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Greenhouse Updates: Feb 23, 2018

Fungus gnats and shore flies observed in some greenhouses this week
Feb 23, 2018

Fungus gnats (Bradysia sp.) and shore flies (Scatela stagnalis) are commonly found in moist environments in greenhouses.

Fungus gnat larvae feed on microbes and decaying matter, but also feed on plant roots. They can be very damaging to seedlings and young plants. They also feed on developing callus and can delay rooting of direct stuck cuttings. Shore flies feed on algae and do not feed on plants. However the presence of adult shore flies can be a nuisance to workers and objectionable to customers.

Adult fungus gnats are small mosquito-like insects with long legs, long antennae and two clear wings with a Y-shaped vein near each wing. Adult shore flies have a robust body with short legs and antennae and about five whitish spots on the wings.

To manage fungus gnats and shore fly adults, monitor them by positioning yellow sticky cards just above the crop canopy. Count them weekly and record the numbers to see if the infestation is increasing or decreasing. Avoid overwatering, clean up algae and clean up any spilled media. Wet growing media encourages the microbes that larvae feed on, so growing dry can reduce numbers of fungus gnats and shore flies.

Biological control of fungus gnats has worked very well for many growers. Beneficial organisms available for control of fungus gnats include:

  • Steinernema feltiae nematodes - very effective for controlling fungus gnat larvae. Applications of nematodes should begin at the start of the crop because they are not good at reducing serious infestations. It is important to make sure that the growing media is moist before applying nematodes because they require moisture for dispersal. Follow the supplier’s application directions.
  • Gnatrol (Bacillus thuringiensis var. israelensis) - provide good control of fungus gnat larvae when applied as a drench.  To be effective, Gnatrol must be ingested by the larvae. For serious infestations, repeated application at high rates may be needed to provide good control.
  • Predaceous mite Strateolaelaps spp. (formerly known as Hypoaspis spp.) - can also give good control. Start releases soon after planting.
  • Predaceous rove beetle Dalotia coriaria (formerly known as Atheta coriaria) - also commercially available. Rove beetles are voracious predators that are mostly active at night. They eat almost any soil-dwelling insect or mite.

Chemical control of fungus gnats involves the use of pesticides that target only the larval stages and those that target the adults. For serious infestations it is important to target both larvae and adult fungus gnats.

Pesticides used for control of fungus gnat larval stages include: 

  • Pylon (Chlorfenapyr)
  • Citation (Cyromazine)
  • Distance (Pyriproxyfen)

Pesticides available for control of fungus gnat adults include:

  • Bifenthrin (Attain TR, Talstar P, Talstar)
  • Chlorpyrifos (DuraGuard ME)
  • Permethrin (Astral)
  • Pyrethrin (Pyganic EC)

Not all products used to control fungus gnats work on shore flies. Beneficial organisms available for control of shore flies include:

  • Steinernema carpocapsae nematodes
  • Dalotia coriaria
  • Strateolaelaps scimitus

Pesticides available for control of shore fly larvae include:

  • Citation (Cyromazine)
  • Distance (Pyriproxyfen)

Pesticides available for control of shore fly adults include Spinosad (Conserve SC, Entrust SC)

Cornell University : Practical Suggestions for Managing Fungus Gnats in the Greenhouse

UMass Extension: Fungus Gnats and Shore Flies

UConn Extension: Biological Control of Fungus Gnats

Geoffrey Njue, UMass Extension