Fungi carried by adult fungus gnats (Diptera: Sciaridae) in Idaho greenhouses

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Forest Pest
Management
Report 94-5 3450
March 1994
FUNGI CARRIED BY ADULT FUNGUS GNATS (DIPTERA: SCIARIDAE)
IN IDAHO GREENHOUSES
by
R. L.
James1,R. K. Dumroese2, D.
L.
Wenny2
ABSTRACT
Eight species of fungi were isolated from external portions of adult fungus gnats (Bradysia sp.; Diptera:
Sciaridae) adults within greenhouses at the USDA Forest Service Nursery in Coeur d'Alene, Idaho and the
University of Idaho Research Nursery, Moscow. Fungus gnats were either collected from open water contain-
ers or standard yellow, sticky traps. The most common potentially pathogenic fungus isolated from adult
gnats was Botrytis cinerea, an important foliar pathogen of several species of conifer seedlings. Three species
of Phoma, another group of potential pathogens, were also isolated. The most common species was P.
eupyrena, which is associated with several conifer seedling diseases. Fusarium, an important root pathogen
of conifer seedlings, was represented by two species: F. proliferatum and F. sambucinum. All isolated fungi
were carried externally on adult gnats. Characteristics and impact of fungus gnat infestations in greenhouses
and control procedures are discussed.
INTRODUCTION
Growers of container forest tree seedlings have often noticed high levels of fungus gnats (Bradysia spp.;
Diptera:Sciaridae) associated with their greenhouse crops. Gnat larvae consume fungi and may cause crop
damage by feeding. directly on roots (Dennis 1978; Hamlen and Wettstein 1978), whereas adults may
disseminate fungal spores which might infect plants (Gardiner and others 1990, Kalb and Millar 1986).
1
Plant Pathologist, TCFPM, stationed in Coeur d'Alene, Idaho.
2
Research Associate and Professor, respectively, University of Idaho Research Nursery, Moscow.
United States
Department of
Agriculture
Forest
Service Northern
Region P.O.Box 7669
Missoula, Montana
59807

Adult fungus gnats are small, dark, mosquito-like insects that do not damage plants (McHugh 1991). Their
larvae are small and maggot-like, with white bodies and distinct black heads. The larvae feed on organic
matter in growing media, including decaying plant debris, as well as seedling roots (Shrimpton 1991). Two
species of fungus gnats are usually recognized in association with seedling crops: Bradysia impatiens and
B. caprophila (Gardiner and others 1990; McHugh 1991). Many growers believe these insects may significant-
ly contribute to decline of seedling growth and performance during greenhouse production. Gnat populations
tend to intensify in portions of containers with moss or liverwort buildup at the tops of cells (McHugh 1991).
Growers may confuse fungus gnats with other flying insects occurring in greenhouses, such as shore flies
.(Scatella spp.)(Baker 1972). These latter insects generally have heavier bodies and are stronger fliers
(McHugh 1991). Adult shore flies have unnoticeable antennae and dark wings with clear spots (King 1990).
Adult fungus gnats appear more mosquito-like, whereas shore flies look more like common flies. Shore fly
larvae can be differentiated by their yellow to brown color and lack of a distinctive head (King 1990, McHugh
1991). Shore fly larvae and adults feed primarily on algae growing on the surface of growing media, walls,
floors, benches, and containers. Unlike fungus gnats, shore flies rarely damage plant material, even though
they may contribute to the spread of pathogenic fungi within greenhouses (McHugh 1991).
Both fungus gnats and shore flies thrive in high moisture environments, particularly those common within
greenhouses (Baker 1972; McHugh 1991). Level of plant damage caused by fungus gnats can vary; their
presence is usually more of a nuisance than a production problem (Robb 1991). Seedling damage may
include stunting and wilting, premature foliage loss, and chlorosis (King 1990), symptoms similar to those
caused by root pathogenic fungi (James and others 1991).
Fungus gnat adults lay their eggs within growing media; after 5-6 days the eggs hatch into larvae which
remain within growing media feeding on seedling roots, fungi and organic matter for about 10-14 days. They
are usually confined to the top portion of plugs where they may cause damage by feeding on roots and
stripping root hairs (King 1990). In severe infestations, larvae can be found tunneling through succulent stems
at or below the ground line (King 1990). Larvae can also feed on foliage, especially near the ground line, and
may create wounds, allowing infection by pathogenic fungi (King 1990). Pupae also form within the media
and adults emerge from pupae after a few days (McHugh 1991). The entire life cycle is temperature
dependent, but usually takes from 2-4 weeks, although cycles may be quicker if gnats feed exclusively on
fungi (Kennedy 1974). Therefore, several insect cycles are possible during greenhouse seedling production.
Little is known about the role of fungus gnats in the epidemiology of plant pathogenic fungi. One study
(Gardiner and others 1990) evaluated the interrelationships of gnats with Pythium root disease on different
greenhouse crops. They found that during Pythium outbreaks, fungus gnat populations were correspondingly
very high. Examination of gnat larvae indicated all forms of Pythium were ingested: mycelium, oospores and
zoospore cysts. Digestive tracts of larvae were often packed with Pythium oospores, which readily germinate
after being excreted. Apparently, fungus gnats were important in spreading Pythium root disease among
different types of host plants within greenhouses (Gardiner and others 1990). Another study (Kalb and Millar
1986) demonstrated vectoring of Verticillium elbo-etrum, an important root pathogen of alfalfa, by adult fungus
gnats (B. impatiens). Investigators in another study found that fungus gnat larval feeding could predispose
alfalfa and red clover seedlings to wilt caused by Fusarium oxysporum f. sp. medicaginis (Leath and Newton
1969). Similar investigations with greenhouse pathogens of forest seedlings have not been done.
Because of their common association with fungi and prevalence of fungi as potential causes of disease in
greenhouse seedlings (James 1984b), an investigation was conducted to identify fungi commonly carried by
adult fungus gnats within greenhouses. Two north Idaho nurseries, the USDA Forest Service Nursery in Coeur
d'Alene and the University of Idaho Research Nursery in Moscow, were sampled in this evaluation.
2

