Donald R. Lewis
Iowa State University
Not many insects are moving at this time of year. Our cold winter temperatures keep cold-blooded animals such as insects in a state of suspended animation - alive but not really doing much of anything.
Exceptions include those insects that find a way to stay indoors with us through the winter. If the house is warm enough for us to be comfortable, then insects will find it acceptable as well. Indoor insects include our best-known household pests such as cockroaches, stored food pests and accidental invaders such as ladybugs and attic flies. Other indoor pests pass the time as our guests on infested houseplants.
One of the easiest houseplant insect pests to recognize is the whitefly. These sap feeders are not really flies - they just look like it. Actually, I think they look more like tiny white moths, but they are the size of gnats, about 1/16 inch long. You might notice them sitting on the underside of an infested leaf, usually the youngest leaves. More likely you will notice the tiny adults when they flutter off the leaf as you water or handle the plant. They are easily disturbed but fly only a short distance before they quickly return to the leaf.
Whitefly adults are multi-taskers. The females eat and lay eggs at the same time. They feed by inserting their short, needle-like beak into the foliage and sucking out the plant juices. They can't bother to stop eating to lay eggs so they turn on their beak as they feed and consequently end up laying eggs in a semicircle around the feeding site.
After five to seven days the eggs hatch into tiny pale green immatures called nymphs. These crawl a short distance before settling down to feed in one place for the rest of their life. Nymphs suck out large quantities of plant sap for two to three weeks and then go into a non-feeding resting stage while they transform to adults. In four to six weeks the entire process from egg to adult is completed and ready to start again.
The immobile immature stages are small, flat, nearly-translucent, and easy to overlook. Consequently large populations can develop before you notice anything is wrong. Sap feeding by nymphs and adults can stress the plants and cause decline of plant vigor. Infested leaves may be stunted or yellow and may drop prematurely.
Typical of most sap feeders, whiteflies must eat large quantities of dilute sap in order to obtain the necessary nutrients. All that liquid and excess sugar ends up being excreted as shiny, sticky honeydew that may detract from the plant's appearance or lead to black sooty mold that grows on the foliage.
Whiteflies are not picky eaters. They have been found on more than 250 species of ornamental and vegetable plants, including many common houseplants. My favorite places to look for whiteflies are on poinsettia, begonia, lantana, hibiscus and angel trumpet.
Whiteflies are wimps. They cannot survive outdoors in Iowa. We may find whiteflies during the summer on flowers, vegetables and even weeds, but come winter they either freeze or starve to death from lack of host plants, whichever comes first. Only indoors, in the relatively tropical climate of our heated homes and offices, can the whiteflies survive a real Iowa winter.
It is very hard to get rid of whiteflies, so the first step is to do all you can to prevent infestations. Carefully check all new plants you purchase and the plants you bring indoors from the garden or patio in the fall. Keep a watchful eye on these plants. Check regularly and frequently for several weeks and be ready to launch the control assault at the first signs of infestation. One option for heavily infested or badly damaged plants is to give up and throw the plant away to minimize your losses and avoid spreading the problem to other plants.
Whiteflies are highly attracted to yellow objects, a behavior that is exploited in gardens and greenhouses by the use of yellow sticky traps. These sticky cards, stakes or tapes catch only the flying adults and are more appropriately used as a monitoring device. I wouldn't expect sticky traps to eliminate a houseplant whitefly problem, and wouldn't rate this as very useful in the house.
Washing infested leaves (especially the undersides) with a moist cloth or sponge is one way to reduce whiteflies, but is labor intensive and inefficient for large plants.
Finally, insecticides are available but control is usually marginal at best. Thorough application to infested leaf undersides is difficult to do and the eggs and nonfeeding resting stages are immune. Nymphs and adults can be sprayed to death, but repeat applications at weekly intervals are usually required. Systemic insecticides that are applied to the soil and taken into the plant via the roots may be available for foliage plants. Apply only ready-to-use insecticides specifically made for houseplants according to label directions.