Whiteflies in the Fall

Some people tell me that they haven’t seen any whiteflies in their gardens this year, but I can tell you from my own observations that those pesky critters, like the principle villains in a horror movie, are out there right now.

If you are new to the area and unfamiliar with the beasts, look for tiny white insects fluttering in the air during the early morning hours.  The heavy fall flights haven’t quite started yet, but there are plenty of young ones storing up energy and getting ready to do their thing.  We start looking for them as the long summer starts drawing to an end.

The sweet potato whitefly usually becomes noticeable to most people towards the end of the August or early September as they flutter around in the early mornings.  Large numbers can usually be seen through September, depending upon how long the hot weather hangs around.  White, greasy stains on the car or truck windshield are often one of the first signs of heavy whitefly populations.  These small insects eat by sucking plant juices up a tube-like mouth part. Feeding by large populations can seriously damage, stunt or destroy a large variety of garden and landscape plants. 

The sweet potato whitefly was first identified as a serious plant pest about twenty years ago.  A tropical, heat-loving insect, it seems to thrive under desert conditions and is a serious threat to many area plants.  It damages plants by removing fluids and nutrients from the plants faster than the plant can replace them.  When susceptible plants are heavily infested, it is important to water and fertilize more frequently to help offset the feeding damage.  Whiteflies also cause feeding disorders in some plants.

These insects have yellow bodies with white wings covered with a waxy powder.  Other types of whiteflies found in Arizona include the greenhouse whitefly, which thrives in greenhouses, and the ash whitefly, which is found mainly on ash and citrus trees, none of which is as serious a threat to good plant health as the sweet potato whitefly.

Sweet potato whiteflies are small insects, about the size of a pinhead, and while they do not fly as gracefully as other insects, they are adept at moving from place to place.  Their style of flight is a simple flutter that serves to suspend them in the air while they either stumble upon a new host or are blown by the wind to a hospitable spot.  Airplane pilots have reported the impact of whitefly insects on windshields as high as 1000 feet above the ground, sometimes in relatively large numbers. 

Temperature is important to these animals.  Cool weather slows its reproductive rate while warm temperatures speed it up. That is why so many are seen at the end of summer and not during the winter.  They are here in the winter, but not in numbers that can easily be seen.

These particular whiteflies feed on a number of different kinds of plants.  Their host range is fairly large, ranging from vegetables, like lettuce, melons and squash to cotton where they contaminate open bolls of lint with their excrement; a clear, sweet liquid that reduces the quality of the cotton fibers by causing them to become sticky.  They also feed on many ornamental plants. 

Plants growing in the fall, however, are the most devastated when sheer numbers of whiteflies overwhelm the defensive mechanisms of many plants and leave them weak and stunted.  There is good news, though.  If the plants can survive through the summer into periods of cooler temperatures, they will quickly recover as the populations decline.

Whiteflies begin their life as they emerge from eggs laid by adults.  The immature nymphs are the most damaging forms because of the amount of nourishment they need to mature, but all stages, including the egg, pull fluids from the plant tissue upon which they rest.

When the adult whiteflies find a suitable host, such as lantana, hibiscus, grapes, annual flowers, herbs, or vegetables, they lay their eggs on the underside of the leaves, particularly along the veins of the leaves.  The eggs hatch into “crawlers” that move around until they find a good place to feed.  Once they have settled down, they molt and become what look like tiny transparent scales.  The immature nymphs continue to feed by sucking juices from the plant.  Eventually the insects emerge as the winged adults, which fly off to new plants.

Some plants are definitely more susceptible to damage than others.  When whiteflies are numerous, they may be found resting on virtually every plant, but they are not necessarily causing damage.  To determine which plants may have a problem, examine the underside of the leaves for immature nymphs or other signs of these insects.  Although the eggs and nymphs are difficult to see with out the aid of microscope or hand lens, the underside of the leaf will have the texture of sandpaper, and probably will have tiny droplets of sticky plant sap.  Plants heavily infested with immature whiteflies may turn yellow and wilt.  Sooty molds may grow in the excrement, called honeydew, causing the plant to turn an unsightly gray or black color.

Whiteflies have been shown to carry a number of virus diseases of annual plants, but they do not seem to infect trees and shrubs.  If an annual plant becomes infected with a virus, little can be done except remove and destroy the plant.

Control requires a series of steps to suppress the populations.  A good first step is to spray regularly the underside of the leaves on susceptible plants with a strong stream of water.  This helps wash off the young immature stages and even some of the adults.  Now is a good time to start doing that because most susceptible plants are starting to show heavy immature populations.

Insecticidal soap sprays are also a good way to kill the young insects.  The soap clogs up their breathing tubes and melts away the waxy outside layers.  If it doesn’t kill them outright, they eventually die from dehydration.  There also insecticide options available.  If you choose one of these, be sure to read the label carefully because some materials cannot legally be sprayed onto edible fruits and vegetables.  Okay, let’s be real here.  It is pretty tough to control whiteflies and even a combination of these options may not give complete control.  The best we can do is to help our plants limp through to the end of the season.

Perhaps the easiest way to avoid whitefly damage is to simply not grow those plants that are most sensitive to whiteflies, or if they are annual plants, when whitefly populations are greatest.  For example, delay planting fall vegetable gardens and annual flower beds until the second week in October, or until the whitefly numbers have dropped off.

While we do not have a really good method to control these garden and landscape pests, we can take hope that the cooler weather that is coming will slow them down once again.

If you have questions, you can reach one of the Master Gardeners at the Cooperative Extension office, 820 E. Cottonwood Lane, Building C, in Casa Grande.  The telephone is (520) 836-5221, extension 204.  The author’s email address is gibsonrd@ag.arizona.edu.

The University of Arizona is an equal opportunity, affirmative action institution.  The University does not discriminate on the basis of race, color, religion, sex, national origin, age, disability, veteran status, or sexual orientation in its programs and activities.

Rick Gibson
Extension Agent, Agriculture
University of Arizona Cooperative Extension
820 E. Cottonwood Lane, Building C
Casa Grande, Arizona 85122
Voice:    (520) 836-5221
Fax:    (520) 836-1750
email:    gibsonrd@ag.arizona.edu