Herb Pests: Fungus Gnats

Herbs are generally viewed as healthy, healing plants. They are often described as “the spice of the garden.” As a result, many people think herb gardens are free from pests. Unfortunately, this is false thinking.

Like all plants, herbs can attract pests of all sorts. Sometimes these plants get massive infestations. While the oils in some herbs may keep pests at bay, many herbs can become pest-ridden quickly.

What are Fungus Gnats?

Herb gardeners often grow their plants in containers, and potted plants are easily plagued by pests. Fungus gnats are the most common pests associated with indoor container gardens. Viewed with the naked eye, they are often mistaken for fruit flies.

In fact, fungus gnats look a lot like fruit flies, those annoying insects that hang around rotten bananas and leaky refrigerators. Fungus gnats are different from fruit flies, but both of these pest relatives require control and management.

According to the entomologists who study these insects, fungus gnats live in the top layers of plant soil. They feed on decaying material, including plant roots. Gardeners who purchase potted herbs may discover fungus gnats in the soil after just a few waterings.

Common Pest Control Solutions

Repotting herbs can help control these pesky insects. The best potting mixes do not contain rich compost or peat, however. Because these ingredients break down quickly, they provide young gnats with plenty of food, which enables them to grow, mature and multiply. Choose potting soils with charcoal and vermiculite instead.

Allowing the plant soil to dry between waterings is another way to control the fungus gnat population. Since these gnats like moist conditions, keeping the top layers of soil dry will keep the insect population under control and discourage infestations.

One way to control adult gnats is to pour a layer of sand over the soil and cover it with cedar mulch. Since sand drains quickly, the new dry layer will confuse the insects. The cedar mulch fragrance may also repel them.

Some gardeners use yellow sticky traps. These are effective for catching adult insects, but they do not work for the larval stages. Bacillus thuringiensis (Bt) products, which are designed for flies and mosquitoes, work well to control the immature stages of fungus gnats. Bt products are species-specific, so gardeners must make sure the product they choose works for the pest they want to control.

Insecticidal soap is another effective weapon for herb garden pest control. Spritzing this soap product lightly over an herb garden can knock down adult gnats quickly. An electrocution light, also known as a bug zapper, is another possible solution. Keep in mind that this light may also kill beneficial insects. Natural controls are the preferred pest control methods.

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