METHODS
Fungus gnats were trapped several times throughout the greenhouse production phase at both nurseries.
Gnats were trapped either in open containers filled with water (University of Idaho Research Nursery) or on
standard yellow, sticky cards (USDA Forest Service Coeur d'Alene Nursery) (figure 1). Periodically, traps were
collected and entire fungus gnat bodies, when possible, were aseptically transferred to agar media in the
laboratory. Standard potato dextrose agar and an agar medium selective for Fusarium spp. and closely
related organisms (Komada 1975) were routinely used. This latter medium is often used to isolate root
pathogenic fungi from conifer seedlings. We were especially interested in determining if and to what extent
fungus gnats were carrying species of Fusarium within greenhouses because these organisms are common
pathogens of container-grown conifer seedlings (James and others 19~1).Selected fungi emerging from
trapped fungus gnats were maintained in pure culture for identification purposes. Whenever possible,
single-spore isolates were derived. Several taxonomic compilations were used for fungal identification (Bar-
nett and Hunter 1972, Domsch and others 1980, Dorenbosch 1970, Nelson and others 1983).
Figure 1--Adult fungus gnat (Bradysia sp.) on a yellow, sticky trap within a conifer greenhouse at the USDA
Forest Service Nursery, Coeur d'Alene, Idaho.
RESULTS AND DISCUSSION
Eight identified species of fungi were isolated from external portions of fungus gnat bodies trapped at the two
nurseries (table 1). The most commonly isolated species were Botrytis cinerea Pers ex Nocca. & Balb. and
Aureobasidium pullulans (de Bary) Arnard. The fungal genus most often encountered was Phoma, with three
isolated species: P. eupyrena Sacc., P. glomerata (Corda)Wolienw.
&
Hochapfel, and P. herbarum Westend.
Two species of Fusarium were infrequently isolated: F. proliferatum (Matsushima) Nirenberg and F. sambuci-
num Fuckel, The other fungal species isolated was Oidiodendron griseum Robak. Unidentified, nonsporulat-
ing fungi accounted for about 8 percent of isolations. In several cases, more than one fungal species was
isolated from a particular adult gnat.
3

