Take That, Fungus Gnats!

Fungus gnats drive many a gardener to madness. Once they begin to breed in your house, they seem to have a desire to dive-bomb your coffee and get into your eyes. These tiny pests are a nuisance, to be sure, and you probably don’t want them around. The adult fungus gnat does not actually feed on the leaves—it only has the function of reproduction—but an infestation will harm your plants. Killing the adults rarely results in control as it is likely you have killed them after they’ve laid their eggs. To solve the problem, focusing on dispatching the larvae is the way to go.

Fungus Gnats Can Cause Damage

Infestations of large numbers of fungus gnats will produce an exponential increase in numbers of larvae, resulting in root damage that then causes yellowing leaves, loss of plant vigor, and often substantially reduced production. The larvae can also be carriers of disease, which is then exposed to all infested plants, and in some cases results in a complete loss of crop. Though fungus gnats are generally more of an annoyance than a threat, their presence should be taken seriously and measures to control and eliminate this pest should be initiated as soon as possible.

"Surface soil that is wet most of the time not only attracts fungus gnats, but does not help your plants."

Management & Control of Fungus Gnats

Once you have confirmed your crop has fungus gnats, there are a few considerations and practices you can employ to prevent and correct the situation. Knowing what attracts these gnats and how they grow and damage your plants is the first step.

Overly moist soil is the first consideration. Surface soil that is wet most of the time not only attracts fungus gnats, but does not help your plants. Root systems need oxygen as well as water and nutrition. Oxygen is essential for the plant’s respiration process which is responsible for converting sugars into energy. Without this energy, your plants are not going to produce much. Because of this, the objective for proper watering is to allow the surface (top three inches) to dry before applying water again. Allowing the lower section of the root zone to go dry is also not good, so deep, infrequent water is a good idea. You will need to sample moisture levels throughout the root zone and adjust watering schedules to achieve this. Different soil compositions will require different watering schedules so remember: one schedule doesn’t fit every situation.

If you are using a grow media like cocoa mulch or potting soil, there are lots of organic material in your media. This not only tends to hold water longer but is the primary source of food for fungus gnats. Organics may be what the plant wants for a living soil, but these gnats are not welcome to that party. One practice that can still provide a great organic soil structure while discouraging gnats is topping off the grow media with a half-inch of fine sand or silt. This material drains rapidly and dries quickly, and the dry surface discourages the adult gnat from laying eggs. The soil beneath the sand will remain adequately moist, however, you will still need to establish the best watering cycle to allow oxygen to penetrate deeper into the soil.

Once fungus gnats have infested the grow media of your crop, prevention is no longer going to do the job. Some type of biological or chemical control will likely be needed. As the objective is to attack the larval stage of this pest, focus on soil treatment. With ornamental plants, chemical control is often the preferred choice. Go to the strongest, most reliable killer as soon as possible. Cyflurthrin is very effective for gnat control. Simply add to the water when irrigating once every three to six months and fungus gnats will not be a problem.

With edible crops, most growers want to start with an organic biological control agent. Bacillus thuringiensis (BT) is a spore-forming bacterium that produces crystal proteins toxic to many species of insects and will usually eliminate fungus gnat larvae. Bacillus thuringiensis is organic and not toxic to humans or animals, though it does kill worms. Again, mixing the recommended amount of BT into the water periodically and drenching into the entire root zone is the best technique. Note that spraying the leaves will not affect the larvae living in the soil.

To rid yourself of the annoying fungus gnat adults, you will find the simple pest strip is the safest, yet most effective technique used to eliminate fungus gnats. Once your soil treatment has become effective, the adults will disappear and you can take down the strips.

Make your gardening more pleasant and productive by preventing these pests from infesting your garden.