Issue: June 17, 2000

Fungus Gnats on House Plants in Summer


What should I do about soil gnats (fungus gnats) in my houseplants? They are taking over the house and going up my nose and driving me nuts. I dislike sprays/poisons and have set out many glueboards (which become covered with the creatures) but the gnats are still winning. HELP!!! Thanks! Kerry Biery Las Cruces


House plants are especially prone to pest problems because their indoor location isolates them from many natural controls for pests. The insect infesting your plants would be somewhat controlled by the rapid drying of the soil outside. We will talk about that means of controlling them later.

I'm glad you have tried the glueboards because they are a useful tool but not a means of eliminating the problem. The glueboards help remove the gnats, reducing their numbers in your home environment. The glueboards catch many but not all of these small insects. By the time they are caught, they have already deposited eggs in the potting soil, so the next generation is already on its way to your home.

There are other measures you can also employ. First, let the soil dry more between waterings. The key is to dry the upper inch or more of the soil where the larvae reside. The larvae are feeding on the fungus in the soil (they are fungus gnats) and reduced moisture should reduce the fungus and food supply for the gnats. This should dcrease the number of gnats after a while. Of course, don't dry the soil so much that the plants are injured.

If, after a couple of weeks, the number of gnats being trapped on the glueboards remains the same, then you may have to go to more drastic measures. There is an organic insecticide based on Bacillus thuringiensis that should be helpful. If this doesn't work, temporarily moving the plants outdoors and using a stronger insecticide may be necessary. After that, the reduced watering schedule should help slow the increase in gnat numbers.

Do not move houseplants outdoors into full sunlight or to a bright location, even if they are in a sunny window indoors. In the drier air outside, they will not tolerate the brighter light outside. Put them in a place protected from the wind so that they will not dry too rapidly. Since the potting soil will dry more rapidly outdoors, be careful that they do not dry excessively.

Apply any pesticide labeled for the purpose of controlling gnats to the soil, and allow the plants to remain outside until the chemical has dried or until the odor of the pesticide is not objectionable. The best time to apply the pesticide is after watering in the early morning. By evening you will probably be able to return the plant to its location inside.

Organic Foliar Fertilizer


Last year I burned a lot of plants using a spray-on fertilizer in the summer. Is there something organic that I can use without hurting my plants?


In defense of the spray-on fertilizers, a lot of people use them effectively. It is important to be careful not to make the concentration too strong and to apply them in the early morning while the temperatures are cool and evaporation minimized. The plants should be well-watered when these materials are applied.

To specifically answer your question: Yes, there are "organic" spray-on treatments. You can purchase fish emulsion fertilizer and, after diluting it according to the directions, apply it to your plants. You can also make compost tea or manure tea by soaking compost or manure in water until the water takes on the brown color of the compost or manure. This may also be sprayed over the plants. Don't make them too concentrated and, as with the commercial spray-on fertilizers, apply them early in the morning to well-watered plants.