Table 1--Fungi isolated from external portions of fungus gnats trapped at the USDA Forest Service Nursery
in Coeur d'Alene, Idaho and the University of Idaho Research Nursery in Moscow.
Fungus Species Percent of lsolations!
Botrytis cinerea 25.0
Aureobasidium pullulans 25.0
Phome eupyrena 16.7
Phoma glomerata 8.3
Unidentified (non-sporulating) 8.3
Phoma herbarum 4.2
Fusarium proliferatum 4.2
Fusarium sambucinum 4.2
Oidiodendron griseum 4.2
1
Based on relative frequency each appropriate fungus was isolated,
e.g., 25 percent of all fungi emerging from adult gnats were
identified as B. cinera.
Previous reports (King 1990, McHugh 1991) have implicated fungus gnats in disseminating spores of plant
pathogenic fungi in the genera Botrytis, Fusarium and Phoma. Potentially pathogenic fungi implicated by
others but not found in our evaluation include Verticillium and Pythium spp. (Gardiner and others 1990, Kalb
and Millar 1986). Botrytis cinerea is an extremely important pathogen of greenhouse-grown conifer seedlings
(James 1984a) and especially causes problems late in the growth cycle when seedling canopies are dense.
This fungus causes disease primarily on the above-ground portion of seedlings (James 1984a), but can also
reside in roots, especially those just below the ground surface (James unpublished). Apparently, fungus gnat
larvae may collect Botrytis spores when tunneling into the base of seedlings or feeding on roots near the
growing medium surface (McHugh 1991). Since Botrytis spores are commonly produced on necrotic foliage
near the base of seedlings (James 1984a), it is also likely that adult gnats become contaminated as they
emerge from growing media and move through seedling crops. The relatively high rate of adult fungus gnat
contamination with Botrytis indicates these insects may be important in translocating this pathogen within
greenhouses.
Aureobasidium pullulans is a ubiquitous saprophytic fungus occurring on many different substrates including
soil, growing media, and the above-ground surface of plants (Cooke 1961, Hermanides-Nijhof 1977). Some
strains of the fungus are especially well adapted to peat habitats (Christensen and Whittingham 1965,- Latter
and others 1967). Surveys indicate that A. pullulans is usually located on the surface layers of soil (Cooke
1970, Kendrick 1963, McLennan and Ducker 1954). This fungus may exhibit a dimorphic yeast-type phase
(Domsch and others 1980); it is a common phylloplane inhabitant and its spores may contaminate any insect
encountering infected foliage (Domsch and others 1980).
4

Although Phoma spp. may be important pathogens of conifer seedlings under certain conditions (James and
Hamm 1985), they are more commonly saprophytic (Domsch and others 1980, Dorenbosch 1970). They often
reside in soil or within growing media, but can also colonize conifer foliage, especially just above the
groundline (James and Hamm 1985). These fungi are similar to Botrytis because they may grow saprophyti-
cally on necrotic foliage near the base of seedlings. It is likely that adult fungus gnats became contaminated
with Phoma spores when moving through infected seedlings. Of the three species of Phoma we isolated, the
most important, from the standpoint of causing seedling diseases, was P. eupyrena. This fungus has been
implicated in important dieback diseases and mortality of young seedlings, especially those grown in bare root
nurseries (James 1983, Kliejunas and others 1983). This species produces common catenulate chlamy-
dospores (figure 2) which remain viable in soil or container growing media for extended periods (James and
Hamm 1985). Chlamydospores also form on necrotic seedling foliage and may be the spore stage being
carried by adult fungus gnats. The other isolated Phoma species are usually less of a problem on conifer
seedlings (James and Hamm 1985). Occasionally, P. glomerata may be pathogenic to conifers (Boerema and
others 1971, James and Hamm 1985). It produces very characteristic dictyochlamydospores (figure 3) that
readily form within soil or on other substrate colonized by the fungus. These 'resting spores' allow the fungus
to remain viable during inactive periods.
Figure 2--Catenulate chlamydospores (arrow) of Phoma eupyrena contaminating an adult fungus gnat
(Bradysia sp.) from the University of Idaho Research Nursery, Moscow.
5

Figure 3--Dictyochlamydospores (arrows) of Phoma glomerata contaminating an adult fungus gnat collected
from the USDA Forest Service Nursery, Coeur d'Alene, Idaho.
One objective of this evaluation was to determine if fungus gnats are important vectors of Fusarium spp.,
which cause serious diseases in container-grown conifer seedlings. Previous investigations indicated that
Fusarium spp. cause damping-off diseases on very young seedlings, probably as a result of seed contamina-
tion (James 1986, James and others 1987). However, Fusarium diseases may occur throughout the growth
cycle and are especially important on larger seedlings (James and others 1987, 1991). Fusarium proliferatum
is much more common on roots as seedlinqs become older. This species is not a common resident of conifer
seed, nor does it routinely cause damping-off. However, it is detected at increasing frequency during the crop
cycle (James and others, unpublished). When seedlings are lifted from containers, their roots may often be
extensively colonized with Fusarium, particularly F. proliferatum. This may occur despite lack of disease
symptoms either above or below ground. However, when disease symptoms are found, this same fungus is
readily isolated from roots of affected seedlings. Likewise, F. proliferatum can be extremely aggressive in
killing young Douglas-fir seedlings in controlled pathogenicity tests; most isolates studied so far are similar
in their high level of virulence (James and others, unpublished). Therefore, from the standpoint of improving
disease control in container nurseries, we are interested in understanding Fusarium epidemiology in general
and that of F. proliferatum in particular. The current evaluation indicated that F. proliferatum was only
infrequently isolated from bodies .ot adult fungus gnats. The other Fusarium spp. isolated from adult gnats
was F. sambucinum; its relative importance in conifer seedling diseases is generally unknown, although it may
be isolated from seed and roots of both healthy and diseased seedlings (James and others 1989).
The other fungus isolated from adult fungus gnats was Oidiodendron griseum. This fungus occurs in a variety
of habitats, but most commonly in wet forest soil (Christensen and others 1962; Singh 1976) and peat bogs
(Barron 1962, Dooley and Dickenson 1971). It is likely that O. griseum was a saprophytic inhabitant of the
peat/vermiculite growing media used in conifer greenhouses, existing in dead organic matter (Domsch and
others 1980).
6

