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Fungus gnats - small delicate black flies about 3 mm long with long antennae and legs. The larvae are legless, about 4-5 mm long when fully grown, and have clear segmented bodies and black heads. Fungus gnats are also known as sciarid flies. Adult wings do not completely overlap at rest.
Shore flies - appear similar to a small housefly and have short antennae and dark wings with characteristic pale spots. Adult wings overlap almost completely when at rest. The larvae are wedge-shaped and do not have a distinct head capsule, but have two spiracles at the tail-end of their body.
Fungus gnats and shore flies are more prevalent in greenhouses, but may also become numerous outdoors. They are usually found just below the soil or growing media surface in association with decaying plant material, moss and algae.
Fungus gnat larvae can cause damage to the fine roots of seedlings and cuttings in nurseries and greenhouse crops. The host range of these insects has not been determined, but is likely to include many plants species.
Shore fly larvae are thought to be algae eaters and are not considered to cause damage to plants.
Larvae of fungus gnats graze on the fine roots and stem tissue below soil level.
Signs and symptoms of fungus gnats include:
Presence of large numbers of small mosquito-like insects
Plants lack vigour and leaves may turn yellow
Small brown scars are evident on roots, and root hairs are eaten off
With heavy larval infestations, plants can be weakened severely and die
Associated root diseases like Pythium
Signs and symptoms of shore flies include:
Presence of large numbers of small housefly-like insects
Brown spots of excrement on leaves
Shore flies feed on algae and do not damage plants, but may vector disease
Ideal conditions for fungus gnat and shore fly outbreaks are high humidity, high organic matter levels in soil or growing media, water-saturated soil or growing media, presence of moss and algae and decaying plant material.
The life cycles of fungus gnats and shore flies are similar. Several hundred eggs may be laid by females on the surface or in cracks and crevices of moist soil or growing media rich in organic matter.
Eggs hatch into tiny larvae (maggots) that pass through several instars and feed for approximately 2 weeks. When larvae have completed their development, a pupa is formed. It takes about 5-6 days for an adult to emerge, and adults live for about 10 days.
The life cycle of fungus gnats takes approximately 3-4 weeks depending on temperature. Many generations are possible in a year.
Both fungus gnat and shore fly adults can be a significant nuisance to greenhouse and nursery workers when high numbers are present.
The excrement of shore flies on foliage of may be undesirable on some ornamental plants, such as cut flowers.
Both fungus gnat and shore fly adults have been implicated in the passive transmission of fungal spores from one plant to another, and thus may assist the spread of some plant diseases.
Damage caused by fungus gnat larvae feeding on roots can cause direct loss of seedlings, and is also likely to promote the development of soil-borne fungal diseases and greater loss of seedlings, cuttings and bulbs.
Early action in preventing the establishment and build up of fungus gnats is important if damage is to be avoided. Once fungus gnats have started to damage plants, it is usually too late to effectively protect them. A combination of control techniques is required for successful management.
Like many other greenhouse pest insects, fungus gnats and shore flies are readily captured on yellow sticky traps. However, there are no guidelines for action thresholds. Yellow sticky traps only provide an indication of the presence and possible build up of these pests.
Monitoring traps should be hung close to the soil or pot surface and inspected weekly. Plants and growing media should also be inspected to confirm the presence of pests.
A number of practices are recommended to prevent or minimise the establishment of fungus gnats and shore flies on greenhouse crops. These include:
Avoid overwatering growing media, and water in such a way that allows the surface of the media to become dry for at least part of the day.
Use insect screens to minimise insect entry into greenhouses
Protect vulnerable seedling plants with insect proof netting
Clean up and remove any moss or algal growth in and around greenhouses
Dispose of old growing media as soon as possible, preferably off the property
Clean thoroughly growing units before establishing new crops
Avoid use of organic fertilisers, and add fertilisers to media on the day it will be used
A variety of natural enemies (predators, parasites and diseases) has been researched to assist with the management of fungus gnats and shore flies.
The predatory mite, Hypoaspis aculeifer is the most commonly used biological control agent for this pest.
Hypoaspis aculeifer is available from BioForce Ltd, who sells the product Hyper-Mite™ .
Hypoaspis aculeifer is most effective when it is used in an Integrated Pest Management programme, when the use of harmful pesticides is avoided and effective crop management practices are adopted.
A range of insecticides are claimed to be effective against fungus gnats and shore flies, although few have specific registration claims for use on greenhouse crops.
Local research has indicated diflubenzuron and Bacillus thruingensis var. israelensis applied as a drench to potted plants can reduce fungus gnat and shore fly larval numbers. Regular applications are recommended.
Pre-treatment of growing media with fipronil or diflubenzuron is also effective in suppressing fungus gnat larvae, and gave better protection of plants than curative treatments applied after plants were established in infested growing media.
Overseas research suggests adult fungus gnats and shore flies are susceptible to a wide range of contact insecticides. Regular applications would be needed to suppress adult populations.
Care should be taken to consider the effect of insecticides on beneficial insects or mites that may be being used in integrated pest management programmes. Consult with BioForce Ltd before applying any insecticide when beneficial insects or mites are being used.
If you are uncertain about the identity of any pest in your crop, or need advice on the management of pests contact BioForce Ltd, email@example.com.