Pest and disease management

Critters Down Under: Shore Flies in Greenhouses

Troy Buechel

Horticulture Specialist at Premier Tech Growers and Consumers

February 6, 2018

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

Shore flies (Sactella stagnalis) are the unquestionable ringleaders 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 the 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).

Shore fly vs Fungus Gnats — 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

Table 1. Comparison Chart of Shore Flies and Fungus Gnats
	Shore Flies	Fungus Gnats
Food source	Exclusively algae and decaying organic matter	Primarily algae
Larvae	Opaque/tannish color No head 2 breathing tubes on hind end	White or translucent color Dark head No visible breathing tubes
Adults	Short antennae Strong fliers 5 light spots on dark wings Resemble fruit flies	Long antennae Weak fliers No spots on wings Resemble mosquitoes

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 (Table 2).

Table 2. Life cycle of shore flies.
Life cycle stage	Length of stage	Comments
Eggs	Hatch within one day	Laid on surface of wet, algae-rich environment
Larvae	Lasts 4-6 days	Feeding on algae takes place
Pupae	Lasts 3-5 days	Last stage in growing medium before emerging as a flying adult
Adults	Live 2-3 weeks	Only stage that does not occur in growing medium

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 media 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 quarantining any crops that show infestation.

Algae on growing medium — 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

Monitoring and Management Strategies

Reduction of Gunfus Gnats for Begonia crop — *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 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.