Fungus gnats, once considered to be a minor pest, are of increasing concern to greenhouse growers. Recent studies have shown that fungus gnats play a role in the transmission of certain fungus diseases including Pythium, Verticillium, Thielaviopsis, and Botrytis. Fungus gnat larvae damage roots as they feed and may tunnel into stems and cause plant death. The presence of adult shore flies and fungus gnats may be objectionable to customers.
Both fungus gnats and shore flies thrive in the damp, moist environment that is common in greenhouses. During cloudy, overcast weather in the spring and fall, growers may see both fungus gnats and shore flies. Correct identification is important because management strategies vary depending on what insect is present.
Dark-winged fungus gnat adults (Bradysia spp. and Sciara sp.) are less than 1/10 of an inch long. They are slender, mosquito-like insects with long legs and many-segmented antennae. Their delicate gray wings have a "Y"-shaped vein near the wing tip.
Shore fly adults (Scatella spp.) have stockier bodies, with shorter legs and antennae than fungus gnats. Each wing has distinctive faint light spots which aid in its identification.
Fungus gnat larvae have slender white bodies, which are less than 1/4 of an inch long, with distinctive black head capsules. Shore fly larvae are yellowish-brown in color, lack a head capsule and have forked spiracles, or breathing tubes, at their rear end.
Fungus Gnat and Shorefly pictures
Fungus gnats will feed on fungi and decaying organic material in the soil. If a fungal food source is not readily available, fungus gnats may feed directly on plants. During the propagation of cuttings, fungus gnats may feed upon the callus. This may slow down or completely inhibit rooting. They can also damage the roots as they feed upon seedlings and potted plants. On many thick and succulent stemmed plants, larvae may tunnel into stems and cause plants to wilt and die. Larval feeding upon young roots encourages the development of root and stem rots such as Pythium. When this occurs, plants will be stunted with reduced growth and leaf drop.
Shore flies feed upon algae and do not directly feed upon plants. However, their frass (droppings) on plants, in addition to their presence, may be objectionable to customers.
Transmission of Root Rot Pathogens
Researchers have recently discovered that Pythium can provide a complete nutritional food source for the fungus gnats to develop from egg to adult. In this study, fungus gnat larvae ingested Pythium and readily introduced the fungus to young cucumber plants as they fed upon the roots. Pythium oospores were transmitted at very low levels from the larval to adult fungus gnats. However, adults may help spread the disease by carrying fungal spores on their bodies as they move from plant to plant.
Shore fly larvae, which feed upon algae, also may help spread root rot pathogens. Researchers have found Pythium oospores in the shore fly larvae's digestive system and frass.
Life Cycle of Fungus Gnats
The life cycle from egg to adult is approximately one month, depending upon temperature. Overlapping and continuous generations in the greenhouse make effective management difficult.
|Developmental Stage||Number of Days|
|Egg-laying period||7 days|
|Egg to adult||30 days|
|Total number of eggs laid||100-150|
High moisture levels encourage the development of both fungus gnats and shore flies. The selection of a potting soil that drains well, and the use of proper watering techniques to avoid puddling will help to prevent fungus gnat problems. Keeping the greenhouse floor as dry as possible and free of debris, spilled potting soil and weeds will help to discourage fungus gnats.
All life stages of shore flies can be found on or near algae. Proper sanitation and environmental modification are crucial to managing algae growth. Reduce the moisture and puddling water on floors, benches and greenhouse surfaces. The greenhouse floor should be level and drain properly to prevent the pooling of water. Avoid over watering crops, especially early in the production cycle, to allow the upper media surface to dry out between irrigations. Several disinfectants can be used as part of a precrop cleanup program and during the cropping cycle for routine control of algae on greenhouse surfaces and walkways.
Early detection of fungus gnats is important, as populations may increase rapidly early in the cropping cycle. By detecting fungus gnats early, growers may successfully use several newly available biorational materials. These materials are generally less toxic and have shorter re-entry periods compared to the more traditional pesticides.
