This happens to a lot of people. It’s a little embarrassing but, to be perfectly honest: I’ve done it. Particularly in the winter months or right before a vacation. I’m here to tell you it’s okay and no one is judging you. You (or your customers) have overwatered your potted houseplants and now you have fungus gnats.
While this is a year-round problem, I think it happens a bit more in the winter. Folks have brought in plants from outside that they don’t want to freeze and they naturally want to water them so they survive. Unfortunately, overwatering that indoor grass can lead to a moist environment perfect for fungal growth. And that fungal growth means food material for fungus gnats. Just like fruit and phorid flies, fungus gnats can seem to spontaneously appear out of nowhere. It is not spontaneous, they've been there. It just took a bit and the right conditions for them to pop up.
Let’s back up a minute and talk about the basics of fungus gnats. There are actually a number of families of these true flies including Sciaridae, Keroplatidae, and Mycetophilidae as well as a few other small orders. This means there are literally thousands of species. Luckily, we don’t need to identify down to species and in most cases, as long as we identify them correctly as fungus gnats (and not fruit flies, phorid flies, moth flies, or any other small fly), narrowing down the habitat and treating is relatively quick and easy.
Once it has been established that the issue is fungus gnats, the trick is hashing out where the infested pot is. Once adult flies are seen, it’s likely not just one pot, it’s many. A high point is that adults don’t fly very well or very far, so the source of the infestation is likely close by. The low point is that most homes aren’t that big and have multiple, scattered houseplants throughout. In some cases of office buildings with lots of plants, it could be a widespread problem across a large area. It’s often a good idea to address all the potted plants at the site.
To deal with fungus gnat issues, just get rid of all the plants. Easy. Wait, the customer doesn’t want to ditch the weed? Fine. All the plants need to be dried out as much as possible for as long as possible. That means NO watering. The longer they can go, the drier the soil can get, the better in order to kill off all the lingering eggs and larvae as well as reducing the fungus. Once the soil has dried down a bit, sticky cards can be used in the vicinity to monitor where the highest concentrations of flies are. This way, you can see the reduction in numbers, and focus on what plants may still be having some moisture/fungus issues. Plus, the sticky traps will reduce the number of flying adults to a small degree so the customer will be seeing fewer.
There are some resources that say to use Bt products, insecticidal soaps, and other DIY remedies, but these rarely do more than temporarily reduce the problem.
Some fun facts on fungus flies:
Some species are bioluminescent as larvae – this is where the term “glow worm” comes from.
These can be very cold tolerant – some species have a natural “antifreeze” to help them survive.
Fossilized flies in amber have been dated back to the Cretaceous period.
Mushroom farms are particularly susceptible to massive outbreaks of fungus gnats – with folks doing a lot more home growing projects there may be mushrooms or other cultivated plants in homes now.
I wish I could say that I’ve never had pot problems: particularly as a pest control professional, it’s a bit awkward to admit I have the occasional pest problem. It just goes to show that it’s a problem that can happen to anyone at almost anytime. If the conditions are right, there’s a high chance of a fungus gnat issue. So chill out, find the budding problem, and don’t try to blow smoke at it. (Jodi and Joe say it's dope to keep them dry.) Adress the source and you can eliminate the flies. If you do continue to have problems (on fungus flies or any other urban pest), give us a call and we can help!
Lagniappe – some species can form large “chains” and be found moving in mass from one location to another: