What are whiteflies? Whiteflies are tiny, soft bodied insects. They are not true flies, but belong to the insect Order Homoptera which means "same wing", both the front and back wings are membranous and appear very similar. Other homopterous insects includes: aphids, scale insects, leafhoppers, and mealybugs. Whiteflies derive their name from white, waxy substances secreted by special glands on their abdomen. The adults coat their bodies, particularly their wings, with this wax-hence the name "whiteflies." The immature stages (nymphs) are also secrete wax in various forms, and the appearance and shape of these waxy strands, plates, and filaments is often useful in identification.
What sort of damage can whiteflies do? Whiteflies can seriously injure plants by sucking juices from them causing wilting, stunting, or even death. Whiteflies also secrete a sticky substance called honeydew. This material can coat any object, leaves, fruit, windshields, concrete walks, etc. on which it falls. In addition, several molds, collectively called "sooty molds" because of their black appearance, can grow on this honeydew resulting in an unsightly, sticky mess. Sooty mold can make fruit unmarketable and can block sunlight from reaching the leaf surface thus reducing photosynthesis. Whitefly adults can also transmit several viruses from diseased to healthy plants. These viruses are important mainly in commercial agriculture, but plants in backyard gardens and the landscape can also be effected. The most economically important whitefly transmitted viruses are the gemini viruses, so called because the virus particle is composed of twin identical appearing spheres. When populations are extremely high, large numbers of adults swarming around may become a pest during outdoor activities.
What is the typical whitefly life cycle? All whiteflies have a similar lifecycle. They pass through four instars or stages from egg to adult. Eggs are usually laid on the under surface of the leaf and may be laid randomly, in circles or arcs, or spirals. Some species of whiteflies deposit extensive amounts of wax around their eggs. The eggs hatch into a stage called a crawler. This is the only mobile stage, except for the adult, in the life cycle. The crawlers wander around the leaf until they find a vein into which they can insert their mouthparts and begin sucking sap from the plant. From this point until they emerge as adults, they are immobile and remain in the same location on the plant. The sessile nymphs pass through two additional feeding stages. The nymphs are usually oval or oval-elongate in shape, and are simple in appearance. Legs and antennae are reduced to only one segment and are generally not even visible. Nymphs may be yellowish or black in color. Yellowish individuals are normally associated with herbaceous plants and black individuals are usually found on woody plants. Fourth stage nymphs are frequently called "pupae" although they are not true pupae like those found in butterflies and moths. The "pupal" stage contains most of the characters used to identify whiteflies and is the only stage from which an accurate determination to species can be made. These characters include wax filaments, spines, pores, hairs, and grooves. Most are visible only with a microscope. since many whiteflies appear similar, an accurate determination to species usually requires an expert in whitefly taxonomy. The adults of some of the more common species appear different enough, however, that the lay person can identify the more common ones (see below). As the "pupa" matures, the yellow body pigment and outline of the adult may be visible. In some whiteflies, such as the silverleaf whitefly, bright red eye spots appear shortly before the adult emerges. When it is ready to emerge, the adult forces open a "T" shaped slit in the top of the shell and crawls out. This empty shell is called an exuvia. If the whitefly nymph was attacked and killed by a parasite, a round hole will be found on the top of the exuvia rather than a T shaped slit.
What are some of the most important whiteflies and how can I recognize them? Probably the most commone whitefly in California is the greenhouse whitefly. It is distributed throughout the state and is commonly found in outdoor plantings (gardens, landscape, and commercial agriculture) inside greenhouses, and occasionally on indoor house plants. It generally has long waxy filaments and a fringe of wax around the margin of the pupa. The adults have no markings on the wings and the wings are held flat over the top of the body. Eggs are usually laid in circles or arcs. They have a large number of hosts, generally attacking herbacous plants. Colonies tend to be rather "waxy."
The silverleaf whitefly is found throughout much of southern California and the San Joaquin Valley south of Stanislaus County. It is also found throughout much of the southern and southwestern United States. Adults are approximately 1/16 inch (1.5 mm) long and readily fly when disturbed. They have white, wax covered wings that are held tent like above the abdomen. The wings, which have no markings on them, do not quite touch each other and the yellowish abdomen is visible through the gap between the wings. The eggs are laid randomly on the under side of the leaf. The nymphs appear as small, slightly raised, oval insects that are white to yellowish in color. Silverleaf whiteflies have very little wax associated with their colonies. They have an extensive host range, and attack over 500 plant species.
