. Fungus gnat larvae in media can also be monitored with potato halves or discs as described in the UConn page. There are no established thresholds, but when a few are being trapped on the cards or fungus gnat larvae found under potato discs and plants are at an early stage of production, some kind of control might be needed. Develop your own action thresholds, such as 10 fungus gnats per card per week, and adjust as needed.

Fungus gnats are best controlled by targeting the larvae. Selecting appropriate controls is a bit of an art. Interest in biological control is growing. Beneficial nematodes (Steinernema feltiae and Heterorhabditis bacteriophora), the soil predatory mite Stratiolaelaps, and the predatory rove beetle Atheta coriaria are among those that target larvae. Gnatrol, derived from a bacterium, is probably most effective against newly hatched fungus gnat larvae. We have been finding some “wild” biocontrols becoming established in greenhouses too, especially hunter fly (Coenosia attenuata), a predator of both larvae and adults. Also common are tiny parasitoid wasps Synacra pauperi (photo at bit.ly/QXtHgc), which attacks fungus gnat larvae, and Hexacola neoscatellae (bit.ly/SsWTM6), which prefers shore fly larvae. Check with suppliers on compatibility with fungicides or insecticides if needed.

Insecticide drenches also work well against fungus gnat larvae. Choosing among the options may depend upon compatibility with biocontrols and other pests present (e.g. aphids). Products include Marathon/Discus (or generics); Safari; Flagship; TriStar; Distance; Pylon; Adept; Citation; and azadirachtin materials (Azatin XL, Aza-Direct, Molt-X). Lightly ‘sprench’ the upper media surface; some can be used as heavier drenches.
 

Some of the same biological controls (nematodes, Atheta) can be used for shore flies, but include all areas where algae is found. There is limited information about insecticides, but controlling algae in the greenhouse with steam or sanitizers (ZeroTol, etc.) keeps numbers down. Apply regularly as treatments have no or little residual activity. If populations get too high a light overhead application of Conserve (or Entrust SC for organic growers) knocks down the adult flies. Several other materials are also labeled. In small areas placing yellow sticky cards or tape horizontally can trap out a large proportion of the population.

Look for some interesting developments concerning management of shore flies and fungus gnats. The battle’s not over yet but it’s not looking too good for the flies.

Disclaimer: Consult and follow pesticide labels for registered uses. No discrimination is intended for any products not listed.

Daniel Gilrein is a frequent contributor to Greenhouse Management and an extension entomologist at Cornell University.