Pest description and crop damage Larvae are small white, legless maggots with black head capsules. Adult midges are delicate, black-bodied insects with long legs and antennae. Their wings have a y-shaped vein along its edge distinguishing them from shore flies which are stout-bodied with short antennae and five white spots on their dark wings. The maggots can feed on decaying organic matter as well as plant roots. Larval feeding causes damage to seedling roots and high populations can kill young plants. Both larvae and adult can move spores of plant pathogens.
Biology and life history Eggs are laid in the container substrate or greenhouse floor. The tiny eggs hatch in 4 to 6 days, larvae feed for another 2 to 14 days, then pupate in the substrate or ground for another 5 to 6 days before emerging as an adult. Adults can live a week to 10 days. There are multiple generations per year. Higher populations are associated with wet, low drainage areas of production. Increasing drainage and maintaining a moss and algae free production area will reduce conducive conditions.
Scouting and thresholds Scouting for fungus gnats is usually done with yellow sticky cards. This will give one an indication of the numbers of adult fungus gnats. Some people place potato slices on the soil or medium to monitor for the larvae. Check roots of puny-looking plants for signs of the larvae.
Management-biological control (larvae)
Natural enemies include:
- parasitic nematode (Steinernema feltiae)-Commercially available. Applied as drench to substrate. Generally will need to be applied more than once.
- predatory mite (Stratiolaelaps)-Commercially available.
- rove beetle (Atheta coriaria)-Commercially available.
- soil-dwelling mite (Hypoaspis miles)-Applied to substrate.
Use yellow sticky traps to assess infestation and control efforts. Often, fungus gnats are a serious pest in propagation facilities, where larvae damage young roots. Infestations are associated with highly organic potting mix. Much of the management is directed at controlling the larval stages in the media and floor. Adult control is generally less effective in controlling populations.
- acephate (Orthene TR) at 0.024 lb ai/1,000 sq ft broadcast (0.6 oz). REI 24 hr.
- acetamiprid (Tristar 30SG) at 0.5 lb ai/100 gal (2.7 oz). REI 12 hr.
- azadiractin (Azahar, Azatin XL)-Insect growth regulator.
- Bacillus thuringiensis israelensis (Bti) (Gnatrol) as drench to substrate to kill larvae. Use a Bt formulation specifically designed for fungus gnats. Generally will need to be applied more than once. Some formulations are OMRI-listed for organic use.
- bifenthrin (Talstar) at 0.168 to 0.335 lb ai/1,000 sq ft. REI 12 hr . REI 12 hr. Restricted use pesticide.
- chlorfenapyr (Pylon) at 0.08 to 0.16 lb ai/100 gal. REI 12 hr.
- cyfluthrin (Decathlon) at 1.3 oz/100 gal. REI 12 hr.
- cyfluthrin/imidacloprid (Discus N/G) at 0.063 lb ai/100 gal foliar; 0.009 to 0.014 lb ai/1,000 ft soil. REI 12 hr.
- cyromazine (Citation) at 2 oz ai/100 gal. REI 12 hr. Apply to larval stage.
- diflubenzuron (Adept) at 0.5 oz ai/100 gal soil. REI 12 hr. Apply to larval stage.
- dinotefuan (Safari 20SG) at 0.1 to 0.2 lb ai/1,000 ft sq; 0.05 to 0.1 lb ai/100 gal. REI 12 hr.
- imidacloprid (Marathon II) at 0.027 oz ai /100 gal foliar. REI 12 hr. Drench and irrigation permitted.
- mineral oil/petroleum distillate (Biocover LS)-Some formulations are OMRI-listed for organic use.
- peppermint/oil of rosemary (Ecotec for WA only)-Some formulations are OMRI-listed for organic use.
- pyrethrins/canola oil (E.B. Stone, Earth-tone, Worryfree)-Adult control.
- pyrethrum/piperonyl butoxide-Adult control.
- pyriproxyfen (Distance) at 0.054 to 0.081 lb ai/100 gal (8 to 12 fl oz) foliar. REI 12 hr.
- s-kinoprene (Enstar AQ) at 0.012 to 0.024 lb ai/10,000 sq ft. REI 4 hr.
- soybean oil (Golden Pest Spray)-Some formulations are OMRI-listed for organic use.