Fungus
Gnats and Shoreflies in Greenhouse Crops
PDF Version (508 KB)
Table of Contents
- Introduction
- Description
and Life History
- Damage
- Management
Strategies
- Related Links
Introduction
Fungus
gnats and shoreflies are small black flies often observed in the vicinity of the
growing media of greenhouse crops. These flies, often regarded as nuisance pests,
can facilitate and transmit root diseases in all greenhouse vegetable and ornamental
crops and reduce the aesthetic quality of ornamental crops.
Description
and Life History
Adult fungus gnats are grey to black, about 3-4 mm long
with long legs, thread-like antennae, and large compound eyes that meet above
the base of the antennae. Adults resemble small mosquitoes (Figure
1) and are weak fliers frequently observed resting on the media surface. Each
female lives about 10 days and lays about 150 oval, white eggs in the organic
matter of the media. Eggs hatch in 2-7 days, depending on the temperature, producing
white larvae that are 4-6 mm long. The larvae have 12 abdominal segments and a
distinctive shiny black head (Figure 2). The larvae feed for
5-14 days before pupating, after which adults emerge in 4-6 days. As with other
insects, fungus gnats become more active and reproduce more quickly at warmer
temperatures. The life cycle can be completed in 21 days at 24°C, compared
with 38 days at 16°C.
Adult shoreflies are similar in size to fungus
gnats but resemble small houseflies, having a dark chunky body with short, bristle-like
antennae and short legs (Figure 3). They are stronger fliers
than fungus gnats, and have five clear spots on the wings. Adult shoreflies prefer
wetter conditions than fungus gnats and are often found in wet areas under benches
and other water-soaked areas. Females lay eggs on algae or moist growing media
and larvae hatch in 2-3 days. Shorefly larvae are cream to muddy brown in colour
(Figure 4), appear headless, and feed on algae and other
micro-organisms found in the media for 3-6 days before pupating. After a further
4-5 days, the adults emerge. Shoreflies take approximately 9-14 days to complete
a generation at greenhouse temperatures. Generally, adults and larvae feed on
algae. However, larvae may feed on roots infected with fungi.
Damage
The
immature stages of fungus gnats generally feed on decaying organic matter, soil
fungi and algae, and can cause direct damage by feeding on fine roots, root hairs
and on tender lower stems. All greenhouse crops can be affected. They also cause
indirect damage by creating entry points in the roots for disease-causing organisms.
Fungus gnats can themselves transmit such disease-causing organisms. For instance,
hardy Pythium spores ingested by the larvae can remain in the gut until the larvae
have developed into the adult flying stage. Various studies indicate that fungus
gnat adults can spread spores of disease-causing fungi such as Pythium and Rhizoctonia
(Figure 5) by flying to non-infected plants and excreting
the spores. Adult flies can also spread Fusarium, Verticillium and other fungi
by means of spores that are caught on their legs and bodies. Organic media such
as peat and cocofibre favour reproduction of fungus gnats.
Immature shoreflies,
by contrast, are semi-aquatic, feed principally on algae, and do not normally
feed on plant parts. However, they will feed on roots infected with fungi and
can therefore spread disease if ingested spores remain viable in the gut until
the adult stage is reached. Research indicates that shoreflies can acquire and
spread Pythium spores in a manner similar to that of fungus gnats. Shoreflies
also cause cosmetic damage to ornamental crops by depositing black drops of excrement
on leaves and flowers.
Figure
1. Adult fungus gnat on sticky card. Note long legs and antennae.
Figure
2. Fungus gnat larva.
Figure
3. Shorefly adult. Note very short antennae, stout body and clear spots
on wings.
Figure
4. Shorefly larva.
Figure
5. Fungus gnats can transmit diseases such as Rhizoctonia shown on this
poinsettia.
Management Strategies
Generally,
control of these flies can be difficult because of the above ground stage, continuous
overlapping generations and short life cycles. To avoid problems associated with
these flies, implement control measures early. Strategies for management of these
flies include monitoring and cultural controls. Whereas biological control agents
(BCAs) can be effectively used for suppression of fungus gnats, the effectiveness
of many of them against shoreflies is uncertain.
