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The Master Gardener Articles
I summered my house plants outside, and have noticed this winter that many of the plants are infested with insects. I want to identify them; what are the insects most likely to attack house plants?
Plants summered outside benefit from natural predators that keep pests under control outside; once back inside, pest populations can build up over the winter months. The common pests on house plants include two-spotted spider mites, scale insects, aphids, mealybugs and fungus gnats. All but fungus gnats feed by sucking sap from plant cells, causing foliar discoloration, leaf drop, stunting, distortion, and even plant death. Learning to identify these pests and how to reduce their numbers is important for successful indoor gardening.
Spider mites are not true insects. Two-spotted spider mites attack foliage plants inside. Mites are difficult to detect, being about one-fiftieth of an inch long, and are light in color with two dark spots on the back. Spider mites suck the contents from leaf cells, causing white or yellow speckles; as feeding continues the speckling coalesces and leaves become yellowish in color and may drop.
Populations of spider mites build up rapidly under warm conditions. One week there may be no signs of mites, and the next the plant is covered! There are overlapping generations of mites and numbers may build rapidly. When numbers are high, the underside of the foliage may be covered with fine silk threads, which collect dust and make the foliage appear dusty. Sometimes, tiny cobwebs can be found in the crotches of branches and where leaves are attached to stems. Spider mites don't travel very far very fast, but can move from plant to plant if leaves of adjacent plants are touching.
Scales, aphids, and mealybugs are related, and all of them secrete a substance called "honeydew." This is a sticky substance that can cover leaves and stems, or even drop from the plant onto objects below. Whenever a table or the carpet below a hanging plant is covered with a sticky substance, one of these insects is usually suspect.
Aphids are easy to detect; they usually occur in high numbers due to high reproductive rates and actively walk around on a plant. They are a sixteenth to an eighth of an inch long and are usually green in color, though they can be another color depending on species. They have a pear-shaped body and long legs. Aphids often feed in groups on the youngest plant growth, and can distort young leaves and stems. There can be both winged and wingless forms of aphids on a plant. This is an insect that can easily travel from plant to plant by walking or flying, so quarantining infested plants until they are "clean" is probably a good idea.
Scale insects are somewhat deceptive; they sometimes appear to be a natural part of the plant stem or mimic plant buds. They can be flat or rounded, are often brown in color, and don't move, making them difficult to detect. Scales are a tiny insect under a hard scale covering; upon hatching, the immature scales crawl around the plant, looking for a place to settle and feed. They secrete a covering over themselves that offers protection.
Mealybugs are a form of a scale insect, only their covering is soft. They are about three-sixteenths of an inch in length, white in color, fuzzy, and look like a tiny cotton ball. They move slowly on the plant, and are often found along leaf veins on the underside of leaves, or in the crotches of branches or where leaves are attached to stems, where they feed.
Gardeners who are a bit heavy-handed with watering may have trouble with fungus gnats. Adults are about an eighth inch in length and hover over the soil in containers. The gnats do not injure plants, but can be a nuisance. Their larvae are tiny maggots that feed on decaying organic matter and fungi in the soil mix, but if these sources are unavailable, they may feed directly on plants. On thick-stemmed and succulent plants, larvae may tunnel into stems and cause plants to wilt and eventually die. Larval feeding on young roots can encourage the development of root and stem rots. If this occurs, plants will be stunted with reduced growth and leaf drop.
OSU Extension embraces human diversity and is committed to ensuring that all educational programs conducted by Ohio State University Extension are available to clientele on a nondiscriminatory basis without regard to race, color, age, gender identity or expression, disability, religion, sexual orientation, national origin, or veteran status.
Keith L. Smith, Associate Vice President for Agricultural Administration and Director, OSU Extension TDD No. 800-589-8292 ( Ohio only) or 614-292-1868.
Revised November, 2006