Controlling Pests in Your Garden With Nature: Fungus Gnats
By Nathan Jackson – Natures Control
Oregon Cannabis Connection
The Pest: Fungus Gnats
Small, dingy gray flies, flying around plants aimlessly. Little white worms with black heads in the soil.
Fungus gnats (families Mycetophilidae and Sciaridae) are very common cannabis pests. They show up mainly in indoor and greenhouse grows, but can be found outdoors, too. They are not the most serious or devastating pest you might get, but they are a nuisance and do not go away on their own. In large outbreaks they can cause root damage and stunted plant growth.
Before doing anything, make sure you are dealing with fungus gnats and not something more serious like whiteflies. Fungus gnats are typically grey or black. They are about 2 mm in length and have long legs. They tend to “flutter” around the soil surface erratically but not pay attention to the leaves. Larvae live just below the soil surface. They look like small, nearly-microscopic, white or transparent worms with black heads.
Fungus gnats are attracted to wet soil and organic material that is starting to decay. They love coco coir-based soils and media. They are less common in hydroton- and perlite-heavy hydro systems, but they can settle into those as well if the conditions are right. Females lay several hundred eggs per week in the moist soil surface. Depending on temperature, the eggs usually hatch within two or three days. Larvae that hatch out feed on fungus that grows on decaying debris in the soil. If they do not find enough food, they will eat the fresh young root tips of your plants. This can expose the plant to other diseases. Within about 10 days the larvae will turn into pupae, and about four days later the pupae will turn into adults. A full generation takes about 17 days, depending on temperature, and there can be multiple overlapping generations per year.
Monitoring for fungus gnats will give you an early jump on them if they show up, and allow you to keep an eye on an infestation as you use control methods. Yellow sticky traps and cards are excellent tools. Fungus gnat adults seem to be attracted to them. You can hang yellow traps from lower foliage and even lay them on the soil surface. The adults will get stuck and die, leaving you with evidence of their existence. If you don’t have any fungus gnats, keep an eye on your traps so you know if they start to show up. If you already have a problem, monitor the cards to see if the population is increasing or decreasing.
Larvae are attracted to potatoes and can be used to determine whether you have a problem. Cut 1/2” slices of potato and leave them slightly buried or on the soil surface and check after 4–8 hours to see if larvae are present.
Controlling an active fungus gnat population is not hard, but requires persistence. A 1/2” layer of sand or a product such as Gnat Nix on the soil surface will discourage adults from laying their eggs in the first place. Products containing cedar oil have shown some limited control. Products containing Bacillus Thuringensis (or BT) seem to be effective.
Beneficial insects may be preferable, especially considering all the new pesticide regulations on cannabis grown in Oregon. Predator nematodes will attack the fungus gnat larvae in the soil. They are one of the least expensive and most effective controls available. Do a few releases every week or two if you have an active problem, or every four to six weeks for prevention. Fungus gnat predators (Hypoaspis miles, also known as Stratiolaelaps scimitus) can also be applied to the soil surface to attack the larvae.
Fungus gnats are a nuisance that most growers will experience at one time or another. With basic monitoring you can detect them early before they become a serious problem. Have some beneficial predators in the garden to attack them as soon as they show up.
Remember, an ounce of prevention is worth a pound of medicine.
Nathan Jackson is the owner of Nature’s Control and Ladybug Indoor Gardens. Located in Phoenix, Oregon, Nature’s Control, has supplied growers with beneficial insects for over 35 years. He can be reached at 541-245-6033 or firstname.lastname@example.org
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