Monitoring
Yellow
sticky cards at the normal position at the top of the canopy will indicate the
presence of these flies. But for earlier detection and greater trapping effectiveness,
such cards are best placed horizontally at the base of the plants. For indication
of levels of fungus gnat larvae, place slices of raw potato on the media and examine
after 24 hours with a magnifying lens (Figure 6).
Figure
6. Use of potato slices can be very effective in monitoring for fungus
gnat larvae and the predatory rove beetle.
General Control Strategies
It
is best to prevent establishment of these flies in the greenhouse by implementing
good hygiene practices, and having good drainage to eliminate puddles and formation
of algae (Figure 7a, b). Minimizing exposure of media surfaces
will block light and thus help prevent growth of algae.
Figure
7. Control of algae on rockwool (7a) and under benches (7b) is important
in the management of fungus gnats and shoreflies.
Biological Control
Several
commercially available Biological Control Agents (BCAs) can be used for managing
fungus gnat populations. These include a bacterial insecticide, Bacillus thuringiensis
subsp. israelensis; a parasitic nematode, Steinernema feltiae; a predatory mite,
Hypoaspis spp.; and a predatory beetle, Atheta coriaria, commonly called the rove
beetle. The effectiveness of these agents has been determined mainly for fungus
gnat larvae and may not be equally effective against shorefly larvae. There are
also several naturally occurring BCAs that are not available commercially, but
are often found in greenhouses where pesticide use has been reduced. Coenosia
attenuata is a predatory fly that attacks both fungus gnats and shoreflies, Synacra
and Hexacola neoscatellae are parasitic wasps that attack fungus gnats and shoreflies
respectively. Some notes about use of these BCAs follow.
Bacillus thuringiensis
subsp. israelensis (Bti): Bti consists of two kinds of spores, one is active
and the other is a storage spore that includes a toxic protein crystal. Once these
spores are ingested by the larva, the alkaline pH in the gut facilitates release
of the toxic crystal. This crystal destroys the gut wall of the insect, allowing
the active spores to pass into the blood stream. The insect then dies from blood
poisoning. Within 24 hours of ingesting Bti, fungus gnat larvae stop feeding and
become limp. Death follows 1-7 days after ingestion. This bacterium does not kill
by contact and only the larval stage is killed by ingesting Bti. Studies show
that the younger larval stages of fungus gnats are more susceptible and therefore
multiple applications must be applied to achieve control. Adults do not feed on
these spores and are therefore unaffected. When using this bacterium, ensure that
the pH of the water used for mixing is neutral or slightly acidic (upper limit
is pH = 7.0). Alkaline water, or the addition of any other substance that will
raise the pH, will render this organism ineffective.
Steinernema feltiae:
These nematodes may provide more rapid control of fungus gnat larvae than the
other BCAs. Pupae are not as susceptible to the nematodes as the larval stages.
The nematodes search out an insect host and enter it though body openings such
as the mouth, anus, and breathing pores called spiracles. Once inside the insect,
the nematodes release a bacterium (Xenorhabdus spp.) which they carry in their
gut. The bacteria develop within the insect, killing it within 48 hr. The nematodes
can then theoretically develop by feeding within the larva (Figure
8). However, the size of fungus gnat larvae may be too small to accommodate
their reproduction. For best results, apply at the end of the day to avoid drying
out and exposure to direct sunlight. Applying them at this time also reduces the
risk of flushing the nematodes out of the growing media by irrigation of the fertilizer
solution over the course of the day, particularly in rockwool. Studies show that
these nematodes move very easily through rockwool, and that within two weeks after
application the majority of nematodes are found in the lower parts of the rockwool
media. Nematodes are most effective when the temperature and pH of the water used
for mixing are the same as those required for optimum crop growth. Applications
should be made weekly for several weeks, depending on pest populations.
