Proper soil moisture key to controlling fungus gnats

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Q. I have been fighting a losing battle with fungus gnats in my houseplants. I have tried sticky cards, butter on a brightly colored surface and, finally, insecticidal soap. I have caught many on the sticky paper, and the insecticidal soap works for about a day, but they are back the next morning in full force. Short of transplanting everything, do you have any solutions?

A. Fungus gnats are tiny dark-colored flies that commonly infest all kinds of houseplants. The adult flies do not damage plants but can be a nuisance because they fly in your face, land in your drinks, fall in your food and generally make pests of themselves. They often congregate near windows.

Their larvae can damage plants because they feed on organic matter in the potting soil, including small feeder roots that plants depend on to absorb water and nutrients from the soil. However, fungus gnats are most damaging to seedlings and freshly stuck cuttings; they are unlikely to harm established houseplants unless the population is extremely high. They are most problematic in our homes through the winter and early spring. These native insects are common outdoors, and we often bring them into our homes when we take plants outside for the summer and then bring them back indoors in fall.

One of the most important factors in getting fungus gnats under control is managing soil moisture. Larvae require a moist environment and may die if you can allow the soil to dry out thoroughly. Avoid overwatering, and do not allow plants to sit in saucers filled with runoff water. If possible, allow the soil to dry out pretty thoroughly between waterings. Never take it to the point where the plant wilts, though. You can also repot plants into fresh potting soil, which will get rid of existing eggs and larvae. Covering the soil surface with a layer of sand discourages egg-laying by adults.

There is a Bacillus thuringiensis var. israelensis (Bti) formulation sold under the trade name Gnatrol that controls fungus gnat larvae. It is used as a soil drench, meaning that you water the plants with it. Ultra-fine horticultural oil, neem and insecticidal soap may be sprayed on the foliage to control the adults.

Q. We have 6-foot-tall hemlocks along the back of our property. The lower branches, 2 to 5 inches off the ground, have had some of the bark chewed off, leaving an exposed area. I bought some Spectracide pruning seal (aerosol) and sprayed it on the chewed areas. No further damage was noted where I sprayed, but other branches were chewed. We have a number of rabbits that roam through our lawn area and leave many droppings and possibly eat flowers in the summer. Deer also live in our area. Is there a humane way to control rabbits? Is the pruning seal the best product I can use to help the trees recover? Can you recommend a good deer repellent?

A. From your description, rabbits, rather than deer, have caused the damage to your hemlocks. Rabbits often strip the bark from lower branches of trees and shrubs in search of specific nutrients. Deer more often browse the tips of branches, removing them completely, rather than stripping the bark.

The pruning sealer probably discouraged further nibbling because the taste and/or smell was unappealing to the rabbits. However, we rarely recommend pruning sealer because it interferes with trees' innate ability to heal damage from pruning cuts and animal damage. Despite the startling appearance of the damage, your hemlocks are able to heal themselves from rabbit damage.

There are a number of animal repellents that can help minimize animal damage, including Liquid Fence, Deer Away, Deer Scram, Plantskydd and many others. Some work because of the smell, others work because of the taste. Many discourage rabbits as well as deer. Rotate among repellents to avoid the animals becoming habituated to them.

Fencing the hemlocks off for the winter is another effective means of protecting them. To protect them from deer, the fence would need to be 8 feet tall. Woven wire fencing or heavy-duty deer netting are most effective. A 2-foot-tall fence of chicken wire or hardware cloth would effectively deter rabbits.


Send questions to Sandy Feather by email at or by regular mail c/o Penn State Extension, 400 N. Lexington Ave., Pittsburgh 15208.


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