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Critters Down Under: Shore Flies in Greenhouses

Thursday, September 15, 2022 | Troy Buechel

Figure 1. Adult shore fly with visible white spots on wings.

Shore flies (Sactella stagnalis) are the unquestionable ringleader of nuisance insects found in greenhouses. While they can cause the spread of diseases and fungal spores in the greenhouse, they don’t inflict damage directly to the crop. In this article, we will look at how to differentiate shore flies from fungus gnats, what kind of damage they cause, and steps to take for prevention and management of infestation.

What are the Differences Between Shore Flies and Fungus Gnats?

Shore flies are often confused with fungus gnats, but there are some noticeable differences. Shore flies closely resemble fruit flies with both short antennae and legs, and have five white spots on their dark colored wings (Figure 1). Shore flies are strong fliers and can quickly spread throughout a crop.

Fungus gnats, on the other hand, have long legs, long antennas and look more like mosquitos. They do not have white spots on their dark wings and are generally weak fliers (Figure 2).

Figure 2. Left: adult fungus gnat, similar to a mosquito. Right: adult shore fly with visible white spots on wings. Notice the fly-like body with very short antennae. Source: https://bugguide.net/node/view/482039

Chart 1. Comparison Chart of Shore Flies and Fungus Gnats

Damage

Shore flies (Sactella stagnalis) feed mostly on algae and decaying organic matter found in the growing medium, but cause no direct damage to greenhouse crops. However, they are known to transmit and spread disease and fungal spores throughout greenhouses, and if ignored may cause significant problems.

Life Cycle of Shore Flies

In warm weather, it takes as short as 9 days from when the egg hatches to the time the newly emerged female is sexually mature and able to produce her own eggs. A female shore fly can lay as many as 300 oblong white eggs at once on the growing medium surface, soil surface under a bench, capillary mats or other areas that are wet and have algal growth.

The eggs hatch in a day, after which the emerging larvae feed on algae. The larval stage lasts 4-6 days. They then enter a pupae stage that lasts for 3-5 days before emerging as adults from the growing medium or soil, which live for 2-3 weeks (Chart 2).

Chart 2. Life cycle of Shore Flies.

Figure 3. Algae growth on the media surface serves as an ideal environment for shore flies to lay eggs, leading to infestations. Drying the growing medium surface will greatly reduce algae and shore fly populations. Source: Premier Tech

Shore Flies Control

Water and Soil Management in Greenhouse

As stated above, shore flies feed exclusively on algae and some decaying organic matter. Therefore, they are most efficiently controlled by minimizing algae growth by avoiding standing water and allowing growing media surfaces to dry between waterings.

Growing medias such as PRO-MIX BX BIOFUNGICIDE + MYCORRHIZAE help control certain insects by suppressing their food sources and plant susceptibility. 

Other control methods include cleaning under benches, discarding used growing media and dead plants, sanitizing benches between crops and quarantine any crops that show infestation.

Monitoring and Management Strategies

Reduction of Fungus Gnats of Begonia crop. Note the reduced population of fungus gnats on PRO-MIX BX BIOFUNGICIDE + MYCORRHIZAE yellow card. 

While they do not offer a thorough solution to infestation, yellow sticky cards are a very effective tool in monitoring shore fly populations and indicate when a chemical control may be necessary.

In Conclusion

The best way to prevent shore fly infestations is to avoid overwatering the growing media by letting it dry completely between waterings. If using PRO-MIX BX BIOFUNGICIDE + MYCORRHIZAE, let it become a light brown or tan color before the next irrigation. All PRO-MIX products undergo rigorous testing during and after manufacturing as part of our Quality Assurance program.

Following the steps of Integrated Pest Management (identify, evaluate, prevent, action and monitor) is always an effective way of preventing and controlling insect infestation.

For more information, contact your Premier Tech Grower Services Representative:

Ed Bloodnick Horticulture Director US-South East	JoAnn Peery Horticulture Specialist US-Central, Canada-Central	Lance Lawson Horticulture Specialist US-West, Canada-West
Troy Buechel Horticulture Specialist US-North East	Susan Parent Horticulture Specialist Canada-East, US-New England	Jose Chen Lopez Horticulture Specialist Mexico, Latin & South America