Surprisingly but culinary herbs and even medical herbs do get pests! While it is true that aromatic oils in many herbs keep the pest away, the tender herbs can become pest ridden.
Herbs grown indoors can be easily plagued by pests. If you have purchased herb plants, you may have been surprised to find Fungus Gnat larvae in the soil.
Who are they?
Fungus Gnats are the most frequent houseplant annoyance. To the naked eye, they look like those annoying little bugs flying around old bananas in the fruit bowl. It is likely that these buzzers actually are not Fruit Flies, but a relative insect known as the Fungus Gnat (Sciaridae).
While Fruit Flies hang out primarily in rotten foods, exposed fruit and in leaky fridges, you will find Fungus Gnats in wet plant soil and in household drains. They are attracted to CO2 (Carbon Dioxide) which explains why they are always right up in your face. They are also attracted to the light so you can often see FG around windows. The good news - Fungus gnats are harmless to humans and animals.
Adult Fungus Gnats live about a week and a half, and in that span can lay up to 200 eggs in moist soil. FG larvae lives in the soil of your plants and feed off decaying material... and occasionally your plant roots. They love moist, loaded with humus, compost-rich potting soils.
To see if you have larvae in your container herbs cut a small potato in a half and set on top of the soil. FG larvae are attracted to the row potato and will move to the surface to feed on it. In a couple days, check the slices. Advice: skip this step unless you have a really strong stomach.
How do I get rid of them!?
To make FG larvae go away pour a generous layer of sand on top of the soil and cover with cedar mulch. FGs are attracted to the top layer of wet soil. Because sand drains quickly, adults are confused by the new dry top layer of soil, even though your plant is good watered. The cedar mulch is ornamental and most insects hate the smell.
Transplant the offending plants can help, make sure you remove as much soil as possible. Potting mixes containing cocoa fibre, charcoal, and vermiculite blends are all helpful since they do not decay as quickly as peat blends do.
You can also purchase a Bt (Bacillus thuringiensis) product for flies and mosquitoes which works good for Fungus Gnat control. Make sure you get the correct type of Bt. Several are available in the market and each is working on a particular type. It is not effective for adult FG though.
How do I kill the adults now?
One way to keep ahead of these little critters is to put up sticky yellow traps to catch the adults. These traps are hung over plants and cause insects to stick to the card like a glue. This does not control larvae in the soil, but will prevent flying adults from laying more eggs. Another solution is electrocution-light. The FG will be attracted to the light and killed upon contact.
I trust you are well armed with information now and next time you see FG hovering around your plants you will know exactly what to do.