It is great to have green living plants in our homes during the cold and gray days of winter. Unfortunately, the indoor winter environment is tough on plants, and often plants under stress are more prone to problems from insects and mites.
Scale insects, mealybugs and spider mites all feed on plant sap and will cause a gradual weakening or decline in the plant health. Pests can increase unnoticed on houseplants through the fall and winter, until you suddenly face a sickly, stunted or dead plant.
The first step in insect and mite control on houseplants is to inspect regularly for pests. It is best to notice pests when they still have a low population and it is easier to gain control. Be sure to visually look above and below leaves as well as on the stem.
The common houseplant spider mite is the twospotted spider mite, a yellowish green mite named for the two large dark spots, one on each side of the abdomen. Spider mites a difficult pest to find since they are tiny and almost impossible to see with the naked eye on the plant. Spider mites do make webs where they are feeding. It is not large webs like a spider would make, but the webbing can sometimes be seen on the surface of leaves or at the juncture of the leaf and stem. Often soil or other debris gets caught in the webbing and so leaf undersides look dirty. You can also examine leaf undersides for crawling mites or their eggs that will look like small shiny spheres.
It is quite a bit easier to know when you have a scale insect problem. Most houseplant scale insects produce a lot of honeydew (their excrement that consists of plant sugars). If you notice a clear sticky substances on leaves or underneath the plant it is a good sign you have scale insects. Look for the actual scale insects on stems and leave undersides. Scales may be from 1/16 to 1/4 inch in diameter and are often brownish in color. Some scales are hemispherical in shape, while others are oval and flat.
Mealybugs are commonly in the axils where leaves join the plant stem. Each mealybug is oval and about 1/4 inch long though the body is concealed by filaments of white wax that may be up to 1/2 inch long. It looks a bit like the plant got in a fight with cotton balls and got bits of them stuck all over.
If mites or insects are present, the next step is to determine the extent of the damage and the value of the plant. It is usually easier, quicker and more convenient to discard a small plant that is heavily infested than it is to return it to good health and appearance. If possible, prune heavily infested portions of larger plants before attempting mite or insect control. Washing or syringing infested plant foliage may reduce light pest populations as will hand picking scales and mealybugs as they are found.
Houseplant insects and mites are notoriously difficult to control. Sprays can be used but success will depend upon thoroughness and persistence. Aerosol or hand pump insecticide sprays made just for houseplants are available at garden centers. Formulated active ingredients include insecticidal soaps, pyrethrin, resmethrin, permethrin, bifenthrin and plant oil extracts. Insecticides must be applied thoroughly on all plant surfaces (above and below leaves) repeatedly (weekly for a month or more) to get good control.