Controlling Fungus Gnats

OC0138
Michigan State University Extension-Oakland County
Controlling Fungus
Gnats
Fungus Gnats
Fungus gnats are small, “mosquito-like,”
gray-black, flies of the Sciaridae fly
family often noticed around house plants
and windows. Found throughout the
United States, most species inhabit fungi
or dead plant materials, but sometimes
young maggots injure plants by feeding
on roots. This occurs particularly if the
maggots become very numerous and
thus limit their food supply.
Host Plants and Damage
Fungus gnats are general feeders. They
attack a wide range of ornamental and
vegetable plants. Some species can be
serious pests in
mushroom houses.
The maggot or
larvae is the
damaging state.
However, the adult
flies are usually
noticed before
larval damage to the plant is apparent.
When maggots become numerous they
strip the roots resulting in loss of plant
vigor and yellowing and wilting of the
leaves.
Description and Life History
T h e a d u l t f l i e s a r e s l e n d e r,
approximately 1/8 inch (2.5mm) in
length, and have long legs and
antennae. They are weak flies but can
run quite rapidly across the soil surface.
During the female’s lifetime, of about
one week, she lays a hundred or more
eggs.
The shiny white oval eggs are
semitransparent and are barely visible to
the naked eye. They are laid either
singly or in strings or groups of 10 or
more in the soil surface, usually near
host plants. They hatch in four to six
days.
The mature larvae or
maggots are about ¼
inch (5.5mm) long with
shiny
black
head
capsules and white
transparent bodies. The
maggot reaches maturity
in about two weeks, when it ceases
feeding, spins a silken cocoon and
sheds its skin. After about a week, it
transforms into a pupae. At the end of
the pupal period, the adult fly emerges
from the soil and starts the cycle over
again. There are many overlapping
generations throughout the year.
Control
For
homeowners,
chemical control of this
pest is restricted to
controlling the adults.
Ideally, larval control
would be the best
approach for achieving
complete cleanup of
fungus gnat problem,
but no insecticides as
soil
drenches
are
registered for home use to control the
larvae. Household sprays containing
pyrethrins or pyrethroids and labeled for
“gnats” or “flying insects” will control
adults. Spraying for adults must be
done several times. One application will
not eliminate the gnat population. Larval
control in the home environment can be
achieved by “trap cropping.” Pots of
sprouting grain often attract female
gnats who lay their eggs near these
plants. After a few days these pots
should be submerged in boiling water to
destroy the eggs and maggots or
discarded outdoors.
This practice
should be repeated every two weeks
until flies are no longer noticed.
Fungus gnats live
in moist, shady
situations
with
decaying organic
material
present,
so
cultural
practices such as
the elimination of dead leaves or other
decaying organic matter and the
avoidance of excessive watering will
greatly reduce their numbers.
Control of Fungus Gnats
Commercial Greenhouses
in
Control of fungus gnats in commercial
greenhouses can be achieved with
Bacillus thuringiensis var. israelensis.
For other pesticide selections, contact
your local supplier for the greenhouse
grower.
Monitoring
Successful fungus gnat control depends
on a monitoring program for detection of
adults. Early detection will result in
quicker suppression. Yellow sticky traps
should be used for detection. Place
trays just above the plants at the
frequency of one per 500-1,000 square
feet. Replace them each week and
count the number of fungus gnats on
each trap.
Warning: Use pesticides with care.
Apply them only to plants, animals or
sites listed on the label. When mixing
and applying pesticides, follow all label
precautions to protect yourself and
others around you. It is a violation of the
law to disregard label directions. If
pesticides are spilled on skin or clothing,
remove clothing and wash skin
thoroughly. Store pesticides in their
original containers and keep them out of
the reach of children, pets and livestock.
The law requires that
pesticides be used as the
label directs. Uses against
pests not named on the
label and low application
rates are permissible
exceptions. If there is any apparent
conflict between label directions and the
pesticide uses suggested in this
publication consult your county Extension
Agent.
Prepared by A.L. Antonelli, Ph.D.,
Extension Entomologist, WSU Puyallup
and D.F. Mayer, Ph.D., Extension
Entomologist, WSU Prosser.
Edited by Dave Smitley, Ph.D., MSU,
Entomology Department.
Would you like additional information?
Additional information is available on-line. Please see MSU Extension-Oakland
County’s publications as well s MSU Extension’s Bulletin Office on campus.
Contact our Plant & Pest Hotline (248/858-0902) for assistance with plant
identification, pests and diseases, weeds, trees and shrubs, lawn, flowers,
fruits, vegetables, grasses and groundcovers, native plants, plant propagation,
and many other gardening topics.
Distributed by MSU Extension-Oakland County, 1200 N. Telegraph Road, Pontiac, MI 48341, 248/858-0880,
www.msue.msu.edu/oakland
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