Fungus gnats, once considered to be a minor pest, are
of increasing concern to greenhouse growers. Recent studies have shown
that fungus gnats play a role in the transmission of certain fungus diseases
including Pythium, Verticillium, Thielaviopsis,
and Botrytis. Fungus gnat larvae damage roots as they feed and
may tunnel into stems and cause plant death. The presence of adult shore
flies and fungus gnats may be objectionable to customers.
Identification
Both fungus gnats and shore flies thrive in the damp, moist environment
that is common in greenhouses. During cloudy, overcast weather in the
spring and fall, growers may see both fungus gnats and shore flies. Correct
identification is important because management strategies vary depending
on what insect is present.
Dark-winged fungus gnat adults (Bradysia spp. and Sciara
sp.) are less than 1/10 of an inch long. They are slender, mosquito-like
insects with long legs and many-segmented antennae. Their delicate gray
wings have a "Y"-shaped vein near the wing tip.
Shore fly adults (Scatella spp.) have stockier bodies, with shorter
legs and antennae than fungus gnats. Each wing has distinctive faint light
spots which aid in its identification.
Fungus gnat larvae have slender white bodies, which are less than 1/4
of an inch long, with distinctive black head capsules. Shore fly larvae
are yellowish-brown in color, lack a head capsule and have forked spiracles,
or breathing tubes, at their rear end.
Feeding Damage
Fungus gnats will feed on fungi and decaying organic material in the soil.
If a fungal food source is not readily available, fungus gnats may feed
directly on plants. During the propagation of cuttings, fungus gnats may
feed upon the callus. This may slow down or completely inhibit rooting.
They can also damage the roots as they feed upon seedlings and potted
plants. On many thick and succulent stemmed plants, larvae may tunnel
into stems and cause plants to wilt and die. Larval feeding upon young
roots encourages the development of root and stem rots such as Pythium.
When this occurs, plants will be stunted with reduced growth and leaf
drop.
Shore flies feed upon algae and do not directly feed upon plants. However,
their frass (droppings) on plants, in addition to their presence, may
be objectionable to customers.
Transmission of Root Rot Pathogens
Researchers have recently discovered that Pythium can provide a complete
nutritional food source for the fungus gnats to develop from egg to adult.
In this study, fungus gnat larvae ingested Pythium and readily introduced
the fungus to young cucumber plants as they fed upon the roots. Pythium
oospores were transmitted at very low levels from the larval to adult
fungus gnats. However, adults may help spread the disease by carrying
fungal spores on their bodies as they move from plant to plant.
Shore fly larvae, which feed upon algae, also may help spread root rot
pathogens. Researchers have found Pythium oospores in the shore fly larvae's
digestive system and frass.
 Life
Cycle of Fungus Gnats
The life cycle from egg to adult is approximately one month, depending
upon temperature. Overlapping and continuous generations in the greenhouse
make effective management difficult.
Prevention
High moisture levels encourage the development of both fungus gnats and
shore flies. The selection of a potting soil that drains well and the
use of proper watering techniques to avoid puddling will help to prevent
fungus gnat problems. Keeping the greenhouse floor as dry as possible
and free of debris, spilled potting soil and weeds will help to discourage
fungus gnats.
All life stages of shore flies can be found on or near algae. Proper sanitation
and environmental modification are crucial to managing algae growth. Reduce
the moisture and puddling water on floors, benches and greenhouse surfaces.
The greenhouse floor should be level and drain properly to prevent the
pooling of water. Avoid over watering crops, especially early in the production
cycle, to allow the upper media surface to dry out between irrigations.
Several disinfectants can be used as part of a precrop cleanup program
and during the cropping cycle for routine control of algae on greenhouse
surfaces and walkways.
Monitoring
Early detection of fungus gnats is important, as populations may increase
rapidly early in the cropping cycle. By detecting fungus gnats early,
growers may successfully use several newly available biorational materials.
These materials are generally less toxic and have shorter re-entry periods
compared to the more traditional pesticides.
