October 2002

Fungus Gnats: Media and Water Control Methods By Marc van Iersel, Denise Olson and Ron Oetting

Fungus gnats are common pests in greenhouses. Their larvae feedon the roots of many different crops, which can cause direct damage to thecrop. The root damage caused by the larvae also makes it easier for fungalpathogens to enter into and infect plants. Finally, movement of adult fungusgnats among plants can spread foliar diseases throughout the crop.

Adult fungus gnats lay eggs on the potting medium, and thelarvae feed slightly underneath the medium surface while they develop intoadults. The larvae primarily feed on fungi, however, they can feed on plant rootsas well, particularly when there is little or no fungus present in the pottingmedium. High moisture levels in the growing medium create favorable conditionsfor many fungi, and consequently, fungus gnats appear to cause more problemswhen the growing medium is kept moist. Because the larvae feed mainly in theupper part of the potting medium, letting the top portion of the medium dry outbetween waterings may reduce fungal growth and, therefore, fungus gnatpopulations. Much of the life cycle of fungus gnats occurs in the soil, andsince this is where most of the plant damage occurs, it seems logical thatproper management of the growing medium may help to reduce fungus gnatpopulations in the greenhouse.

Greenhouse managers have a choice of many different types ofgrowing media, and the optimal choice depends on the type of crop being grown.Growing media is available in many different textures, ranging from finelytextured (normally containing vermiculite), which is commonly used forseedlings, to more coarsely textured media (normally with bark) used for pottedplants. In addition to the texture of the medium, the components of variousmedia differ, and there are more choices available now than ever before.Because of problems associated with the supply of peat, as well asenvironmental concerns related to its harvesting, the industry has been lookingfor alternatives to peat. Coconut coir, a by-product of the coconut industry,has rapidly gained popularity as a component of soilless media. Its chemical andphysical characteristics are fairly similar to those of peat, and it has beenpromoted as a replacement for peat in growing media. Also, coconut coir oftenhas been marketed as a growing medium component that can inhibit thedevelopment of fungus gnats. However, there appears to be little research-basedinformation to confirm this claim. We conducted several studies comparing howcoconut coir and peat affect the development of fungus gnat populations.

Does coir inhibit fungus gnat development?

In a laboratory study, we determined how peat and coiraffect the development and survival of fungus gnats. Peat, sterilized peat andpeat with added yeast (as a food source for the fungus gnats) were comparedwith coconut coir, sterilized coir and coir with added yeast. IndividualStyrofoam cups were filled with this media, and 20 fungus gnat eggs were addedto each cup. The number of fungus gnat adults emerging from the different mediawas recorded after two weeks. The counts were continued for an additional twoweeks, at which point almost all the adults had emerged.

Very few adult fungus gnats emerged from either peat or coirwhen a food source was not provided, independent of whether the peat or coirhad been sterilized (See Figure 1, page 20). When yeast was added, about 12 (60percent) of the 20 eggs in each cup developed into adults. The number of adultfungus gnats that had emerged was similar for both peat and coir. Thedevelopmental time from egg to adult was also similar for peat and coir(approximately 18 days in the presence of yeast). We conclude from this studythat fungus gnats develop equally well in peat and coir and that they need afood source to complete their development from egg to adult, regardless of thepotting media. This contradicts the widely held belief that coconut coirinhibits fungus gnat development.

Composition, texture and water management

In a greenhouse study, we looked at how different growingmedia (different textures and made with either coir or peat) affected thedevelopment of fungus gnats. Chrysanthemums were grown in 6-inch pots filledwith one of several potting media. Three different media (Redi-Earth, MetroMix366 and MetroMix 510) were used. Redi-Earth is a fine-textured medium used forgermination and is a mixture of peat and vermiculite. MetroMix 366 has anintermediate texture, made with peat, vermiculite, pine bark and a little bitof bark ash. It is used for both bedding plants and potted plants. MetroMix 510is a coarse-textured growing medium. It is made with the same components asMetroMix 366 but contains less vermiculite and more bark and bark ash. Althoughthese potting media normally are made with peat, the Scotts Company custom-madethese three media for our study with coir instead of peat.

In addition to potting medium composition and texture, wealso looked at the effect of water management on fungus gnat development. To dothis, five different levels of soil moisture were tested. Chrysanthemums werewatered only when the water in the growing medium decreased to 90, 71, 52, 34or 15 percent of the total amount the pots could hold. At 90-percent soilmoisture, the growing medium is almost saturated with water, while 15 percentrefers to a very dry growing medium.

At the start of the experiment, approximately 50 fungus gnateggs were applied to each pot, and 20-30 days later, the number of both larvaeand adult fungus gnats were recorded. We also looked at how the plants grew inthe different media and how almost saturated to very dry conditions affected plantgrowth.

The results of the greenhouse experiment were notstraightforward. Depending on the texture of the growing medium, coir eitherreduced or increased the number of fungus gnat larvae (See Figure 2, page 20).In the fine-textured media (Redi-Earth), coir resulted in more than three timesas Á many larvae as peat. On the other hand, in medium- (MetroMix 366)and coarse- (MetroMix 510) textured media, peat resulted in substantially morelarvae than coir.

The amount of soil moisture affected the number of larvae inpeat-based media, but not in coir-based media (See Figure 3, left). Very wet ordry conditions resulted in fewer fungus gnat larvae than intermediate moisturelevels (71 and 52 percent) in peat-based media. However, when the level ofmoisture is high in the potting medium, there are other problems associatedwith the greenhouse crop, such as lack of oxygen in the root zone. Also at verylow (15 percent) moisture levels, plants may wilt, and growth is inhibited. A34-percent moisture level in the growing medium appears to be a good targetvalue to minimize fungus gnat problems and to maintain good plant growth. Ourresults show that chrysanthemums, and presumably most other greenhouse plants,can grow well with a 34-percent moisture content. Therefore, letting thegrowing medium dry out between waterings can help maintain good plant growthand reduce fungus gnat development, especially in peat-based media.

Desirable medium and soil moisture

The choice of the growing medium to use is complicated bythe fact that coir reduced fungus gnat populations in some cases but increasedit in others and vice versa for peat. Our results suggest that peat may be thebetter choice in the fine-textured media, while coir may be the better choicein more coarse media, such as those used for bedding plants and potted plants.Coir certainly is not a cure-all for fungus gnat problems. However, letting thegrowing medium dry out between watering may help, but won’t eliminate, fungusgnats. Insecticide applications may be necessary when larger populations offungus gnats occur.

You may have to experiment in your own greenhouse todetermine which growing media minimizes problems with fungus gnats. If you arecurrently using a medium in which fungus gnats are a problem, replace the peatwith coconut coir (or vice versa) and allow the top 1-2 inches of the pottingsoil to dry between waterings.