Greenhouse - Shore Flies and Algae

Shore flies can be a nuisance in greenhouses and have also been implicated in disease spread. Algae control and water management is critical for reducing shore flies as algae is their major food source. The following is an article on the subject.

Shore flies (Scatella stagnalis) thrive in wet areas with algae where they can feed and lay their eggs. Shore flies are often confused with another greenhouse pest found in wet areas, the fungus gnat (Bradysia spp.). Unlike fungus gnats, shore flies have robust bodies, short antennae, spots on their wings, and larvae without head capsules. Shore flies are also stronger fliers than fungus gnats which live mainly on the surface of the soil. Because shore flies do not feed on plant tissues like the fungus gnat, they are considered more of a nuisance pest. However, shore flies are capable of spreading diseases like Pythium.

Eliminating breeding areas and preventing the development of algae in the greenhouse is crucial for managing shore fly populations. Do not allow standing water- especially water containing fertilizer. Practice good sanitation by keeping the greenhouse free of debris. Monitoring for larvae is difficult, but by examining algae covered areas with a good hand lens, you can find the white, wedge-shaped larvae. Monitor adult populations using sticky cards. Control the algae first. If controlling algae alone is not effective in suppressing shorefly populations, then treatments should be directed at the larval stage. Soil drench treatments include: Distance (pyriproxyfen) and Adept (diflubenzuron). Make sure the chemical is applied to a depth of one inch or more. Note: Biological controls such as predatory mites, parasitic nematodes, and Gnatrol (Bacillus thuringiensis) drenches used for fungus gnat larvae are not effective on shore flies.

Several products that can be used to control algae include:

ZeroTol Broad Spectrum Algaecide/Fungicide (hydrogen dioxide product labeled for use both on greenhouse surfaces and on plants)
Physan 20 Algaecide, Fungicide, Bactericide Virucide, (quaternary ammonium compound for use on greenhouse surfaces and on orchids, roses, and African violets)
Green-Shield Algaecide, Fungicide, Bactericide Virucide (quaternary ammonium compound for use on greenhouse surfaces)
GreenClean Granular Algaecide (Sodium Carbonate Peroxyhydrate or use on greenhouse surfaces)
Terracyte Broad Spectrum Algaecide, Fungicide (sodium carbonate peroxyhydrate for use on greenhouse surfaces and on plants)
Triathlon Algaecide, Fungicide, Bactericide (quaternary ammonium compound for use on greenhouse surfaces).

Reprinted from the article "Algae in the Greenhouse Guarantees Shore Flies" in the February 24, 2006 issue of the Greenhouse TPM/IPM Weekly Report from University of Maryland Cooperative Extension.