Yellow sticky cards may be used to monitor adult populations. Sticky cards are more effective in detecting adult fungus gnats when they are placed just above the growing media. Horizontal card placement is more effective than vertical card placement.
Fungus gnat larvae are usually found in the top inch or so of growing media. However, larvae may sometimes be found near the bottom of the container. Potato discs, one inch in diameter and ½ inch thick, may be placed on the soil surface to monitor for larvae. Place the discs with the cut surface face down on the growing media. (Potato discs tend to be more effective when the cut surface stays moist and does not dry out.) Mark the pots with flagging tape to facilitate the location of the cut potatoes for frequent inspection. Check both the growing media and underside of the potato discs for larvae. Replace during weekly scouting. Use potato discs during cool, moist weather conditions to monitor fungus gnat larvae. During periods of warm weather at the end of the bedding plant season, potato discs may "melt out."
Treatment is most effective against the young larvae early in the cropping cycle. Bacillus thuringiensis subsp. israelensis is an insect pathogen sold under the trade name of Gnatrol for greenhouse use. This particular type of B. t. is active against flies (Diptera). B. t. consists of viable endospores and crystals that must be ingested by the larvae. After the larva feeds, its gut is paralyzed. Then the larva stops feeding and dies.
Gnatrol should be applied as a soil drench to thoroughly wet the soil of containers, raised beds or under benches where the larvae are found. Gnatrol is only effective for 48 hours so repeat treatments may be needed. One to three applications may be necessary depending on population levels. Gnatrol should not be combined with fertilizers or fungicides containing copper or chlorine.
Parasitic nematodes enter the insect's body through openings in the exoskeleton. The nematodes multiply inside the host insect and release a bacterium that is toxic to the host. The nematodes complete their life cycle within a few days. Large numbers of infective stage nematodes are produced that will search for new hosts.
These beneficial nematodes can be applied as a drench to the growing media and to soil under the benches. Adequate soil moisture is needed in order for the nematodes to move within the soil surface. Remove screens and filters from the sprayer before treatment. Two of the more common species available for greenhouse use include: Steinernema carpocapsae (ScanMask) or Steinernema feltiae (Nemasys).
Hypoaspsis miles is a small predatory mite that feeds on fungus gnat larvae and is sold in one-liter containers in a sawdust mixture. These mites may be sprinkled over the soil surface or mixed into the media before planting. H. miles is a scavenger that will survive in the absence of fungus gnats and will feed on the young developing larvae. H. miles will feed upon thrips pupae, as well. These soil-dwelling, predatory mites will reproduce in the greenhouse. In one study, when the mites were introduced at planting time, control lasted from 6 to 8 weeks. However, these predatory mites are not compatible with many conventional chemicals, but are compatible with B.t. and beneficial nematodes.
Consult the most recent edition of the New England Floricultural Crop Pest Management and Growth Regulation Guide: A Management Guide for Insects, Diseases, Weeds and Growth Regulators for more specific up-to-date recommendations.
By Leanne Pundt, Extension
Educator, Commercial Horticulture, University of Connecticut
Casey, C. Ed. 1997. Integrated Pest Management for Bedding Plants. A Scouting and Pest Management Guide. Cornell Cooperative Extension IPM Pub No. 407. 109 pp.
Gardiner, R.B. Jarvis, W. B. & J. L. Shipp. 1990. Ingestion of Pythium sp. by the larvae of the fungus gnat, Bradysia impatiens. Annuals of Applied Biology. 166(2):205:212.
Goldberg, N. P. & M.E. Stanghellini. 1990. Ingestion-egestion and aerial transmisson of Pythium aphanidermatum by shore flies. Phytopathology. 80:1244-1246.
Harris, M. 1993. Fungus Gnats: They're more
than just a nuisance . GrowerTalks. 56(9):49-58.
This information was developed for conditions in the Northeast. Use in other geographical areas may be inappropriate.
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