Control pests on indoor plants during winter

By Dan Gill
LSU AgCenter Horticulturist
This time of the year, when we have brought many of our container tropical plants inside for the winter, we need to be on the lookout for pest problems. When they happen, indoor pest infestations can be devastating if not dealt with promptly, effectively and safely.
Indoor outbreaks of insect pests can spread rapidly and cause tremendous damage because of the indoor environment. There is no rain to wash off insects, the temperatures are never too warm or too cold, and no natural predators are inside to help control insect populations once they get started. Insects spread rapidly because we often group houseplants together in well-lit locations close to windows or glass doors. We also do our share of spreading pests around by handling infested plants and then handling healthy plants.
Close and regular inspection of your indoor plants is the best defense against pest outbreaks. Three of the most common pests that appear indoors are mealybugs, scales and spider mites. If you can identify these problems in the early stages, you can reduce the amount of damage that occurs and prevent their spread to plants that are not yet infested.
Mealybugs are small, oval, soft-bodied insects usually less than 1/8-inch long, distinctly segmented and usually covered with a powdery or cottony waxy secretion. They are sucking insects, feeding on the plant’s sap much the way mosquitoes feed on our blood. Look for cottony masses in the growing points of plants, in their crowns, under their leaves and where the leaves join the stem.
Plants heavily infested with mealybugs will appear unhealthy. The leaves may have a shiny appearance and feel sticky, and the new growth may appear weak and deformed. Many older leaves will begin to turn yellow and drop off.
Scales are related to mealybugs and are also sucking insects. They are covered with a dome-shaped waxy coating that is most often white, tan or brown, depending on the type of scale. Once they are large enough to notice, they will have settled in one place and no longer move. You may notice the symptoms of scale before you actually see them. Like mealybugs (and many other sucking insects), scales will often cause plants to have shiny, sticky leaves. Even the floor or table where the plant sits may become sticky. This is the result of the accumulation of honeydew (a sweet, sticky excretion of the scale) on surfaces under the plant. If the population of scale insects surpasses the plant’s tolerance, the plant will begin to lose vigor, and leaves will turn yellow and drop off.
Spider mites are very tiny (most are not even visible to the naked eye), and the damage they cause is initially very subtle. This makes early detection difficult, and so populations are generally out of control and damage is extensive before the indoor gardener notices a problem. Initial damage to the foliage causes it to appear dull, faded and unhealthy. As damage increases, new growth may be faded, stunted and deformed, and older leaves may become faded, develop brown edges and begin to drop off. High populations of red spider mites will form fine webbing on the plant.
Virtually every plant we grow indoors is susceptible to one or more of these pests. When a pest problem is detected, prompt action is called for. First, isolate the infested plant or plants. All three of these pests are contagious. Always wash your hands after working with an infested plant, especially if you are about to handle healthy plants.
Remember that no natural controls exist indoors, so if the pest is to be eradicated, you’re going to have to do the job yourself. If you would prefer not to use a pesticide, physical control is worth a try but requires effort and persistence. Spraying the plant once a day with a strong stream of water (getting under the leaves especially well) will usually get rid of spider mites. Continue spraying for at least a week.
You can try removing mealybugs with a cotton swab dipped in rubbing alcohol, but the process is tedious, must be done repeatedly and often fails to do a complete job of control.
On small plants, you can try to control scale by dislodging them individually with your thumbnail (yes, this is tedious), then wiping the leaves with a damp cloth. Repeat once a week for as long as necessary.
If you decide to use pesticides, choose materials that are labeled appropriate for use on indoor plants and are safe to use on the plant you intend to treat. Mealybugs, scales and mites are all controlled by oil sprays, which kill pests by suffocation and are very low in toxicity. Year Round Spray Oil has a label for use indoors. Many insecticidal soaps and products containing pyrethrin also have labels for indoor use. They are excellent for mites and good on mealybugs but not very effective on adult scales. You can often find these in products premixed and ready to spray. Use pesticides cautiously and follow label directions precisely. Whatever product you choose, several applications will be necessary for complete control in most situations.