Controlling Fungus Gnats

Think this belongs in LOTR - a fungus gnat by Lord V
Think this belongs in LOTR - a fungus gnat by Lord V

TheGardenLady received this question from Devon.

Do you have any tips on controlling fungus gnats.  I have some in a herb planter, so i don’t want to use chemicals because I’d still like to be able to consume the herbs.

Fungus gnats look like tiny mosquitos that are about 1/8 inch in length. They lay their eggs in cracks on the soil surface which hatch into larva within 6 days and begin feeding on plant roots. After feeding for about 2 weeks, they pupate in the soil and emerge in less than a week as adults, to begin the cycle all over again. Usually when you first get them they are in the egg stage- in the soil of plants that have been outside for the summer or in damp bags of potting soil or from the nursery where they were bought. It is the larva stage that can do damage to young plants and seedlings by feeding on the new, tender roots. Their feeding stresses the plants and provides an entryway for disease pathogens.

Fungus gnats are usually a problem in the house or greenhouse. When you buy herbs, you should check to see that the soil is free of this problem. Fungus gnats can be a real problem to eliminate. Hopefully, you are finding them early so that you can eradicate them. If you have a really bad infestation, it might be easier to get rid of the plants and get new, healthy plants.

It is smart to practice IPM – which stands for Integrated Pest Management- as this is the most user friendly method of getting rid of pests on your plants. Stay away from chemical pesticides if you can. With fungus gnats, it is not really necessary for the home gardener to use any chemicals.

First of all fungus gnats like dampness. Are you over watering your herbs? Most popular herbs are native to areas that are dry. These herbs like sunny dry conditions when raised in your garden or when in pots. Herbs do not like over watering. Many herbs are drought-tolerant. Moisture is needed especially when the herbs are seedlings and moisture is needed to maintain active growth but don’t over do it. Water herbs thoroughly and then allow the soil to dry out somewhat before watering again. Plants should be watered early enough in the day that leaves can dry before nightfall.

It is advisable to get yellow sticky traps. For some reason insects are attracted to the yellow. You can see from the number of fungus gnats sticking to the traps just how bad the infestation is to plan your next step. Commercial greenhouses keep these traps all over to monitor their insect problems. Yellow sticky traps are sold in hardware stores.

Always keep plant areas clean. Get rid of weeds and dead debris which can be a breeding place for pests like fungus gnats. If the plants are in pots, see that the weeds and debris are discarded and not left near the pots. Don’t use fresh composted organic matter or potting mixes that contain fresh compost unless it was pasteurized first. In addition to feeding on plant roots, fungus gnat larva will consume organic material in the soil. Don’t use mulches or moist and decomposing grass clippings or organic fertilizers on the soil. Clean up free- standing water and eliminate any plumbing or irrigation system leaks. And be sure that the soil is well draining.

Put a moist slice of potato and keep it moist- on the soil. The potato surface sometimes attracts the feeding larva which can be used to collect and dispose of larva and to gauge when the larva are actively feeding.

There are some predators that you can buy. Two types of beetles feed on fungus gnat larva – rove beetles and ground beetles and there are some nematodes that will destroy the larvae. You will have to check for these products on line or in catalogs. And then there is a biological insecticide Bacillus thuringiensis subspecies israelensis (Bti) (the product is sold as Gnatrol) that can be applied to control fungus gnat larvae in container media. Bti applied as the label tells you to provides temporary control and is toxic only to fly larvae, such as mosquitoes, black flies, and fungus gnats. Repeat applications are needed for long-term control.  See here.