Critters Down Under: Shore Flies
Friday, October 2, 2020 | Troy Buechel
“Figure 1. Adult shore fly with visible white spots on wings.
Notice the fly-like body with very short antennae.
Shore flies are the unquestionable ring leader of nuisance insects found in the greenhouse.
Shore flies (Sactella stagnalis) feed mostly on algae as well as decaying organic matter found in the growing medium, but cause no direct damage to greenhouse crops. They are known to transmit and spread disease throughout a greenhouse and if ignored may cause significant disease problems.
Shore flies are often confused with fungus gnats. 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 (Chart 1).
|Shore Flies||Fungus Gnats|
|Food source||Exclusively algae and decaying organic matter||Primarily algae|
|White or translucent color
No visible breathing tubes
5 light spots on dark wings
Resemble fruit flies
No spots on wings
“Chart 1. Comparison Chart of Shore Flies and Fungus Gnats”
The life cycle of shore flies can be as short as 9 days during warm weather with the adults living for 2-3 weeks. A female shore fly can lay as many as 300 oblong white eggs at one time 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 and then the emerging larvae feed on algae. The larval stage lasts 4-6 days. Then they enter a pupae stage that lasts for 3-5 days before emerging as adults from the growing medium or soil (Chart 2).
|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|
“Chart 2. Life cycle of Shore Flies.”
“Figure 2. 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 Horticulture.”
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. Other control methods include cleaning under benches and discarding used growing media and dead plants, sanitizing benches between crops and quarantine any crops that show infestation. Yellow sticky cards are a very effective tool in monitoring shore fly populations and indicate when a chemical control may be necessary. By minimizing algae in the greenhouse, a grower will minimize the population of shore flies.
For more information, contact your Premier Tech Horticulture Grower Services Representative:
Jose Chen Lopez
PRO-MIX® is a registered trademark of PREMIER HORTICULTURE Ltd.
The avid gardener will tell you that the best soil amendment for vegetable gardens and flower beds is compost. Compost is an excellent source of nutrients and organic matter that can help in breaking-up heavy clay soils or improve water retention of sandy soils.