B2 – Shore fly

Introduction

Shore flies do not damage herb plants by feeding on them, as they feed largely on algae. However, the presence of the flies on plants at marketing can cause crop rejections by supermarkets and they may also leave unacceptable faecal spots on foliage. The flies can be a nuisance to nursery staff and may spread plant diseases such as Pythium and Thielaviopsis. They are commonly found in wet conditions which encourage algal growth, e.g. in propagation houses and in glasshouses and tunnels used for producing pot herbs, particularly those grown with ebb and flood irrigation or in hydroponic systems.

 

Recognition and host plants

Shore flies in protected UK crops were previously reported as Scatella stagnalis, but recent identification of shore flies in HDC-funded project PC 239 have confirmed shore flies on herbs, celery and ornamentals as S. tenuicosta (Bennison, 2007). The adult is a small, robust black fly, about 4mm long, often seen sitting on plants, compost, bench or floor coverings. The antennae are short and stubby and each wing has five pale spots, giving a ‘domino’ effect (Fig. 1). The females lay eggs onto the surface of damp compost or any other substrate which is supporting fresh, active algal growth. The eggs are small (about 0.4 mm long) and white, with a net-like pattern on the surface (Fig. 2). The three larval stages are transparent, allowing the brown gut contents to be seen through the body wall, thus the larvae are difficult to see on the surface of the compost or substrate (Fig. 3). The pupae are brown and tapered at both ends, and both larvae and pupae have two spiracles (‘breathing tubes’) at the rear end, which have black tips (Fig. 4).

Shore flies breed very quickly in warm temperatures, and can occur in large numbers in wet areas where fresh algae grow. The flies are often more numerous on slower-growing herbs e.g. mint, basil and thyme, because these allow more algal growth on the growing medium exposed to light in the early production stage.

Symptoms

In addition to the presence of adult flies, faecal spotting can be seen on the leaves (Fig. 5) and this can also cause marketing problems.

Sources of infestation and favourable conditions

Shore flies breed in wet areas where algae grow. The adults are active flyers and migrate onto young herb plants with fresh algal growth on the compost. Shore fly population increase is favoured by warm temperatures. Very few shore flies complete their development below 10°C (Jacobson, 1997). Research in Finland determined the biology of shore fly at various temperatures. The minimum temperature for development from egg to adult is 6.4°C. The development time from egg to adult decreases with increasing temperature, e.g. this takes a mean of 15.9 days at constant 20°C, 11.4 days at constant 25°C and 10.1 days at fluctuating summer glasshouse temperatures of 23-34°C (mean 28.5°C). The population can double in 5.3 days at constant 25°C (Vanninen, 2005).

Integrated Pest management (protected herbs)

Cultural control

• Shore flies feed and breed on algae, so minimising algal growth is key to reducing fly populations. Avoid over-watering and keep benches, gutters, floor coverings and irrigation equipment as clean as possible (Fig. 6).
• Horticultural fleece may be used over the herb plants in the early stages of production, to reduce shore fly egg-laying on the growing substrate. However, the fleece must be removed as the plants grow.
• Yellow sticky traps will catch adult flies (Fig. 7), and horizontal traps are more effective for shore flies than vertical ones. These can be placed on or above benches or floors. Long ‘curtain’ sticky traps should be used with care, as they will also catch large numbers of flying beneficial insects e.g. parasitic wasps.
• A large suction trap positioned above potted herbs on moving benches on one UK nursery, successfully removes shore flies from the crop just before packing (Fig. 8).

Biological control

The only biological control agent currently marketed for the control of shore flies is the predatory beetle, Atheta coriaria. Other biological control agents have been tested against shore flies or have potential for commercial use:

1. Atheta coriara
The ground-dwelling predatory ‘rove’ beetle, Atheta coriaria is commercially available for the control of both sciarid 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. 9). 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. 10). 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 prey, including sciarid and shore fly eggs and larvae.

Research by ADAS showed that direct releases of Atheta as either adults or as mixed life-stages did not control shore flies on potted mint at a commercial nursery (Bennison & Green, 2007). However, subsequent research in HDC-funded project PC 239 and PC 239a developed rearing-release units for Atheta, whereby the predators are reared and maintained in large numbers in the glasshouse at very little cost (Bennison 2010). Using the rearing release units on one commercial herb nursery (Fig. 11) reduced numbers of sciarid flies on parsley, but control of shore flies could not be properly tested as they were controlled by a natural shore fly parasitioid, Aphaereta debilitata (Bennison, 2007 and see below). However, growers have subsequently reported that use of the predator and the rearing-release system on both protected herbs and ornamentals have given encouraging results against shore flies on commercial nurseries.

