Fungus gnats and shore flies are common pests of propagation nurseries and greenhouse crops.
Adult fungus gnats are small (3-4 mm long) black flies that resemble mosquitoes. They have long legs and long antennae. They are weak fliers and are often seen resting or walking on the surface of the growing media. The larvae are slender white or translucent maggots with a distinct black head capsule. They grow to a length of 6 mm.
Adult shore flies are similar in size to fungus gnats but resemble small houseflies. They have a dark, chunky body with short antennae, short legs and red eyes. They are stronger fliers than fungus gnats. They prefer wetter conditions than fungus gnats and are often found around wet areas on or under benches. Shore fly larvae do not have a head capsule. They are cream or brown coloured and grow to about 6 mm. Both the larvae and pupae of shore flies have a pair of breathing tubes (spiracles) on their hind end.
Fungus gnat larvae live in the growing media and generally feed on decaying organic matter, soil fungi and algae. However, they can also feed on fine roots, root hairs and tender lower stems, causing direct damage to plants. Damage is most extreme in seedlings and young plants, but can also occur in mature plants if numbers are high. Adult fungus gnats can transmit fungal diseases and the wounds produced by the larvae provide entry sites for soil-borne pathogens.
Shore fly larvae are semi-aquatic and feed mainly on algae. They do not normally feed on plants, but they will feed on roots already infected with fungi. Like fungus gnats, adults shore flies can also spread fungal diseases.
Both fungus gnats and shore flies can cause cosmetic damage to ornamental crops because they leave unsightly feacal droppings (flyspecks) on leaves and flowers.
Sticky traps can be used to monitor (and mass trap) populations of adult flies. Both fungus gnats and shore flies are attracted to yellow sticky traps, but there is evidence that shore flies are more attracted to blue traps.
The best management strategy is to prevent these flies from establishing and thriving in the greenhouse. This can be done using a combination of cultural and biological control measures.
Important cultural controls include:
- Eliminating breeding areas such as standing water on or bellow benches, infested growing media and plant debris
- Avoiding over watering and over fertilising plants to reduce the amount of algae growing in containers
- Implementing good hygiene practices
Commercially available biocontrol agents include:
- Predatory mites (Hypoaspis spp.)
- Parasitic nematodes (Steinernema feltiae)
- Predatory beetles (Dalotia coriaria)
- A bacterial insecticide (Bacillus thuringiensis subsp. israelensis) sold under the trade name Vectobac
Naturally occurring biocontrol agents that are not available commercially (e.g. parasitic wasps) can be found in glasshouses where pesticide use has been reduced.