The Winter Gnat Scourge

For those new to Foothills gardening or even those who need a quick reminder, you may be wondering where the seemingly endless and bothersome invasion of tiny gnats in your house may have come from.  Who knew that when you brought in summer plant containers or got a new houseplant, you may also have been putting out the welcome mat for these tiny gnats?  These pests can also be found in terrarium moss on soils or light mulch around plants. Even if you’ve let the soil stay too moist in a favorite house plant, you were setting yourself up for this invasion.   Take heart as everyone deals with Soil Fungus Gnats at some point.  It doesn’t matter if it’s a houseplant or an outside garden plant – all are subject to this annoying pest.  Just remember, they are harmless to humans!



Fungus Gnats are small



These tiny insects are attracted to vegetables and herbs in addition to African violets, cyclamen, foliage plants and holiday poinsettias where they can do serious damage.   Fall and winter months are especially active.  The gnats, only 1/16” long can be easily mistaken for fruit flies, but are totally different.   Once indoors, the larvae in an infected plant feed on roots and root hairs, that when damaged may result in yellowed & wilted leaves, see photo. 



Orange Mint with Fungus Gnats





Being inside over winter months is perfect for their environmental needs.  House temperatures are conducive to their lifecycle breeding, while cooler temperatures near windows allows  water sources in soil to evaporate more slowly. Yes, these pests do drink water!!!  Typically, an infected plant may hatch larval eggs about every 4-6 days.  The lifespan of this gnat is around 10 days.  They are drawn to light as adult fliers, so you can check window sills or lamps for evidence.  In those ten days it can be hard when you’re trying to read or watch TV by a favorite lamp and these tiny critters use the opportunity to practice dive bombing you!

If you suspect you may have an infestation in a plant, here’s the best way to confirm your suspicions:

1)      Place a slice of raw potato into the soil of a suspect plant.  Larvae from this fly are attracted to the enzymes in the potato.  Check it after 24 hrs. or longer to see what’s going on.

2)       If larvae are found on the potato slice, you know the plant is infected.

The easiest and simplest control method is to let your plant/ container soils dry out between watering, if you’re able to do this without damaging the plant.  Caution … It may well become a battle of patience!  



If replanting infected plants, remember to use sterile soil medium. Bio-Insecticides are effective w/a soil drench, but yellow sticky sheets/tape work just as well for most of us.  For further information check out CSU Fact Sheet 5.584 on the CSU extension site, http://www.ext.colostate.edu/pubs/insect/05584.html