January 7, 2016 – Kenosha News
Question: I read your column in December about aphids and other insects on houseplants. It didn’t mention anything about gnats in the dirt and on the leaves which are the problem in my houseplants. I read online about whiteflies. Do you think my plants are infested with whiteflies or is it a different insect? What can I do to can get rid of them? F.H.
Answer: Small gnat like insects infesting your plants are most likely whiteflies or fungus gnats. You need to take a close look at the insects and plants to decide if you have one or both of these pests.
Fungus gnats are 3/16 inch long, dark brown to black and have two wings. They are poor flyers so you often see them sitting on top of the potting soil or lower leaves of the plants. Fungus gnat larvae feed on decaying organic matter, fungi and plant roots.
Fungus gnats can be controlled by making their environment less hospitable. Keep the potting soil on the dry side and reduce fertilization. Heavily infested plants can be re-potted. Carefully wash the old potting soil off the plant roots before re-planting in fresh clean soil mix. Clean and rinse pots if you are reusing them. Make sure you get rid of the old potting soil – add it in your compost pile.
The other possibility is whiteflies. Whiteflies are 1/16th inch long with four milky white wings. When disturbed, the adults fly around the plant, quickly forming a cloud of insects. Adults and immatures are found on the undersides of the leaves but adult flies may rest anywhere on the plant. The nymphs are pale green, flat, and do not fly. Plants infested with white flies have poor growth, yellow leaves, and may have sticky, shiny leaves.
Whiteflies are difficult to control. Whitefly infested plants must be treated every five to ten days for several weeks or even months.
Insecticidal soap is a safe option especially indoors. Spray foliage and stems thoroughly to the point of dripping every five days. Insecticidal soap should not be used on some plants so check and follow label directions.
Azadirachtin, which is the active ingredient from the neem tree, is natural insecticide you might try. Several other insecticides, including horticultural oils, are labeled for whiteflies on houseplants. Always read and follow label directions for all types of insecticides.
Barb Larson is horticulture educator for the Kenosha County University of Wisconsin Extension. She holds a master’s of science in horticulture from the University of Wisconsin-Madison. If you have a plant or gardening question, email her at email@example.com or call 262-857-1942.