Yellow sticky cards may be used to monitor adult populations. Sticky cards
are more effective in detecting adult fungus gnats when they are placed
just above the growing media. Horizontal card placement is more effective
than vertical card placement.
Fungus gnat larvae are usually found in the top inch or so of growing
media. However, larvae may sometimes be found near the bottom of the container.
Potato discs, one inch in diameter and ½ inch thick, may be placed
on the soil surface to monitor for larvae. Place the discs with the cut
surface face down on the growing media. (Potato discs tend to be more
effective when the cut surface stays moist and does not dry out.) Mark
the pots with flagging tape to facilitate the location of the cut potatoes
for frequent inspection. Check both the growing media and underside of
the potato discs for larvae. Replace during weekly scouting. Use potato
discs during cool, moist weather conditions to monitor fungus gnat larvae.
During periods of warm weather at the end of the bedding plant season,
potato discs may "melt out."
Treatment Options
Treatment is most effective against the young larvae early in the cropping
cycle. Bacillus thuringiensis subsp. israelensis is an insect pathogen
sold under the trade name of Gnatrol™ for greenhouse use. This particular
type of B. t. is active against flies (Diptera). B. t. consists of viable
endospores and crystals that must be ingested by the larvae. After the
larva feeds, its gut is paralyzed. Then the larva stops feeding and dies.
Gnatrol™ should be applied as a soil drench to thoroughly wet the
soil of containers, raised beds or under benches where the larvae are
found. Gnatrol™ is only effective for 48 hours so repeat treatments
may be needed. One to three applications may be necessary depending on
population levels. Gnatrol™ should not be combined with fertilizers
or fungicides containing copper or chlorine.
Parasitic nematodes enter the insect's body through openings in the exoskeleton.
The nematodes multiply inside the host insect and release a bacterium
that is toxic to the host. The nematodes complete their life cycle within
a few days. Large numbers of infective stage nematodes are produced that
will search for new hosts. These beneficial nematodes can be applied as
a drench to the growing media and to soil under the benches. Adequate
soil moisture is needed in order for the nematodes to move within the
soil surface. Remove screens and filters from the sprayer before treatment.
Two of the more common species available for greenhouse use include: Steinernema
carpocapsae (ScanMask) or Steinernema feltiae (Nemasys).
Hypoaspsis miles is a small predatory mite that feeds on fungus gnat larvae
and is sold in one-liter containers in a sawdust mixture. These mites
may be sprinkled over the soil surface or mixed into the media before
planting. H. miles is a scavenger that will survive in the absence of
fungus gnats and will feed on the young developing larvae. H. miles will
feed upon thrips pupae, as well. These soil-dwelling, predatory mites
will reproduce in the greenhouse. In one study, when the mites were introduced
at planting time, control lasted from 6 to 8 weeks. However, these predatory
mites are not compatible with many conventional chemicals, but are compatible
with B.t. and beneficial nematodes.
Consult the most recent edition of the New England Floricultural
Crop Pest Management and Growth Regulation Guide: A Management Guide for
Insects, Diseases, Weeds and Growth Regulators for more specific
up-to-date recommendations.
This information was developed for conditions in the Northeast. Use in
other geographical areas may be inappropriate.
For pesticide recommendations call the UConn Home
and Garden Education Center at 877-486-6271.
References
Casey, C. Ed. 1997. Integrated Pest Management for Bedding Plants. A Scouting
and Pest Management Guide. Cornell Cooperative Extension IPM Pub No. 407.
109 pp.
Gardiner, R.B. Jarvis, W. B. & J. L. Shipp. 1990. Ingestion of Pythium
sp. by the larvae of the fungus gnat, Bradysia impatiens. Annuals of Applied
Biology. 166(2):205:212.
Goldberg, N. P. & M.E. Stanghellini. 1990. Ingestion-egestion and
aerial transmisson of Pythium aphanidermatum by shore flies. Phytopathology.
80:1244-1246.
Harris, M. 1993. Fungus Gnats: They're more than just a nuisance . GrowerTalks.
56(9):49-58.
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