Don’t Let Fungus Gnats Plague Your Interiorscape Accounts

Unlike mealybugs or spider mites, fungus gnats or fruit flies can be twice as difficult to combat with their mobile abilities.

Since they are attracted to computer screens and can hover for hours, they are instantly noticed and annoying even the most patient person. It’s extremely rare that I get client complaints for mealybugs or spider mites, but every year, especially during the warm, rainy season, I always get a few calls concerning gnat issues. If you ignore this flying problem, you will have a very good chance of the client asking for all live plant material to be removed. With over fifteen years of dealing with these pests, I’ve learned a few things to help combat these tiny, winged demons.

Common Culprits

Starting my trade with two years of commercial greenhouse experience, I know just how fast fungus gnats can breed in moist soil. The most common type of plant to carry the gnat larva are flowering varieties. Violets, Chrysanthemums, and poinsettias that are grown on solid mats make the best breeding habitats for these flying pests. I’ve literally had thousands swarming around my head when trying to box hundreds of poinsettias for shipping. Since blooming foliage usually requires consistent moist soil, it’s the perfect environment for gnat larva to develop into adults – again and again. These flowering plants are the first I check when trying to locate a gnat infestation. If you prefer to be proactive, treating flowering material by chemically drenching the soil or adding the fungus gnat larva’s natural predator, the nematode, before installation will help prevent breeding environments and will save you and your client much frustration.

Search for Saturated Soil

After I go through the flowering material, I inspect all containers for over-watering. Foliage growing in moist soil – or worse – sitting in an inch or more of water can become a breeding ground for everything from mosquitoes to fruit flies. All it takes is for one to get inside, find that moist spot, and lay eggs to drive the occupants insane. Making sure all excess water is removed, placing dry soil on top of overly saturated soil, and instructing the service tech to reduce watering is the best way to prohibit any more contamination after treatment.

A Lingering Problem

What if you’ve taken every measure possible to find and treat infested plants yet your client is still complaining about gnats? Unfortunately, this happens quite often because flying insects utilize other breeding environments aside from potted soil. I’ve had pest control vendors blame my plants in spite of me taking every proactive measure with no sign of infestation. While I’ve wanted to argue with the pest control contractor, it won’t solve the client’s issue and may get the both of you fired. Here are a few additional steps I’ve discovered that could be the answer.

  1. Place sticky traps around the top soil of every plant. Since you can’t always see the pests hovering around, this is the best way to determine which, if any, plants are infested. The following week, you should be able to see at least a few tiny black flies stuck to the cards and then you will know which plants need to be treated or removed.
  2. While all the company plants maybe fungus gnat free, there are often personal plants sitting around the office that could be the culprit. After getting permission, I’ll inspect every personal plant and often find someone is killing their dish garden with kindness – over-saturating the soil and causing the gnat infestation.
  3. During my first year, I was going crazy at one three-story location that continually had gnat issues. My plants were treated, had clean sticky traps, no personal plants were allowed, and yet there were gnats. Then I thought of the drain traps. The part of the drain trap that curves often holds debris, creating yet another popular breeding location. I asked the building manager if someone from maintenance could be responsible for pouring half a cup of bleach down all the drains – especially the kitchen sinks – at least once a week. A couple weeks later, the gnat issue disappeared. And thirteen years later, I’m happy to report that the building never had another outbreak like that again.
  4. Check for leftover food and/or rotting fruit. Most companies collect trash and dispose at least nightly. This prevents most larva from having enough time to develop and spread from unwanted food. But, I’ve had a few cafeteria locations where I’ve seen ripe fruit, such as bananas, sitting in an open container with fruit fries hovering all around it. If timely disposal of trash and fruit left out in the cafeteria aren’t the issue, it could be deteriorating food in a personal office causing the nuisance.
  5. Puddles from outside containers and moist outdoor landscape environments can also breed gnats that are small enough to sneak inside through revolving doors, cracks in windows, or vent systems. This is often the case with lobbies and reception areas where people are continuously coming and going. The only solution I’ve found to combat an insect during flight, is to add ultraviolet bug lights in these spaces. Much like an over-sized nightlight, it’s a simple device that plugs into an outlet, illuminating a purplish coiled light that attracts the insect to the sticky board placed behind it. While this device won’t catch all flying pests, I’ve found it has worked as a good deterrent allowing employees to focus on their jobs rather than flying pests.

For a more in depth look at the seven most common indoor plant pests, check out this free Professional’s Field Guide on Plant Pest Control authored by Suzanne Wainwright-Evans, known as the Bug Lady.

Sherry has been part of the interiorscape industry for over fifteen years, starting at an entry level job at North Florida's largest greenhouse and currently owning two horticulture companies. At UMaine, Sherry majored in English where she worked part-time writing scripts for a local college TV studio.

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