“God in his wisdom made the fly.. and then forgot to tell us why”. Odgen Nash’s short poem on the fly aptly describes the relationship of whiteflies to gardeners. While the short part is correct, the fly part is not. You see whiteflies are not flies at all, but are actually plant-parasitic Hemipterans. The classification is Classification Kingdom Animalia (Animals) Phylum Arthropoda (Arthropods) Subphylum Hexapoda (Hexapods) Class Insecta (Insects) Order Hemiptera (True Bugs, Cicadas, Hoppers, Aphids and Allies) Suborder Sternorrhyncha (Plant-parasitic Hemipterans) Superfamily Aleyrodoidea Family Aleyrodidae (Whiteflies). Ok, Why whiteflies this month? My annual trip into my crawlspace to get our pool equipment take me past a mature and very healthy Rhododendron plant. As I was opening the door to the crawlspace, I found myself covered in whiteflies. I had a dark shirt on and they must like that color. I stopped and shook the branches and went WOW! That’s a lot of whiteflies. Time to get out the sprayer. More on that later.
Whiteflies are small sap-sucking insects that feed on plant juices. They excrete sticky honeydew and due to their constant feeding cause yellowing and death of leaves. Large outbreaks are difficult to manage especially when populations are high. In addition this insect can develop resistance to pesticides due to its quick life cycles. Plant feeders like whiteflies do need proteins in order to lay viable eggs. Since plant juices are high in carbohydrates (fructose sugars) and very little protein, they feed constantly to get enough protein in their diet. This is why the sweet substance called “honeydew” is produced. They ingest more plant juices than they can digest and excrete a sweet substance. Ants love this honeydew and it is their primary diet in the summer months. If you have ever parked your car under a tree and came back and there is “sap” all over it… it is probably honeydew from the hemitperans (aphids, scale insects, mealybugs and whiteflies) living in the trees you parked under.
Whiteflies normally lay their tiny oblong eggs on the undersides of leaves. The eggs hatch, and the young whiteflies gradually increase in size through four nymphal stages called instars. The first nymphal stage (crawler) is barely visible even with a hand lens. The crawlers move around for several hours before settling to begin feeding. Later nymphal stages are immobile, oval, and flattened, with greatly reduced legs and antennae, like small scale insects. The winged adult emerges from the last nymphal stage (sometimes called a pupa, although whiteflies don’t have a true complete metamorphosis). All stages feed by sucking plant juices from leaves and excreting excess liquid as drops of honeydew as they feed.
Whiteflies use their piercing, needlelike mouthparts to suck sap from phloem, the food-conducting tissues in plant stems and leaves. Large populations can cause leaves to turn yellow, appear dry, or fall off plants.
Feeding by the immature sweetpotato whitefly, Bemisia tabaci, can cause plant distortion, discoloration, or silvering of leaves, and may cause serious losses in some vegetable crops. Some whiteflies transmit viruses to certain vegetable crops. Whiteflies are not normally a problem in fruit trees although their populations can build up in citrus, pomegranate and avocado. Several whitefly species occur on ornamental trees and shrubs, but most are uncommon because of natural controls such as parasites and predators. Most whiteflies on trees have limited host ranges. Low levels of whiteflies are not usually damaging. Adults by themselves will not cause significant damage unless they are transmitting a plant pathogen. Generally, plant losses do not occur unless there is a significant population of whitefly nymphs.
Control measures would be directed toward the immature and adult stages. If you choose to use insecticides, insecticidal soaps or oils such as neem oil may reduce but not eliminate populations. Systemic insecticides may be more effective but can have negative impacts on beneficial insects and pollinators. There are biological controls as well as Organic measures. Since whiteflies can develop resistance quickly, consult your Rutgers Extension Agent for current cultural, biological, organic, least toxic and chemical control. There are some new chemistries for resistance whitefly populations.
Editor’s Note: William A. Kolbe, BCE is a Board Certified Entomologist for Viking® Pest Control based out of Warren, NJ. He has a Bachelor’s Degree in Entomology with a minor in Ecology from the University of Delaware. Bill is a member of The Denville NJ Community Gardens. He can be reached at 800-618-2847 or visit www.vikingpest.com