Extension Educator, Horticulture
Scratches on the car. An extra five pounds on the scale. Four foot tall weeds in the garden. Insects on houseplants. They appear overnight. We are sure they weren't there yesterday. The sources of some problems are obvious -- the car gremlin, the fat fairy and zombie weeds that creep into our gardens at night.
Insects on houseplants have their sources too. This time of year houseplant insects can reach high numbers. If plants spent the summer outdoors, many natural predators such as ladybugs and lacewing larvae kept the pest populations small. As soon as we bring the plants indoors, the predators are gone and pests can reach huge populations in a near perfect nursery. Insects may also spread from new plants brought home or from Aunt Alice's sick spider plant that you are nursing back to health.
Symptoms of houseplant pests are: yellow or brown leaves; dropping leaves; stunted or distorted growth; and sticky leaves or counters around plants. Sapfeeding insects excrete a sweet sticky substance called honeydew that can also grow a dark sooty mold. Honeydew can drip onto the table and floor. If it feels sticky under plants, look for pests.
Some of the main houseplant pests are: aphids; mealybugs; scale; and spider mites.
Aphids are small, globular juicy-looking insects. They are usually green, but may be any color. Aphids are always found in herds crowded on the new leaves. At certain times of the year aphids have wings. If you look close they appear to have two tiny exhaust pipes, too. For quick getaways I guess. Aphids can be found on just about any indoor or outdoor plants, but can be easily squished or washed off.
Mealybugs and scale fall into the category of "it doesn't look like a living thing since I can't see any head, legs or eyes". Mealybugs are about 1/8 of an inch long covered with white waxy strands like tiny balls of sticky cotton. Mealybugs are usually found where a branch or leaf connects with the main stem especially on jade plants.
Scale are brownish "helmets" less than an 1/8 of an inch long. Scale's favorite lunch stands include fig trees and ferns. Scale can produce lots of sticky honeydew.
Spider mites appear as tiny specks that move. Fine webbing can be seen on the undersides of leaves and where leaves or branches attach. Spider mites enjoy Norfolk Island pines, gardenias, ivy and most other houseplants.
Luckily houseplant pests are treatable. One product, insecticidal soap often sold under the brand name of Safer's, can be very effective for all of these pests. Insecticidal soap has very low toxicity for us, our pets and for beneficial insects. Insecticidal soaps do have precautionary statements on the label. As with all pesticides read and follow all label directions.
Insecticidal soaps are specially formulated to be used on plants unlike dishwashing detergent. Since insecticidal soaps are contact killers, good coverage of insects is essential. Be sure to spray the entire plant especially on the undersides of leaves.
Most insects require weekly spraying of insecticidal soap for at least three weeks. Scale is difficult to control and requires weekly spraying for 9 to12 weeks. Early control of pests is the best defense. If possible prune off some of the infested branches to reduce the populations and isolate bug-ridden plants from other plants. Some houseplants may be so heavily infested it may be better to feed your compost pile and buy a new plant. For more info on houseplants, check out http://urbanext.illinois.edu/houseplants/