How to Remove Pesky Gnats From an Indoor Basil Plant

With a sunny windowsill and a bit of care, many herbs adapt to indoor life without a hitch. Annual basil (Ocimum basilicum) is somewhat trickier, needing temperatures consistently in the 70- to 80-degree Fahrenheit range. Like most indoor herbs, basil often attracts swarms of tiny, flying fungus gnats. The long-legged insects deposit eggs in the potting mix, where their root-chewing larvae may threaten the plant's health. To remove a fungus-gnat infestation, eliminate the larvae before they mature into egg-laying adults.

As annoying as they are, adult fungus gnats don't harm basil plants. They actually serve as a warning sign of overwatering, because they show up only when white fungus appears on the surface of excessively wet potting mix.

Each adult deposits 100 to 300 tiny white eggs in the fungus. The larvae eat the fungus, the decaying potting mix and plant debris. If those run out, they help themselves to the roots.

After two to three weeks, they pupate in the potting mix and emerge to lay their own eggs. Unless treated, an indoor basil may host adults, eggs and larvae all year.

Battle fungus gnat larvae where they live by drenching the basil's soil with commercially available Steinernema feltiae nematodes. The microscopic roundworms penetrate the pests, release toxic bacteria that dissolve them from the inside out, feed on the resulting broth and reproduce.

Water the basil well and move it out of direct sunlight. Sprinkle the nematodes, which are packaged in a dry carrier such as vermiculite, over the surface of the potting soil and water lightly. Use the amount of carrier the label recommends for the size of the pot.

After the nematodes dissolve and devour their original host larvae, they and their offspring move on to fresh ones. Repeat applications are rarely needed.

Bacillus thuringiensis var. israelensis bacteria starve gnat larvae by paralyzing their digestive tracts. To treat a heavy infestation, mix 2/3 to 1 1/2 teaspoons, or the Bti brand's recommended amount, of granules per 1 gallon of soft water.

Make enough solution to thoroughly wet the top 1 inch of the basil's potting soil, and repeat the application weekly for two more weeks. After that, cutting back to monthly applications with 1/4 to 1/3 teaspoon of granules per gallon of water prevents future fungus gnat problems.

When handling the granules, wear a long-sleeved shirt, long pants, protective eyewear, a respiratory mask and waterproof gloves. They'll protect against potential skin, eye or lung irritation.

Predatory Hyposapsis miles mites are packaged in carriers of vermiculite and peat. They feed and reproduce in the top 1/2 inch of potting mix, where each of them eats between one and five gnat eggs or larvae per day.

Each package contains adults, larvae and eggs. Moisten the potting mix, let the plant stand for 10 minutes and release enough mites to cover the surface. Store the remaining ones between 45 and 65 degrees Fahrenheit for up to two weeks.

Sometimes repotting is the only way to rid an indoor basil of fungus gnats. Lift the plant from its pot, hold it over a trashcan and shake the old potting soil off the roots. Replace that mix with a sterilized, quickly draining one.

If the pot has no drainage holes, substitute one that does. After repotting, check frequently and water only if the top 1 inch of potting mix is dry.