Issue: October 30, 2004

Houseplant insects

As I was taking houseplants indoors from my garden where they spent the summer, I noticed that they had no insects. I remembered that this was usually the case, but in the middle of the winter insects would suddenly appear in great numbers. How can they appear suddenly when there are none when I bring them indoors in the fall? What can I do? In mid-winter it is too cold to take them outside and spray them with an insecticide.

Even though you don't see the insects, they are probably there (at least on some of the plants). While houseplants are outside, there are many predators that control the population of insect pests. When we bring them indoors, these predators are usually absent, so natural controls fail to keep pest populations low. When you bring them indoors, there may be small insects under the leaves or in other hiding places. There are probably insect eggs present on the plants.

I called Dr. Carol Sutherland, NMSU Extension Entomologist, about your question. I asked her if the insect eggs were dormant in the autumn like some plant seeds are dormant. She said it was possible, so eggs present on the plants would not hatch immediately but would delay hatching. However, she thought that a more important factor in the appearance of the insects was the slow increase from a few insects in hidden locations to large populations that suddenly seem to become visible. Once the plants with their few insects are indoors, the absence of insect predators allows for steady increases in plant populations.

Some of the most common indoor houseplant pests include spider mites, which are very small and not observed until they have developed very large populations. Aphids and mealy bugs are also common houseplant pests that can easily hide under leaves, between the leaf and the stem or below the soil line. Once populations reach a certain level, they are forced into more visible locations, but by that time their numbers allow rapid population explosions.

This information allows better management of pests. Treat with indoor safe insecticides (insecticidal soap, washing with strong mists of water) to maintain insect populations at low levels, preventing the sudden explosion of insect populations that you have seen in the past. Some insects may require more toxic chemicals treatments, and by treating in the autumn when there are more frequent warm days, the plants may be taken outside for treatment. They can be returned indoors after all the chemicals have completely dried. This is better than waiting for the insect population to explode before beginning to treat and not having the option to take the plants outside for treatment.

Treat the plants for any visible pests before you bring them indoors. Watch very carefully and treat before the pest population becomes large. Always choose insecticides labeled for the pest to be controlled and the environment in which the treatment will be applied. (Do not use an insecticide labeled for outdoor use on indoor plants.) Be sure you understand the label directions and then carefully follow the directions when you treat your plants.

Indoor gardening can make winter much more pleasant for gardeners, but proper management of insects is important to maximize this enjoyment. It is also important for the safety of our families and our plants.

Marisa Y. Thompson, PhD, is the Extension Horticulture Specialist, in the Department of Extension Plant Sciences at the New Mexico State University Los Lunas Agricultural Science Center, email: desertblooms@nmsu.edu, office: 505-865-7340, ext. 113.

For more gardening information, visit the NMSU Extension Horticulture page at Desert Blooms and the NMSU Horticulture Publications page.

Send gardening questions to Southwest Yard and Garden - Attn: Dr. Marisa Thompson at desertblooms@nmsu.edu, or at the Desert Blooms Facebook.

Please copy your County Extension Agent and indicate your county of residence when you submit your question!