How to Combat Whiteflies

Question:
Can you suggest the best methods to get rid of white flies, both in a New York City garden, and then as a precaution before bringing plants indoors for the winter?

Answer:
Sure enough white flies are one frustrating little pest, aren’t they? The main problem that sets white flies apart from the other insect pests is that they can produce so many generations a year. According to reference books here in our library, most species require 20-30 days for a complete life cycle, and that length of time can be even shorter during the warmer summer months. With numerous overlapping generations in a year infestations can happen quickly and can be very overwhelming, but I am more than happy to share with you my tips for helping to control them.

First, as I like to do with all insect questions, let me describe the pest and the damage so you know more clearly what you are looking at. Whiteflies are minute sucking insects that hide and feed on the undersides of leaves. If you shake a plant with whiteflies you will see them fly up and make a little fluttering cloud. Adults lay eggs on the undersides of leaves and these eggs are gray or yellow and roughly the size of a head of a pin. The larval nymphs emerge only a few days later as translucent little scales, also on the undersides of the leaves, and begin feeding off the plant. As they suck out the plant cells to feed they continue to go through different growth stages until they become adults only a couple weeks later. As you might guess, this feeding weakens and stresses out the plant. A good tell-tale sign of whitefly damage is the sticky honeydew that they secrete and leave on the leaf surface. This sugary honeydew can lead to sooty mold, a black fungus that grows on the honeydew covering leaves and fruit. Sooty mold is not likely to kill your plant, but it sure is ugly.

If you have whiteflies on plants that you still have outside, I would go ahead and treat the plants with an insecticidal soap right away. Safer brand insecticidal soaps and other such soaps you should be able to find at your local garden center or any larger retail store (Lowe’s, Home Depot, etc.). Read the instructions and spray and coat the infested plants as best you can. Remember that spraying the undersides of the leaves is crucial since that is where whiteflies spend most of their time. I believe the bottle will recommend spraying the plant to the point of runoff. As long as the leaves of the plants are not too fragile, like certain ferns might be, you can repeat application of the insecticidal soap weekly until you see the situation getting better. In addition to spraying, any physical removal of eggs or nymphs that you can do will be a big help as well. A soapy paper towel and/or swab with rubbing alcohol will help as you attempt physical removal.

Finally, when you move the plants back inside before frost, put out whitefly traps. You can buy them prepackaged or you can make your own. Get a piece of florescent yellow paper and cut it down to 2” x 2” squares. Coat the paper with petroleum jelly. Then use a cut-down plant stake, or some such support, to place the traps in your containers. Since they are not the most aesthetically pleasing, place them in the back of the pots so they are not such an eyesore. The bright color attracts the whiteflies and they get stuck to the petroleum jelly. Throw out and replace as necessary. This way you can keep track of the pests through the winter and you can tell if there are enough of them that another application of insecticidal soap is necessary.