Here they come again

 Are you and your houseplants troubled by fungus gnats, those pesky bugs that look like fruit flies, or mini mosquitoes? They come with the territory and your houseplants are their territory, or to be more precise, the soil in which your plants are sitting. If you’ve never been bothered by them, either you’re lucky, or all you grow are cactus and other desert plants — we’ll get to that later.

Fungus gnats are not a threat to animals or children, but they can seriously upset an adult. You’ll know you have them when they rise to greet you as you approach your plants, and they’ll head for your nose because after damp soil they have a fondness for the carbon dioxide you’re emitting. Fruit flies, however, are attracted to fruit rather than soil. Leave a grape on the kitchen counter too long and fruit flies are soon rolling it out the door. Their bodies are orange or brown and a little plumper than the gnats.

The only function of a fungus gnat is to mate and then lay tiny eggs on the soil of houseplants. Adults live about seven to ten days and deposit eggs on the moist soil surface or in soil cracks. Females lay from 100 to 300 eggs in batches of 2 to 30. Eggs hatch in four to six days; larvae feed for 12 to 14 days. The pupal stage is about five to six days. So as fast as you wipe out one batch, there’s another group hatching. The gnats don’t live long. After a week or so it’s off to the undertaker but then new ones arrive and it becomes an endless cycle — unless you take action.

Since you take loving care of your plants and never forget to water them, you’re providing perfect conditions to support them. They love that damp soil full of organic matter, which is why fungus gnats aren’t attracted to the sandy cactus soil of succulents. 

Soaking the soil with a variety of solutions is often suggested, although some can be just as toxic to the user. A safe one is mosquito dunks, which contain BTI, a type of bacteria that kills mosquito larvae, and it is effective on the fungus gnat larvae.

Other advice will recommend you let the plants dry out as much as you dare between waterings, at the same time fluffing up the soil surface with a fork, allowing it to dry quickly. This makes the environment uninhabitable for eggs and larvae. That’s fine if you only own one houseplant, otherwise the task becomes tedious and time consuming. Watering only from the bottom, however, can also help.

An easier solution that I’ve found effective is to add a layer of grit or fine gravel to the soil surface. This simulates a desert, and acts as a barrier to the egg-layers. It’s an attractive mineral mulch. Add a yellow sticky trap to catch the frequent fliers and that usually takes care of fungus gnats. You might even catch a few fruit flies too.