Erl Svendsen and Bernadette Vangool
Saskatchewan Perennial Society
Aristotle proposed that some life spontaneously generated, or that living things came into being from non-living elements. He felt that this was certainly true for some flies. Louis Pasteur, on the other hand, demonstrated that spontaneous generations did not occur and hence we have pasteurization. But then, how do you explain the sudden appearance of swarms of fungus gnats that can occur around your houseplants?
Perhaps they aren’t spontaneously generated, but it sure doesn’t take long for a few small no-see-ums to develop into a cloud of fungus gnats that lift off when you disturb your houseplants the least little bit. And they’ll soon be found throughout your home, landing in your coffee or any open beverage. They can literally become so numerous that you become afraid of taking a deep breath for fear of sucking a few down. This “spontaneous generation” of fungus gnats can happen to even the most experienced of houseplant care-givers.
Back to the beginning – how can a few fungus gnats develop into such a huge problem? For a start, each female fungus gnat can lay up to 300 eggs over a period of 8 to 10 days. The eggs hatch in 4 to 6 days. The ¼ inch (6 mm) long, shiny, black-headed, white larvae feed for about 2 weeks before developing into pupa and emerging as adults 4 to 6 days later. The larvae eat decaying organic matter (peatmoss, dead roots, etc.) as well as fungi in the media – hence their name. When the number of larvae increases, they begin to eat living plant parts and your plants soon become affected. Moisture and temperature determine the development rate and survival of the larvae.
Fortunately, there are several methods that can be used to control fungus gnats. If your plants can tolerate it, keep the media on the dry side. This will limit the number of larvae that survive. (Bottom watering may help as the larvae typically stay in the top layer of soil). This strategy will reduce the number of fungus gnats but not eliminate them entirely.
Trying to kill the adults is difficult as they are airborne, and it is easy to miss some to many with a contact insecticide. An insecticidal soap containing pyrethrum is effective against adult gnats and is considered a low-risk pesticide (to humans that is). Regardless of the insecticide, avoid spraying pesticides within your home. Rather, do it in a well-ventilated area: outdoors or in a garage. You can use yellow sticky traps in combination with contact insecticides to eliminate surviving adults, but there will always be some that are smarter (or colour blind?) and survive.
Better to try to kill the larvae where they are trapped in the pot and can’t get away A larval control method used in recent years is a product called Bti or (Bacillus thuringiensis israelensis) which is sold as ‘mosquito dunks’, available at garden centres. Bti will attack larvae of mosquitos, and ‘true flies’, but is harmless to humans, pets, dragonflies and wildlife. Mosquito dunks are small donut shaped pucks. If treating pots with gnat infestations, dissolve one puck in a gallon of water, and water your plants with it. A friend was trying out this product and had good results with only dissolving a quarter puck. She replenished the container with water when it was half empty and used this diluted mixture for watering additional trays of seedlings. Her gnat problem in her tray upon tray of garden starter plants was over.
An ounce of prevention is worth a pound of cure. Always quarantine new plants coming into your home to ensure they don’t bring along unwanted visitors. This is especially true for houseplants you’ve put outside for the summer, tender bulbs and roots (e.g., canna lily) you’ve dug up to store indoors overwinter; or fresh herbs from your garden you’ve decided to grow indoors. Yellow sticky traps work great for monitoring. Always store opened potting media in airtight containers, this will starve any larvae that may be present in the medium. Remove any fallen leaves, flowers or fruit from the top of the media. Finally, avoid keeping your plants too moist, as this creates an ideal habitat for a new infestation.
This column is provided courtesy of the Saskatchewan Perennial Society (SPS; saskperennial@hotmail.com). Check our website (www.saskperennial.ca) or Facebook page for a list of upcoming gardening events.