Figure
8. Steinernema feltiae nematodes can provide effective control of fungus
gnats.
Hypoaspis: This predator is a soil-dwelling
mite (Figure 9) that feeds mainly on the young larvae of
the fungus gnat, very little on eggs, and probably not at all on pupae. They do
not diapause but will become inactive if media temperatures fall below 15°C.
If these predators are released in the seedling stage, make a second release after
planting out in the main house when there is increased likelihood of available
food to sustain them. It is best to release these predators before fungus gnat
populations are established. The advantage of using this predator is that it provides
season long suppression, and will also feed on other small insects in the growing
media, such as springtails, and thrips pupae.
Figure
9. Hypoaspis predatory mites feed on soil-dwelling organisms such as
fungus gnat larvae, and other pests such as thrips pupae.
Atheta coriaria:
This beetle is a relative newcomer to the biological control arsenal for fungus
gnats. Laboratory studies indicate that it holds much potential as a control agent
for fungus gnats and shoreflies. The adult is a small black beetle, 3-4 mm long
(Figure 10). There are three larval stages that vary from
white in the earlier stages to yellow-brown in the final stage. The adult and
all larval stages are predatory. The rove beetle is very active, establishes easily
and spreads quickly throughout the greenhouse. It often establishes naturally
and resident populations will maintain a permanent presence. Potato slices used
to monitor fungus gnat larvae are also very effective in detecting the presence
of Atheta adults and larvae.
Figure
10. The rove beetle Atheta coriaria is a soil-dwelling predator that
feeds on the eggs and larvae of fungus gnats and shoreflies.
Coenosia
attenuata (also called the hunter fly or tiger fly) is a greyish, predatory
fly in the same family as the housefly (Figure 11). It
is larger than the shorefly and the adult preys on other flying insects, catching
them in flight. It feeds on fungus gnats and shoreflies, but also on other flying
insects such as leafminer and, to a lesser extent, whiteflies. Coenosia larvae
live in the soil and are generalist predators on other soil-dwelling organisms
such as fungus gnat and shorefly larvae.
Figure
11. Coenosia attenuata is a predatory fly, the adult of which feeds on
flying insects such as fungus gnats and shoreflies, and whose larva feeds on soil-dwelling
organisms. The figure shows the size comparison between a shorefly on the left
and Coenosia on the right.
Synacra is a parasitic wasp that
lays its eggs into fungus gnat larvae. The wasps develop and emerge from the fungus
gnat pupae. The adult wasp is similar in size to the fungus gnat but has a typical
wasp-like appearance with a pinched "waist" and a long, tapered abdomen
(Figure 12). Although its effectiveness in controlling
fungus gnat populations has not been documented, large numbers of these BCAs are
often found on sticky cards in greenhouses.
Figure
12. Synacra is a parasitic wasp that lays its eggs into fungus gnat larvae.
Hexacola
neoscatellae is a parasitic wasp that feeds on shoreflies (Figure
13). It is smaller than a shorefly and can be found in large numbers on yellow
sticky cards in greenhouses with resident shorefly populations. It is black in
colour and has an almost spherical abdomen, in contrast with the more elongated
abdomen of Synacra (Figure 14).
Figure
13. Hexacola neoscatellae is a parasitic wasp that lays its eggs into
shorefly larvae.
Figure
14. Comparison in size between Synacra (left) and Hexacola. (right).
Chemical Control
Use of pesticides to control fungus gnats is targeted
against the larval life stages living in the growing medium. Best results are
usually obtained when applied early in the crop production cycle, which is when
fungus gnat populations are often at the worst. As the crop matures, the growing
medium dries out more quickly and the developing root system is less prone to
feeding damage. There are several pesticides registered for control of fungus
gnats and shoreflies, some of which are compatible with biological control programs.
Please refer to OMAFRA Publication 370, Production Recommendations for Greenhouse
Floriculture, and Publication 371, Growing Greenhouse Vegetables.
Related
Links
