In Florida and elsewhere, silverleaf whiteflies, Bemisia argentifolii, are major pests of tomato, peppers, squash, cucumber, beans, eggplant, watermelon and cabbage. They cause economic damage in several ways. Heavy infestations can cause seedling death, or reduction in vigor and yield of older plants, due simply to sap removal. As whiteflies feed, they excrete honeydew, a sticky waste product. Sooty mold growing on this honeydew can reduce photosynthesis and fruit quality.
Feeding by immature whiteflies is associated with several physiological disorders. Tomato irregular ripening was first noted in Florida in 1987. Tomatoes on heavily infested plants fail to completely develop external color. On fruits that appear normal, the internal tissue may be white, hard, and unripe. Squash silverleaf disorder is another issue caused by whiteflies. This problem affects many cucurbits including squash and pumpkins. Feeding by immature whiteflies causes developing leaves to take on a silvery appearance. Fruit that develops on silvered plants may be bleached and of lower quality.
Silverleaf whitefly also vectors several serious plant-pathogenic geminiviruses. In Florida, the tomato yellow leaf curl virus, tomato mottle virus, and bean golden mosaic virus are major problems.
Survival And Spread
Adults whiteflies typically fly short distances moving around on the same plant or over to adjacent plants. Migrating individuals that develop on senescing plants may migrate up to several miles. Migrating individuals usually develop on plants that are senescing. These migrations can be massive and lead to infestation of nearby crops.
Mating occurs as soon as the whiteflies have expanded and hardened their wings, usually within a few hours. The female lands on a plant, tests the suitability of the host by probing the leaf with her mouthparts and ingesting a small amount of sap. If the host plant is deemed acceptable, she will insert her mouthparts into the phloem. While she is feeding, she may lay eggs — often in a semicircular arrangement — as she swivels her body around her feeding site. Females can lay between 66 and 300 eggs, depending on host plant and temperature. At 77°F, the eggs will hatch in six to seven days.
The first nymphal instar is capable of limited movement and is called the crawler. It is oval in shape and usually moves only a few centimeters in search of a feeding site, but can move to another leaf on the same plant. After they have begun feeding, they will molt to the second nymphal instar.
The second, third, and fourth nymphal instars are immobile. The nymphs secrete a waxy material at the margins of their body that helps adhere them to the leaf surface. Nymphs are flattened and oval in shape. The second and third instars each last about two to three days.
The red-eyed nymphal stage is sometimes called the “pupal stage.” There is no molt between the fourth nymphal instar and the red-eyed nymphal stage but there are morphological differences. The fourth and red-eyed nymphal stages combined lasts for five to six days. The stage gets its name from the prominent red eyes that are much larger than the eyes of earlier nymphal instars.
Adult whiteflies emerge through a slit in the integument of the last nymphal instar. The remaining white, transparent “shell” is called the exuvia. If the shell has a round hole in it rather than a slit, an adult parasitoid emerged.
Whiteflies rarely reach outbreak levels in natural environments or in agricultural situations where no insecticides are applied. In these situations, natural biological controls keep Bemisia below economically injurious levels. The most common parasitoids attacking Bemisia are minute parasitic wasps. Bemisia populations also are reduced by predation. Insects that prey on whiteflies include lacewings, minute pirate bugs, and bigeyed bugs. Several Coccinellid species also are whitefly predators. During warm wet weather, whitefly populations can be drastically reduced by natural entomopathogens. Beauveria bassiana is commercially available.
Several cultural controls can reduce Bemisia populations and their impact on crops. One important control method is sanitation. During the growing season, geminivirus-infected plants can be rogued out and destroyed. After harvest, crop residue should be destroyed immediately to reduce the survival of large numbers of whiteflies over the crop-free period.
Host plant resistance offers some hope. Tomato and bean breeders are developing varieties with resistance to geminiviruses, but commercialization is still some way off. Colored plastic mulches may be effective in reducing whitefly populations and geminivirus incidence.
Bemisia can be difficult to control with insecticides for two reasons; the insects infest lower leaf surfaces and are difficult to reach with insecticide sprays, and they have developed resistance to many insecticides.
In the mid-’90s, the introduction of new class of neonicotinoid-based systemic insecticides by Bayer CropScience (Admire and Provado, imidicloprid) gave growers a new tool in whitefly and geminivirus management in tomato. Over the past few years, labeling has been expanded to include a range of vegetables and second and third generation nicotinoids (Platinum, Actara [thiamethoxam, Syngenta] and Venom [dinotefuran, Valent U.S.A.]) on the market. Applied at planting, these materials have demonstrated excellent long-term residual control of whiteflies and whitefly vectored geminiviruses. Unfortunately, studies have shown that whiteflies in some areas already are showing elevated tolerance to neonicotinoid compounds. Recently, concerns about effects on bees has caused EPA to propose new regulations on neonicotinoid usage.
After the residual effects of soil-applied nicontinoids abate, growers may turn to a variety of materials to suppress whitefly populations. These include insecticidal soaps and oils and insect growth regulators like Knack (pyriproxyfen, Valent U.S.A.). In recent trials, pymetrozine (Fulfill, Syngenta) has been demonstrated to be effective in preventing viral transmission by whiteflies. Movento (spirotetramat, Bayer CropScience) and Oberon (spiromesifen, Bayer CropScience) have given excellent control of whiteflies in university trials.
Organic growers can use biocontrols like Mycotrol (Beauveria bassiana, BioWorks), insecticidal soaps, oils, and Neem-based materials (note: use of Neem products is provisionally allowed but regulated – check OMRI for status) for whitefly management.
Gene McAvoy is Regional Vegetable Extension Agent IV with the University of Florida, Institute of Food and Agricultural Sciences (UF/IFAS)