2. Aphaereta debilitata
Two shore fly parasitoids, Aphaereta debilitata and Kleidotoma psiloides occur naturally on nurseries using IPM. A. debilitata has been found in large numbers on several UK herb nurseries, and parasitised up to 99% of shore fly pupae on mint in an experiment in HDC-funded project PC 239 (Bennison, 2007). The adult parasitic wasps are small and dark, about 1.5 mm long, with long beaded antennae (Fig. 12). The females have paddle-shaped wings, but those of the males are very reduced and the latter are unable to fly. The adults can be found on the surface of compost or on benches or floor coverings where shore flies are present. The females lay each of their eggs inside a shore fly larva, and the parasitoid develops inside the body of the larva and pupa. When fully developed, the Aphaereta adult emerges from the parasitised shore fly pupa. The biology of Aphaereta has been studied in Finland, where it has also occurred naturally and contributed to control of shore flies on mint (Vanninen 2005). Research on both Aphaereta and Kleidotoma hasalso been done at York University. The parasitoids are not yet commercially available.

3. Steinernema feltiae
Insect-pathogenic nematodes, including Steinernema feltiae,were first developed as compost drenches for control of ground-dwelling pests, and they are commonly used on protected herbs against sciarid flies. Research in this project tested repeated drenches of S. feltiae at two application rates against shore fly on potted mint on a commercial nursery. The nematodes did not reduce the numbers of shore flies at the marketing stage when compared with untreated controls (Bennison & Green, 2007). This result agreed with those of Finnish researchers, who reported that a much higher application rate than that used for sciarid fly control was needed for effective control of shore fly on mint. The effective rate for shore fly control is considered too expensive for commercial use.

4. Hypoaspis miles and H. aculeifer
The ground-dwelling predatory mites Hypoaspis (Stratiolaelaps) miles and Hypoaspis (Gaeolaelaps) aculeifer are used for the control of sciarid flies, and are generally regarded by growers as giving some reduction of shore flies. However, research in this project demonstrated that H. aculeifer at the rate recommended for sciarid fly controldid not reduce numbers of shore flies on potted mint at a commercial nursery when compared with untreated controls (Bennison & Green, 2007). This result agreed with those of Finnish researchers, who reported that a much higher application rate of H. aculeifer than that used for sciarid fly control was needed for effective shore fly control.

5. 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 2.5-4 mm long and resemble small house flies http://web.agrsci.dk/plb/iobc/sting/sting29.pdf). The adult catches its prey, e.g. adult shore flies, in flight, and leaves the prey bodies behind when it has sucked out the body fluids. 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.

6. Insect-pathogenic fungi
The insect-pathogenic fungus Beauveria bassiana has been shown to infect shore fly larvae (Jacobson 1997). Beauveria bassiana (Naturalis-L)is now approved as a biopesticide in the UK, but only as a foliar spray for control of whiteflies and thrips (see B5 and B6). However, naturally-infected shore fly adults and pupae have been found in herb crops in the UK. Shore flies naturally infected with another rare insect-pathogenic fungus, Torrubiella sp., have also been found on one herb crop in the UK (Fig. 13). Both pathogens were investigated in a Horticulture LINK project, HL 0193. (Chandler, 2009, 2010 & 2011).

 

Monitoring within IPM

• Numbers of shore fly adults can be monitored using yellow sticky traps (See Section A – Principles of IPM). Horizontal traps placed on benches or floors are more effective in trapping shore flies than vertical traps.
• 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).

 

Chemical control (protected herbs)

There are no pesticides with approval or Extension of Authorisation for Minor Use (EAMU) for the control of shore flies on protected herbs.

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.

 

Further information

Bennison, J. & Green, K. (2007). Protected herbs: Best Practice Guidelines for integrated pest and disease management. Final report for Defra project HH3118TPC and HDC project PC 210. (available from the HDC).

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 rearingthe predory 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 within waste-based growing media. Annual and final reports to HDC for HortLINK project HL0193.

Jacobson, R. (1997). Protected lettuce: control of sciarid and shore flies. Final report for HDC project PC 101.

Vanninen, I. (2005). Parasitoids as biocontrol options of shore flies (Dipt., Ephydridae). Abstracts of the International workshop on implementation of biocontrol in practice in temperate regions – present and near future. Research Centre Flakkebjerg, Denmark, November 1-3, 2005. http://www.center-biologisk-bekaempelse.dk/abstract-bog.doc