I bought a plant online that came with an infestation of soil gnats, also called fungus gnats. They look like tiny mosquitoes but they don’t bite. They’re a nuisance to you but not harmful to you or your dogs and cats. However, they can, and will multiply fast. They can eat roots & vegetation if they go unchecked.
They're kind of like an iceberg. What you see isn't nearly as big as what's below the surface. The adults fly around, it's the eggs and larvae just under the soil surface that you should worry about.
The worst thing you can do is ignore them, like I did. Their lifespan is just a few days; "how bad can it get?" Ugh, It was almost like a cloud of them hovering over the plants. Yes, plants, as in plural. They easily migrated & found a new, larger home in my nearby Meyer Lemon tree.
The best thing to do is re-pot the plants with new, organic potting soil. Unfortunately, my nearby Meyer Lemon tree is way too big to consider re-potting.
I tried the small saucers of beer, apple cider vinegar and wine numerous 'experts' recommend online. They all did a good job attracting & drowning the adults. I don’t recommend the apple cider vinegar in any room you spend much time in. Phew! But these liquid 'traps' barely made a dent in the thriving throng of mini-mosquitoes that had invaded my home.
I bought the yellow stickers. They were more effective than the various liquids at trapping the little buggers but again, just the adults. Not to mention kind of gross looking.
I bought mine on Amazon because there was a free next day delivery option and I wanted them ASAP. $5.99 for a pack of 8 stickers. Not too bad of a deal.
Just trapping adults doesn’t solve the problem. It's the larvae & eggs in the top layer of the soil. I tried pouring organic insecticidal soap. It seems to have slowed them down but they came back with a vengeance about a week later.
Here’s What Works - Get Rid of Soil Gnats Permanently
Sand. There’s sand made for soil cover. I bought some on Amazon. I paid $4.18 for a 5 pound bag. They come in a few different color choices. I’m sure most garden centers carry it. Since I have Amazon Prime, shipping was free and again, next day delivery was awesome.
Simply spread a layer of about 1/2 inch of sand over the top layer of your containers soil. You may need to take out a little soil to make room. Make sure to spread the sand all the way to the edges.
Sand doesn’t have any organic matter. The gnats can’t lay eggs in it. They can’t get through it and the larvae trapped in the soil underneath the sand eventually die.
Just be careful when watering the plant. The water can soak the sand down into the soil. In may case, it’s a lemon tree, a little sand will just help the drainage which is perfect. I bought an extra couple of bags anticipating this. You also don't want watering to create any gaps in the sand where the soil is exposed.
Honestly, the best thing about the yellow sticky traps was putting them in after I put the sand on the top layer. It helped trap the alarmingly large swarm of frantic adults all over the sand.
A decorative & functional solution. Just be a little patient. These soil gnats can live up to 10 days. You'll still see the ones that were already out floating around for a few days.