Barbara Brand
Adams County Master Gardener
The recent freeze warnings triggered bringing my outside houseplants, cacti, and other succulents inside for the winter. This act also can bring unwanted hitchhiking pests inside. A few days before coming inside, I usually give them close examination for visible critters such as mealybugs, aphids or scale that like to hide out under leaves or in leaf-stem intersections. Any plants having an obvious pest problem get sprayed with a blast of water first. If I find any pests, I try hand picking with alcohol swabs. As a last resort I use a spray with an insecticide labeled for a specific pest such as scale or mealybugs. If spraying an insecticide, always read the label first and do not just grab a "kills all" insecticide. Not all Insecticides treat all pests. Read and follow the label to ensure efficacy.
There is one obnoxious hitchhiker not always visible, but who will show up once inside - the fungus soil gnat. In fact, I very recently received a question about how to get rid of these pests. These gnats thrive in moist soil and love potted houseplants. You will know you have them when you see very tiny delicate, grayish or black, mosquito-like flies (1/8 inch long) flying around your plants, especially when watering. Fungus gnat larvae usually are located in the top 2 to 3 inches of the soil, depending on moisture level, and primarily feed on plant roots, fungi, and decaying plant matter.
Gnats are not strong fliers and emerge from soil when disturbed. Adult fungus gnats actually do not damage plants or bite people. But, the gnat’s presence is considered a real nuisance and their larvae are what actually damage a plant. In addition to seeing gnats flying around, plants may exhibit symptoms of severe infestations such as sudden wilting, loss of vigor, poor growth, and yellowing. Any plant is a target.
While fungus gnats can come in from the outside in plants you bring in for the winter, they also hitchhike home in newly purchased plants. Before purchase, poke up soil near the base of a potential new plant and look for glossy larvae or flying gnats. Regardless, it is a good thing to isolate any new plant for a few weeks and especially those coming in from outside.
Adult gnats live about one week and can lay up to 300 eggs in moist potted plant soils. Within
4-6 days tiny larvae emerge and begin feeding on plant roots during their two-weeks as larvae. Their pupal stage lasts 3-4 days then young adults leave the soil and begin the next generation. The entire life cycle from egg to adult may be completed in as little as 3 weeks depending on temperature. Because of this, plants can host each stage: egg, larvae, pupae, and adult in multiple generations at once.
Treatments aimed at the adults, eggs and larvae require different approaches. Yellow or yellow and blue sticky card "traps" are very effective at reducing a flying gnat infestation. These small cards are relatively inexpensive and perch in a plant’s pot hanging from a stick. One card is good for a large area, lasts for months, and can trap numerous gnats and flying pests. They are a very good approach to capturing the adults, particularly adult females, thus reducing the number of larvae in the next generation. I keep them in plants all year to monitor for their presence. By doing that, I do not see gnats flying around anymore. Another strategy to minimize fungus gnat problems is to cover the pot’s top surface with a layer of clean sand. That is another way to discourage adult gnats from burrowing and seeking organic matter for feeding and breeding.
For severe infestations, insecticides can be used on the soil’s surface since this is where new adults will emerge. The most effective treatments are those that are persistent for up to several days. Read insecticide labels to determine if they treat fungus gnats. Be sure to follow directions. Insecticides such as those containing insecticidal soaps, oils, and neem do not provide sufficient long-term control of gnat adults. Insecticides should be a last resort. It is much easier, and more environmentally friendly to plants inside your house to use hanging sticky traps to catch these unwanted hitchhikers.
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