PSI: The case of the growing medium pest

Fungus gnats (Bradysia spp.) are a major insect pest of many greenhouse-grown horticultural crops including vegetables and herbs. Fungus gnats are one of the few insect pests encountered by greenhouse producers in which the damaging stage (the larva, in this case) is located in the growing medium. They are particularly problematic under the excessively moist conditions that occur during propagation when young transplants, cuttings, plugs or seedlings are initiating root development. The primary means of dealing with fungus gnat populations is either the use of insecticides (contact or insect growth regulators) or biological control agents (predatory mite, rove beetle or entomopathogenic nematode). Both of these strategies are effective, but the implementation of appropriate cultural, sanitation, physical strategies and regularly scouting crops is also important. This may result in a reduction in insecticide inputs, and enhance the success of both insecticides and/or biological control agents. This article discusses identification, biology, damage and management of fungus gnats in greenhouse production systems associated with vegetables and herbs.
 

Identification, biology, and damage

Adult fungus gnats are winged, approximately 3.0 to 4.0 mm (0.12 to 0.16 inches) in length, with long legs and antennae. Adults live from seven to 10 days, and primarily aggregate near the growing medium surface. Females deposit 100 to 200 eggs into the cracks and crevices of the growing medium that hatch into white, translucent, legless larvae that are about 6.0 mm (0.23 inches) long. The life cycle consists of an egg, four larval instars, pupa, and adult. The life cycle can be completed in 20 to 28 days depending on growing medium temperature. Larvae are located within the top 2.5 to 5.0 cm (1.0 to 2.0 inches) of the growing medium or inside plant tissue such as the crown or base. Fungus gnat larvae feed on plant roots including root hairs, and organic matter in the upper 2.0 cm (0.78 inches) of the growing medium. However, larvae may be distributed throughout the growing medium profile, even at the bottom of containers near drainage holes. Furthermore, fungus gnat larvae may emerge from the growing medium and feed on leaves lying on the surface of the growing medium or tunnel directly into plant crowns.

Fungus gnat larval feeding directly damages developing root systems and interferes with the ability of plant roots to absorb water and nutrients. Larvae may cause indirect damage during feeding by creating wounds that allow entry of certain soil-borne plant pathogens. Fungus gnats may be present in bagged growing medium, which means that pasteurization or treating the growing medium with heat may be required to avoid issues. Both larvae and adult may also transmit fungal diseases such as Pythium spp., Fusarium spp., Thielaviopsis basicola (black root rot), from infected to non-infected plants.
 

Management

Greenhouses that have water accumulating and algae present typically have higher fungus gnat populations, resulting in damage to greenhouse-grown horticultural crops. In general, fungus gnats are more abundant in greenhouses with soil floors than those with concrete floors. What has been consistently recommended as a means of suppressing fungus gnat larval populations is to allow the growing medium to dry out occasionally, especially the upper 2.5 to 7.7 cm (1.0 to 3.0 inches). The “dry” surface may be less attractive to ovipositing females, and even if eggs are laid in the growing medium, they will fail to hatch because of the low moisture content. However, allowing plant material to become “too dry” is usually not a feasible option in greenhouses particularly during propagation.

Always use a well-drained growing medium that will minimize conditions associated with “too much moisture” and avoid algae growth, both of which are conducive to fungus gnat development. Alga is a prime breeding medium for fungus gnats (and shore flies). There are a number of ways to prevent the build-up of algae such as avoiding over-watering or over-fertilizing plants, and using commercially available disinfectants (e.g. ZeroTol, Green-Shield or KleenGrow). In addition, installing a landscape or fabric barrier, which is a geotextile, non-biodegradable material, underneath benches, will minimize problems with algae growth.

Another strategy is to place yellow sticky tape among the crop, just above the crop canopy, to capture fungus gnat adults flying around. This method has been used successfully by greenhouse producers in the United States and worldwide. Some greenhouse producers place yellow sticky tape below benches, with soil floors, to capture adults that emerge from the soil. Some greenhouse producers actually attach yellow sticky tape or more rigid yellow colored boards — similar in color to yellow sticky tape — to irrigation booms that move back-and-forth continuously, day and night, which can be very effective in capturing fungus gnat adults. The disposal of “old” growing medium will also help reduce problems with fungus gnats. Immediately remove “old” growing medium from within the greenhouse or place into a container with a tight-sealing lid. If the lid is not secured, adults may fly out and migrate back to the main crop.
 

Treatment

A number of insecticides including insect growth regulators and microbial compounds are effective against the larval stages of fungus gnats. Be sure to read labels to determine those insecticides that may be used on vegetables and herbs. In addition, various biological control agents including the soil-dwelling predatory mite, Stratiolaelaps scimitus; rove beetle, Dalotia coriaria, and entomopathogenic nematode, Steinernema feltiae are effective in suppressing or regulating fungus gnat larval populations when released/introduced early in the crop production cycle.

Raymond Cloyd is a professor and extension specialist in horticultural entomology/plant protection in the Department of Entomology at Kansas State University. His research and extension program involves plant protection in greenhouses, nurseries, landscapes, conservatories and vegetables and fruits.