B3 – Sciarid flies

Introduction

Sciarid flies, also known as fungus gnats, can be serious pests of herbs. The larvae damage the roots of young plants, leading to poor vigour, wilting and even death. Adult sciarid flies can also carry important fungal pathogens e.g. Pythium. Sciarids are often present as secondary pests on plants affected by root disease, the larvae feeding on the rotting plant tissue and fungi.The presence of the flies on plants at marketing can cause crop rejections by supermarkets.

 

Recognition and host plants

Sciarid flies damaging herbs and other glasshouse crops are usually Bradysia difformis (formerly known as Bradysia paupera), but other species of Bradysia and some Lycoriella spp. may also cause damage. The adult is a small, black, gnat-like fly, about 2-3 mm long, with iridescent wings (Fig. 1), often seen resting on the compost or pots, or flying weakly around the plants. They are also commonly found on yellow sticky traps used for pest monitoring (Fig. 2). Sciarids can be distinguished from shore flies and other black flies by their long legs, long, beaded antennae and a distinctive Y-shaped vein on each wing. The adult females are attracted to damp compost or substrate in which to lay their eggs, particularly highly organic composts and those containing ‘green’ waste. The flies seem to be particularly attracted to pots of freshly germinated parsley, which are favourite host plants. However, a wide range of young herb plants can be attacked. The larvae are transparent legless maggots, up to 5 mm long, with shiny, spherical black head capsules (Fig. 3). They can be found around the roots of host plants and sometimes on the surface of the compost. The green or brown gut contents of the larvae can be seen through the transparent body wall. The pupae develop in the compost, then emerge into the next generation of adults.

Symptoms

Young seedlings and cuttings are very susceptible to damage by the larvae, which weaken growth and can cause plant death. Sciarid larvae can tunnel up the stems of young cuttings. Plants with well-established roots are less susceptible to damage, but heavy infestations can cause plant wilting and death. Larvae can also feed on the lower leaves resting on the compost (Fig. 4), and this damage can be confused with that made by slugs.

 

Sources of infestation and favourable conditions

Sciarid flies can occur all year round on protected herbs, especially in warm, moist conditions e.g. in propagation houses. However, they are more of a problem in spring and summer, when warm temperatures allow faster increase in numbers, especially when more frequent watering is needed, which favours development of the pest. The development time from egg to adult is around three weeks at 21°C, but faster at higher temperatures e.g. only 17 days at 25°C. The adults, although weak fliers, can fly from infested areas to newly emerged seedlings or young cuttings.

 

Integrated Pest Management (protected herbs)

Cultural control

• Strict nursery hygiene will reduce breeding areas for sciarid flies, e.g. prompt removal of badly infested or diseased plants, keeping bench and floor coverings clean, and keeping stocks of compost covered.
• Avoid over-watering and maintain adequate ventilation.
• Yellow sticky traps used for monitoring will catch adult flies, but insufficient numbers will be caught to reduce sciarid fly populations. Long ‘curtain’ sticky traps will catch more adults, but should be used with care, as they will also catch large numbers of flying beneficial insects e.g. parasitic wasps.

 

Biological control

Commercial biological control agents
Several biological control agents are commercially available for the control of sciarid flies. Times and rates of release within an Integrated Pest Management (IPM) programme should be planned carefully. If necessary, seek advice from the supplier or a consultant.

1. Steinernema feltiae
This insect-pathogenic nematode is commonly used on protected herbs against sciarid flies. The nematode suspension is applied as a compost drench. The nematodes swim to find a sciarid fly larva and enter the body, where they multiply and release symbiotic bacteria that kill it. The dead sciarid larva rapidly decomposes, releasing the nematodes, which can then infect other larvae (Fig. 5). The nematodes need to be applied carefully, following all supplier recommendations, e.g. the suspension of nematodes should be kept agitated during application, and the compost kept moist so that the nematodes can survive and move within the compost. The nematodes are effective in compost between 10-30°C, depending on product.

2. Hypoaspis miles and H. aculeifer
The ground-dwelling predatory mites Hypoaspis (Stratiolaelaps) miles and Hypoaspis (Gaeolaelaps) aculeifer will feed on sciarid fly eggs and larvae. They will also feed on other ground-dwelling invertebrates e.g. springtails and thrips pupae. The adult mites are up to 0.8 mm long, off-white with a pale brown shield covering most of the upper surface of the body (Fig. 6). Both H. miles and H. aculeifer need moist compost, soil or substrate and temperatures above 15°C. The optimum temperature for H. miles is 25°C and that for H. aculeifer is 22°C.Both species are very active and can be seen running on the substrate surface or under pots and trays.

3. Macrocheles robustulus
This predatory mite is similar to Hypoaspis species but larger. It can occur naturally in glasshouses and became available in 2010. Like Hypoaspis spp., Macrocheles prefers moist soil compost with a loose structure and temperatures of 15°C or above. The mites feed on sciarid eggs, larvae and pupae.

4. Atheta coriara
The ground-dwelling predatory ‘rove’ beetle, Atheta coriaria is commercially available for the control of both sciarid flies and shore flies. The adult beetle is small, about 3-4mm long, dark brown and shiny, with short wings and an upturned rear end (Fig. 7). The adults are very active and can both run and fly, but often remain hidden within the growing substrate and under pots. The female beetles lay eggs in the compost or soil and the eggs hatch into the first stage larvae, which are small and milky white. There are three larval stages, the oldest larvae being about 3-4 mm long and brownish yellow (Fig. 8). When fully developed, the oldest larvae build pupation chambers in the growing medium out of soil particles held together with silk, thus they are not readily visible at this stage. Both adults and larvae feed on a range of ground-dwelling prey, including sciarid and shore fly eggs and larvae, and thrips pupae.

