Fighting fungus gnats

Not only are fungus gnats a nuisance, they may also present real problems in plant production. Fungus gnat larvae feeding can cause damage to plant growth and spread diseases such as fusarium, pythium and other soil-borne diseases. It’s essential that growers take appropriate steps to ensure their greenhouses are not conducive to fungus gnat development.

Fungus gnat adults are flies approximately 1/8 inches long with long legs and long, thread-like antennae. The adults are generally found near the surface of the soil or potting medium. They are weak fliers, so they do not wander far when flying. However, they can spread with plant material and growing media. The larvae feed on fungi and decaying plant material in the potting medium. Heavy populations of fungus gnat larvae can also feed on plant roots and be quite devastating to rooted cuttings.

Damage caused by feeding
The primary food of fungus gnat larvae is plant pathogenic or saprophytic fungi. It was long thought that larval feeding resulted in a site for plant pathogens to infect roots, and while this does contribute to infection, it’s now known that fungus gnats actively transmit fungal spores from diseased plants to healthy plants.

Feeding, which causes root and stem injury, reduces the plant’s ability to take up water and nutrients from the growing medium, resulting in stunted plants that do not grow as quickly as pest-free plants. Fungus gnat larvae can bore into the base of stems and feed on callus tissue and roots, restricting root growth and providing an entry point for disease-causing pathogens. Tender young plants, especially slow growing seedlings, can be destroyed by larval feeding.

Fungus gnat larvae feeding often begin during the mid-late stages of propagation. This is especially true in direct stick programs and in propagation areas that have not been properly sanitized.

Integrated crop management to control fungus gnats
Integrated Crop Management (ICM) combines all aspects of crop inputs, including integrated pest management, to achieve the needs of producers and consumers. For pest and disease control, biological control agents (BCAs), or beneficial insects, should be integrated with physical and chemical controls to create an overall management system.

Cultural control tactics
Cultural control options include maintaining a well-drained, clean, weed-free area around your crop to help eliminate breeding areas. Avoid overwatering plants, since fungus gnats require moist to very moist soil or substrate for development. Potting media should be stored dry, and pots and production areas must be well-drained. Fungus gnats can exist on soil, fungi and algae on and under benches. Periodically inspect developing callus and roots for larvae, and monitor adult populations by placing yellow sticky cards several inches above the plant canopy throughout the crop.

Biological and chemical control tactics
Compatibility between BCAs and other plant protection products must be carefully planned according to cropping systems, environmental conditions, previous chemical applications and rotations, and the pest history on specific crops or in greenhouses.

There are several BCAs that control fungus gnats and are compatible with chemical controls. The entomopathogenic nematode, Steinernema feltiae, will infect the larval stage of the fungus gnat when applied to the growing media. S. feltiae can be applied anytime during the crop cycle. The predatory mite, Hypoaspis miles, and the rove beetle, Atheta coriara, also work well to control fungus gnat larvae.

If fungus gnat larvae are detected or adult populations are starting to build, apply an insect growth regulator as a drench. Pyrethroid sprays can be used to control adults and can be applied as needed once the mist cycle is reduced to the point that allows the foliage periodic intervals of drying. Pyrethroids are not compatible with A. coriara.

The ICM chart below is a sample approach for controlling fungus gnats based on using Citation insect growth regulator. Similar programs can be built depending on the products used.
 

To see a bigger ICM Program for Fungus Gnat Control table, click the image above.

Marla Faver is a technical services representative for Syngenta Plant Protection.