Fungus Gnats: Media and Water Control Methods

October 10, 2002 - 09:43

Three professors put peat and coconut coir to the test for fungus gnat control.

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

Adult fungus gnats lay eggs on the potting medium, and the
larvae feed slightly underneath the medium surface while they develop into
adults. The larvae primarily feed on fungi, however, they can feed on plant roots
as well, particularly when there is little or no fungus present in the potting
medium. High moisture levels in the growing medium create favorable conditions
for many fungi, and consequently, fungus gnats appear to cause more problems
when the growing medium is kept moist. Because the larvae feed mainly in the
upper part of the potting medium, letting the top portion of the medium dry out
between waterings may reduce fungal growth and, therefore, fungus gnat
populations. Much of the life cycle of fungus gnats occurs in the soil, and
since this is where most of the plant damage occurs, it seems logical that
proper management of the growing medium may help to reduce fungus gnat
populations in the greenhouse.

Greenhouse managers have a choice of many different types of
growing media, and the optimal choice depends on the type of crop being grown.
Growing media is available in many different textures, ranging from finely
textured (normally containing vermiculite), which is commonly used for
seedlings, to more coarsely textured media (normally with bark) used for potted
plants. In addition to the texture of the medium, the components of various
media differ, and there are more choices available now than ever before.
Because of problems associated with the supply of peat, as well as
environmental concerns related to its harvesting, the industry has been looking
for alternatives to peat. Coconut coir, a by-product of the coconut industry,
has rapidly gained popularity as a component of soilless media. Its chemical and
physical characteristics are fairly similar to those of peat, and it has been
promoted as a replacement for peat in growing media. Also, coconut coir often
has been marketed as a growing medium component that can inhibit the
development of fungus gnats. However, there appears to be little research-based
information to confirm this claim. We conducted several studies comparing how
coconut 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 coir
affect the development and survival of fungus gnats. Peat, sterilized peat and
peat with added yeast (as a food source for the fungus gnats) were compared
with coconut coir, sterilized coir and coir with added yeast. Individual
Styrofoam cups were filled with this media, and 20 fungus gnat eggs were added
to each cup. The number of fungus gnat adults emerging from the different media
was recorded after two weeks. The counts were continued for an additional two
weeks, at which point almost all the adults had emerged.

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

Composition, texture and water management

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

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

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

The results of the greenhouse experiment were not
straightforward. Depending on the texture of the growing medium, coir either
reduced 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 times
as Á many larvae as peat. On the other hand, in medium- (MetroMix 366)
and coarse- (MetroMix 510) textured media, peat resulted in substantially more
larvae than coir.

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

Desirable medium and soil moisture

The choice of the growing medium to use is complicated by
the fact that coir reduced fungus gnat populations in some cases but increased
it in others and vice versa for peat. Our results suggest that peat may be the
better choice in the fine-textured media, while coir may be the better choice
in 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 the
growing medium dry out between watering may help, but won't eliminate, fungus
gnats. Insecticide applications may be necessary when larger populations of
fungus gnats occur.

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

About The Author

Marc van Iersel is associate professor of floriculture and Ron Oetting is professor of entomology at the University of Georgia, Griffin, Ga. Denise Olson is assistant professor of entomology, North Dakota State University, Fargo, N.D. They can be reached by phone at (706) 583-0284 or E-mail at

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