Research in HDC-funded project PC 239 and PC 239a developed a rearing-release system for Atheta, whereby the predators are reared and maintained in large numbers in the glasshouse at very little cost to growers (Bennison, 2010). Using the rearing release units on a commercial herb nursery (Fig. 9) reduced numbers of sciarid flies on parsley(Bennison, 2007).

Naturally-occurring biological control agents
In addition to commercially-available biological control agents, several natural enemies of sciarid flies often occur on herb nurseries, particularly where IPM is used. They can sometimes occur in sufficient numbers to play an important role in controlling the pest:

1. Synacra sp.
A parasitic wasp, Synacra sp. can occur naturally on nurseries using IPM. The adults can be found on yellow sticky traps used for pest monitoring, or on the surface of the compost. The adult is brown, about 2-3 mm long with a very narrow ‘waist’, long beaded antennae and paddle-shaped wings. The females lay their eggs through crevices in the compost or soil, into sciarid fly larvae. The immature Synacra develops inside the body of the sciarid larva and later emerges as an adult from the sciarid pupa.

2. Coenosia attenuata
The ‘hunter fly’, Coenosia attenuata, is a naturally-occurring predator of various pests, including sciarid flies, shore flies, leaf miners and whiteflies. The greyish adult flies are 3-4 mm long (larger and stouter than sciarid flies) and resemble small house flies (http://web.agrsci.dk/plb/iobc/sting/sting29.pdf). The adult catches its prey, e.g. adult sciarid and shore flies, in flight, sucks out the body fluids and discards the emptied prey bodies. The presence of Coenosia is likely if scattered, dry ‘husks’ of prey insects are readily found, e,g, on the plants. Coenosia larvae are predatory on ground-dwelling prey, e.g. sciarid and shore fly larvae. Research is being done in the USA on the potential of Coenosia as a commercial biological control agent.

3. Insect pathogenic fungi
A naturally-occurring fungus, Furia sciarae, that attacks sciarid larvae, has been found in recent years on several nurseries growing herbs, protected ornamental bedding and pot plants where the growers are using IPM. The infected larvae rise to the compost surface to die and they have an opaque, milky white appearance (Fig. 10). This fungus has been investigated in a Horticulture LINK project, HL 0193 (Chandler, 2009, 2010 & 2011).

Another fungus, Metarhizium anisopliae (Met52) is commercially available and has an Extension of Approval for Minor Use (EAMU) for control of sciarid fly larvae when used as a mulch or a pre-planting soil treatment in protected and outdoor herbs.

Monitoring within IPM

• Numbers of sciarid fly adults can be monitored using yellow sticky traps (see Section A – Principles of IPM).
• Sciarid larvae can be monitored by examining the compost surface, around the roots and inside the pot after tapping out the plant.
• Sciarid larvae killed by nematodes quickly decompose, and cannot be seen in the compost. Efficacy of nematodes is best assessed by monitoring sciarid fly numbers.
• Hypoaspis spp. can be seen running over the substrate or under pots and trays, but efficacy is best assessed by monitoring sciarid fly numbers.
• Atheta adults and larvae can sometimes be seen under pots or in/on compost. A ‘bait pot’ technique using an artificial food source for Atheta has been developed in HDC project PC 239 (Bennison,2007 & 2010).
• Sciarid fly larvae infected by the naturally-occurring fungus, Furia sciarae can be seen on the compost surface. It is difficult to find sciarid fly larvae infected by Met52.

 

Chemical control (protected herbs)

There are no pesticides with approval or Extension of Authorisation for Minor Use (EAMU) for the control of sciarid flies on protected herbs. Various pesticides such as pyrethrins and the pyrethroid insecticides cypermethrin and deltamethrin would kill adult flies, but control measures for scarid fly populations are best targeted against larvae. Pyrethroid insecticides are harmful to biological control agents for up to three months after application, thus they are incompatible with IPM.

• Full details for the use of biological control agents and compatibility of pesticides are available from biological control suppliers or consultants. Also see Section A – Principles of IPM.
• Pesticide approval information in this guideline is current at 29 April 2013.
• Regular changes occur in the approval status of pesticides arising from changes in pesticide legislation or from other reasons. For the most up to date information, please check with a professional supplier or the CRD website http://www.pesticides.gov.uk/.  General enquiries on pesticides and detergents are now being handled by the Defra Helpline (as of April 2013): Tel: 08459 33 55 77.
• Always follow label recommendations or statutory conditions for use on Extension of Authorisation for Minor Use (EAMU) notices of approval.
• Always follow instructions for Pesticide Resistance Management guidelines given on the label or EAMU.
• Growers must hold a paper or electronic copy of the current EAMU before using any product under the EAMU arrangements. Any use of a pesticide with a EAMU is at grower’s own risk. Relevant EAMUs are sent to HDC members by HDC, or are available from CRD (see above) or from consultants.
• Use pesticides safely.

 

Further information

HDC Factsheet 08/02. Control of sciarid flies in protected ornamentals.

Bennison, J. (2007 and 2008). Protected herbs, ornamentals and celery: development of an on-nursery rearing system for Atheta coriaria for reduced cost biological control of sciarid and shore flies. Annual reports for HDC project PC 239.

Bennison, Jude (2010). Grower system for rearing the predatory beetle Atheta coriaria. HDC Factsheet 06/10.

Chandler, D (2009, 2010 & 2011). New approaches to microbial control of insect pests in protected crops and their interactions with waste – based growing media. Annual and final reports to HDC for HortLink project HL0193.