A range of flies can be found on and around growing media in glasshouse and polytunnel ornamental crops as well as plants in interior landscapes.
The two most common flies are the shore fly, mainly just a nuisance although their black frass can reduce plant quality, and the sciarid fly, which is a damaging pest. Sciarid larvae feed on plant roots and have the ability to transmit a range of root rot diseases that affect growth and sometimes lead to plant death.
Sciarid flies, which are also known as fungus gnats, are attracted to organic matter in the growing media, algae growing on spilt peat and decaying plant tissue. They are especially troublesome in pot-plant crops such as poinsettias and in the warm, humid environment of the propagation house where life cycles are quick to complete.
There is a reasonable choice of both chemical and biological controls for sciarid fly, but creating the environmental conditions that discourage flies goes a long way towards keeping the pests at manageable levels.
How to recognise them
Adult sciarid flies (Bradysia paupera) are 3-5mm long, black, with long, tapering antennae. They run or "hop" more than they fly over the compost surface and are most often encountered in heated glasshouses but can be found outdoors.
Larvae are legless and translucent, 6-10mm in length, with dark, shiny heads. They inhabit
mainly the surface layer of the growing medium so are difficult to find, although you may see them underneath leaves that are close to the media surface.
Shore flies (Scatella stagnalis or S. tenuicosta) are slightly longer, broader and have a fluffy appearance. They have stunted antennae and white spots on their wings. The larvae feed solely on algae.
Both types of fly lay their eggs in moist potting media or wet patches of soil under gutters or around the edges of polytunnels. They hatch after several days. Larvae feed for about four weeks before pupating.
Plants attacked by sciarid fly larvae are weakened and stunted. Young roots are eaten away. Larvae can sometimes mine into cuttings. Feeding damage creates entry points for root diseases such as Pythium spp. Adult flies have been shown to carry fungal spores.
Treatment: biological control
Hypoaspis miles or Macrocheles robustulus (predatory mites) feed on sciarid fly larvae in the surface layer of soil or growing media. They can survive for several weeks in the absence of prey so are a good choice for a preventive treatment or where pest populations are at low levels. A minimum temperature of 15°C is needed for mites to be active and for eggs to hatch.
Both will have some activity against shore flies.
The parasitic nematode Steinernema feltiae attacks sciarid larvae, including those that have tunnelled into stems. Another parasitic nematode, Steinernema carpocapsae, which is usually applied to control caterpillars and a range of soil-borne pests, can provide limited control on shore fly larvae. Both are applied as a drench (apply within a crop’s irrigation regime) and require soil and compost temperatures of between 14°C and 30°C.
Adults and larvae of the predatory rove beetle Atheta coriaria eat sciarid fly, shore fly and thrip larvae, and will kill more than they need where pests are at high levels. Introduce as a mixture of beetles at different stages of development — they are effective over a range of temperatures.
Species of the naturally occurring fungus Conidiobolus have also been found to control larvae. Infected larvae turn white, move up to the soil surface and die.
Treatment: cultural control
Large numbers of flies can be caught on sticky yellow traps (including roller traps) that are sticky on both sides.
Avoid over-watering. Sub-irrigation in nurseries and interior landscapes helps to keep the compost surface dry, preventing sciarid flies from breeding. Alternatively, top dressing, mulching or incorporating a percentage of PAS 100 green compost into a mix will have the same effect. Shore flies feed on algae so try and keep capillary matting and paths clean.
Treatment: chemical control
Active ingredient Chlorpyrifos
IRAC code 1B
Formulations Various including Dursban WG, Equity (Dow)
Action(s) Contact and ingested organophosphorus insecticide. Incompatible with biological controls.
Active ingredient Deltamethrin
IRAC code 3
Formulations Bandu (Headland), Decis, (Bayer), Decis Protech (Certis)
Action(s) Contact pyrethroid insecticide with residual activity. Incompatible with biological controls.
Active ingredient Diflubenzuron
IRAC code 15
Formulation Dimilin Flo (Certis)
Action(s) Selective insecticide. Apply to a small area first to check for phytotoxicity. Compatible with biological controls.
Active ingredient Metarhizium anisopliae strain F52
Formulation Met52 granular
Action(s) Bioinsecticide with an EAMU for mulch application around established ornamentals. Compatible with most biological controls.
Fully updated by Dove Associates.
Use plant protection products safely. Always read the label and product information before use.
* EAMU required for use in ornamental plant production outdoors and/or under protection.
Dove Associates shall in no event be liable for the loss or damage to any crops or biological control agents caused by the use of products mentioned.