Wednesday, February 13, 2008

Greenhouse - Fungus Gnats

Fungus gnats can ge a problem in the greenhouse when plants are kept too moist. The following is an article on the subject reprinted from the University of Maryland Greenhouse TPM/IPM Weekly Report

We are experiencing warmer than normal temperatures that heat up the greenhouse for several days, followed by cloud cover and rainy weather. This weather is ideal for fungus gnats which can feed on the roots of pansies. We have also had samples from growers where the larvae have bored into the stems, usually when the plants have been held too wet during cloud cover periods.

Many of you have pansies in the greenhouse at this time of the year and are getting ready to move them out into the market in the next 3 - 4 weeks (hopefully). Take care with your watering practices, applying the water when it is predicted to be sunny and warm. Try to water in the morning so that the plants can dry down during the day, and keep the plants on the dry side during the cloudy and wet weather.

We suggest monitoring for fungus gnat larvae in the soil by placing a potato wedge on the soil. Inspect it every couple of days using a 10 -16 X magnifier to examine the side that you had on the soil surface. The fungus gnat larvae are clear (early instar) to slightly whitish as they go into the later instars. They will have a black head capsule.


Bti sold under the name Gnatrol can be applied. It is recommended that you make three applications at 7 - 14 day intervals if using Gnatrol. The label recommends that the first application be made at a higher rate followed by two additional applications at lower rates.

Distance applied as a soil drench gives excellent long term control of fungus gnats. Distance is an insect growth regulator that prevents larvae from maturing. The kill is not quick, but it is very effective.

Entomopathogenic nematodes- we have had excellent success using the beneficial nematode, Steinernma feltiae, applied as soil drenches to control fungus gnats.

Reprinted from the February 8 edition of the 2008 Greenhouse TPM/IPM Weekly Report from the University of Maryland Cooperative Extension. Go to for the whole newsletter.

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