FUNGUS GNAT CONTROL
Several approaches for reducing fungus gnat populations in greenhouses are available. yellow, sticky cards
can be used to monitor fungus gnat populations because adults are attracted to the yellow color (Parrella
1987). When little plant damage occurs from fungus gnats, direct control measures are unnecessary.
Reducing the proportion of organic matter in seedling growing media may help limit buildup of gnat popula-
tions (McHugh 1991). Since excessive moisture is required for maintenance of high gnat populations, wet
areas should be eliminated and moisture drainage from growing media improved when possible (King 1990,
Shrimpton 1991). It is especially important to avoid overwatering and provide adequate ventilation to ensure
that greenhouses dry out between irrigations (McHugh 1991; Robb 1991). Overall sanitation, such as removal
of greenhouse weeds and sterilizing surfaces of benches, floors, and walls between seedling crops will help
control gnat populations (King 1990, Robb 1991). If necessary, shore flies can best be controlled by reducing
algal growth by minimizing excessive moisture and by incorporating chemicals such as Agribrome® into
irrigation water (King 1990, McHugh 1991). Application of hydrated lime or copper sulfate to greenhouse
floors can also reduce algal growth (McHugh 1991).
Adult fungus gnats may be controlled using yellOW, sticky ribbons. Past experience (Shrimpton 1986)
indicated that populations can be controlled successfully in greenhouses by placing these ribbons at a
density of one per ten square feet.
Chemical pesticides should usually be applied only in response to either very high insect levels or noticeable
seedling damage (McHugh 1991). Pesticide applications on a routine basis during the growth cycle are
unnecessary and not recommended (Hussey and others 1969). Several pesticides that have given good
control of adult fungus gnats include diazinon, bendiocarb, acephate, and oxamyl (King 1990).
The larval stage is easy to control (King 1990) and biological control formulations are either currently available
or being developed for use against greenhouse populations of fungus gnat larvae. Parasitic nematodes
(Steinernema carpocapsae) are available under various trade names (i.e., Exhibit®) from several companies
(King 1990; McHugh 1991). Nematodes are applied as an aqueous drench directly onto the surface of
growing media; nematodes feed on gnat larvae just below the groundline. Another biocontrol agent is a brown
mite (Hypoaspis miles) that feeds on gnat eggs and larvae (McHugh 1991). Mites are produced as a mixture
of eggs, nymphs, and adults, and are sold mixed with a combination of vermiculite and peat which can be
used as the container growing medium. Mite preparations can also be used to treat greenhouse floors and
benches. A formulation of Bacillus thuringiensis called Gnatrol® has also proven effective in greenhouses
(King 1990). In general, parasitic nematodes give longer control because they are active for up to 6 weeks,
wherease B. thuringiensis treatments only last a few days. Therefore, several applications of B. thuringiensis
are necessary to control successive gnat generations.
Another approach to reducing populations of gnats in greenhouses is application of insect growth regulators
such as Dimilin® (King 1990). This material is currently being developed as an alternative to more commonly
used chemical pesticides.
CONCLUSIONS
In conclusion, our evaluation indicated that fungus gnat adults may be important carriers of potentially
pathogenic fungi in conifer seedling greenhouses. Although we only evaluated organisms carried passively
on bodies of adults, it is possible that some of these or other fungi might be disseminated shorter distances
by larvae. Specific fungal species may also be preferentially fed on by gnats. Correlations between high
fungus gnat populations and extensive damage by fungal diseases in conifer greenhouses have not been
documented. Nevertheless, many growers feel it is important to limit gnat populations to ensure production
of healthy seedling crops.
7

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