Whiteflies
Species & Identification
Whiteflies are common pests of over 500 ornamental plant species in greenhouses. Adult whiteflies are moth-like and covered in a white, waxy powder. Adult females are around 1/16 inch long. Nymphs are flattened and look like a miniature scale insect.
The greenhouse whitefly was the primary species until the mid-1980s; since then, the sweetpotato whitefly has become the primary species. There are two biotypes of the sweetpotato whitefly. The B-biotype (also known as the Middle East-Asia Minor 1 or MEAM1 biotype) is more common and was formerly classified as the silverleaf whitefly. The Q-biotype (or the Mediterranean or MED biotype) was introduced to the United States in 2004 and is resistant to many insecticides. Bandedwinged and citrus whiteflies are occasionally found in greenhouses, but seldom become damaging. Citrus whitefly can be quite numerous on gardenias in the landscape.
The primary way to distinguish between the sweetpotato and greenhouse whiteflies is the appearance of the pupa, or final stage nymph. A sweetpotato whitefly pupa has sloped sides when viewed laterally and no fringe of waxy filaments around the edge. The greenhouse whitefly pupa has straight sides when viewed laterally and has a fringe of wax filaments along the edge that completely encircles the top surface. Adult sweetpotato whitefly is smaller than greenhouse whitefly and holds its wings tight against the body in a “roof-like” manner. Greenhouse whitefly adults hold the wings almost parallel to the leaf surface. There is no way to morphologically distinguish between the B- and Q-biotypes of sweetpotato whiteflies. If you suspect the presence of Q-biotypes, samples should be collected and sent for genetic analysis to confirm biotype.
Damage & Symptoms
Whiteflies feed by injecting enzymes into plant tissue and removing sap from the phloem, which reduces plant vigor. Feeding occurs exclusively on the underside of leaves. They also produce honeydew, which encourages the growth of sooty molds. The most common symptoms of whitefly feeding are stem blanching, chlorotic spots, leaf yellowing, and shedding. At high enough populations, plants can die.
Life Cycle
Whitefly life cycle is completed in 2.5 to 3 weeks under normal greenhouse conditions but can reach 2 months when cooler. Each female produces about 50 eggs in cool conditions and up to 400 eggs at higher temperatures. Eggs are deposited on the underside of leaves. New eggs are white to light beige but darken just before hatching. Newly hatched immature whiteflies move around for a few hours before attaching to the plant and feeding. The rest of the immature stage is immobile.
Scouting
Yellow or blue sticky cards, placed near doors and new plants, can provide information on the presence and movement of whiteflies. Randomly select 10 plants per 1000 square feet of greenhouse space to thoroughly inspect for the presence of whiteflies on the underside of leaves. Honeydew and sooty mold can alert a scout to plants that may be infested by sucking insects, such as whiteflies. Using a 10x hand lens will aid in observing all life stages of whiteflies on plant leaves.
Cultural Control
Use screens with mesh size < 0.19 mm to exclude whitefly adults from entering greenhouses. Maintain a constant weed management program and remove unsold plants to reduce alternative hosts and refuge sites for whiteflies. Do not keep personal plants in the greenhouse. Keep stock plants clean with a persistent pest management program to produce whitefly-free cuttings. Always inspect all plugs and liners for whitefly nymphs and eggs. Reject infested seedlings and liners from propagators. Plant liners and seedlings can be treated with a spray or dip of biopesticides or insecticides prior to planting.
Biological Control
Several biological control methods have been successful in controlling whitefly populations. Any biological control measure should be started preventatively to achieve success. Several parasitoids are available. Encarsia formosa can control the greenhouse whitefly, Eretmocerus mundus can control the sweetpotato whitefly, and Eretmocerus eremicus can attack both. Predatory mites (Amblyseius swirskii), predatory bug (Dicyphus hesperus), lacewings, and a beetle (Delphastus catalinae) can attack both whitefly species as well. The biopesticides Beauveria bassiana, Isaria fumosorosea, and Chromobacterium subtsugae are available as well. In poinsettias, dipping cuttings in horticultural oil or a mix of insecticidal soap and Beauveria bassiana has reduced whitefly populations for up to 8 weeks after planting.
Chemical Control
The most effective long-term option for controlling whitefly populations has been a medium drench or foliar spray with neonicotinoids. However, insecticide resistance has been developing due to over-reliance on neonicotinoids, especially in the sweetpotato whitefly Q-biotype. Medium drenches with flupyradifurone, spirotetramat, and cyantraniliprole are effective alternatives to neonicotinoids. Foliar insecticide applications can be made to control adults and young immatures. Foliar applications should be repeated weekly until whitefly populations are reduced. Insecticide resistance is highly likely to develop in whitefly populations. As such, one should rotate insecticide modes of action between each application. If Q-biotype sweetpotato whiteflies are suspected, do not apply acephate, bupofezin, neonicotinoids, novaluron, and pyriproxyfen.
