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Fungus gnats

Larvae of fungus gnats, most commonly Bradysia species (family Sciaridae), can be a problem in container nurseries where populations may build up rapidly under favorable conditions. Although it is the larvae of fungus gnats which damage seedlings, adults are most conspicuous. They are small (2.5 mm), dark, mosquitoe-like flies, distinguishable from the often abundant shore flies (family Ephydridae) by clear wings, long legs and long segmented antennae (Figure 72). Adults run along the top of styroblocks or fly over seedlings, and although they do little direct damage, they have recently been shown to act as vectors for pathogenic fungi such as Botrytis and Fusarium.

Hosts and damage

Fungus gnats breed and populations increase in container nurseries if algae, decaying organic matter, and moisture are abundant. Larvae infest container cavities with no apparent host preference. Although larvae generally feed on fungi and organic matter, they are attracted to seedling roots predisposed by stress, and often accompany the root pathogen Fusarium. They prefer to feed on the upper roots, consuming root hairs and small rootlets. In heavy infestations, fungus gnat larvae (Figure 73) will strip the main roots leaving only the vascular tissue, and will sometimes girdle entire stems. Seedling wilting and sudden loss of vigor results. Well-established and vigorously growing seedlings are usually unaffected by the larvae.

Life history (Figure 74)

Under favorable conditions, fungus gnats complete their life cycle in 3 weeks, permitting rapid population build-up. Adults are attracted to moss and other organic matter on styroblocks or greenhouse floors, where they lay their eggs. Larvae emerge and are legless, semi-transparent, milky-white worms with black heads, reaching 5 mm in length.

Management

Sanitation and good drainage are the best methods of fungus gnat management. Keeping styroblocks, benches, and greenhouse floors free of mosses, algae, and excess water helps prevent population build-up. Producing vigorous healthy stock and avoiding over-watering makes seedlings less attractive. Population levels can be monitored with the use of yellow sticky traps (Figure 75). If fungus gnats persist and damage results, an insecticide drench may be necessary.

Selected References

Rutherford, T.A., D.B. Trotter, and J.M. Webster. 1985. Monitoring fungus gnats (Diptera: Sciaridae) in cucumber greenhouses. Can. Ent. 117: 1387-1394.

Shrimpton, G. 1983. Biology and control of some insect pests in B.C. Forest Service seedling nurseries. M.P.M. thesis. Simon Fraser Univ. Dep. Biol. Sci., Burnaby, B.C.

Wilkinson, J.D. and D.M. Daugherty. 1970. The biology and immature stages of Bradysia impatiens (Diptera: Sciaridae). Ann. Entomol. Soc. Amer. 63: 656-660.

Summary

Fungus gnats

Principal, locally grown hosts

Host age and season when damage appears

 

Nursery type and location

 

 

 

     

Bareroot

 

Container

 
 

Age

Season

Coastal

Interior

Coastal

Interior

All species

1+0
2+0

Throughou t growing season

No

No

Yes

Yes


Figures

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Click on this image to see a larger version     Figure 72. Adult fungus gnat (male).

 

 


Click on this image to see a larger version     Figure 73. Fungus gnat larva with damaged spruce germinant. Note the larva's semi-transparent body and black head.

 

 

 

 

 


Click on this image to see a larger version     Figure 74. Life history of fungus gnats (3-week life cycle; many generations per year).

 

 

 


Click on this image to see a larger version     Figure 75. Adult fungus gnats caught on a yellow sticky trap.