The sweetpotato whitefly appears identical to the silverleaf whitefly and the two can only be distingushed from each other by a whitefly taxonomist. Their host range is similar to that of the silverleaf whitefly.
The ash whitefly is found throughout much of California. This whitefly produces large quantities of wax. The "pupa" has a very thick, waxy coat over the middle of the back as well as long fringes of wax along the edges. The adults have plain white wings with no markings and their bodies are covered with lots of wax. Broadleaf trees and shrubs serve as their main hosts.
The bandedwinged whitefly adult can be recognized by the two irregular zigzag smoky-gray lines transverse each of the front pair of wings. No bands are present on the hind wings. Eggs are usually laid randomly but occasionally may be deposited in neat circles. The nymphs have a marginal fringe of translucent wax filaments, and the dorsal medial area of the skin turns brown. This dark area on the top of the nymph readily distinguishes the bandedwinged whitefly from the greenhouse whitefly. Hosts include many common weeds and ornamentals.
The iris whitefly adult is slisght larger than the previously described whiteflies. The body and wings are quite waxy and the wings are held flat over the body. The wings are more rounded at the tip than are those of some other species, and they have a dark spot in the center of each back wing. The eggs are laid in distinctive circles and large amounts of wax can be found around both the eggs and the nymphs. Hosts include: iris, gladiolus, and other landscape plants, many vegetables, and cotton.
The giant whitefly is a native of Mexico and has only recently (1992) been found in the United States. The adults may be up to 3/16 inch (4 mm) long and it is one of the largest whiteflies found in North America. The wings overlap across the back and are mottled with black markings. Giant whitefly produces copious amounts of wax. The adults deposit spirals of wax as they walk on the leaves and the eggs are often laid amongst these waxy deposits. The nymphs produce long, hairlike filaments of wax up to 2 inches (50 mm) long. A large number of ornamental and agricultural plants serve as hosts.
How do whiteflies buildup so rapidly? Whiteflies, like many other organisms, increase in numbers exponentially, that is, 2 becomes 4, 4 becomes 8, 8 becomes 16 and so on. A single female whitefly may lay from 150 to 300 eggs during her lifetime. During the hottest part of the summer, whiteflies may mature, that is develop from the egg stage to an adult that is ready to lay more eggs, in as few as 16 days. In the absence of natural enemies, a few individuals can become thousands in a very short period of time.
What plants do whiteflies attack? Some whiteflies, such as the silverleaf whitefly and the greenhouse whitefly, attack a very large number of plants. Silverleaf whitefly for example, has over 500 hosts. Other whiteflies are restricted on their hosts and attack only a few plants. Host lists for the more common whiteflies are available under the description of each whitefly listed above. Simply click on "host" under each individual whitefly discussed to view some of the more common plants attacked.
How can I control whiteflies? Whiteflies are often very difficult to control. Their location on the underside of the leaf makes it difficult to deliver insecticides to the site where they are found. Also, whiteflies rapidly develop resistance to the insecticides used against them. If you have a whitefly problem, contact the University of California Cooperative Extension office in your county for the latest information on whitefly control or refer to the University of California Statewide Integrated Pest Management Project's Pest Management Guidelines or Fact Sheets. Remember, insecticides can be dangerous. If you chose to use an insecticide, read and follow the label directions carefully. Some whiteflies are currently under effective biological control (see below).
Is biological control effective in combating whiteflies? Whiteflies are attacked by a number of predators and parasitoids. Whitefly nymphs are eaten by green lacewings, lady bugs, minute pirate bugs, big eyed bugs, and damsel bugs. In addition, whiteflies are frequently attacked by a number of native and introduced parasites. The ash whitefly is now under very effective biological control throughout California. University of California scientists discovered a tiny parasitic wasp that feeds on the developing nymphs. This wasp has been released throughout the state and ash whitefly is now largely under effective biological control.
Are there methods other than insecticides and biological control available to manage whiteflies? Washing plants, particularly ornamentals, can sometimes be used to control whiteflies. Spraying (washing) with Safer-Soap® helps dislodge the whiteflies from the leaves. Follow label directions carefully. Plants should be washed at least weekly and more frequently during periods of heavy infestations.
Where can I obtain additional information on whiteflies? In addition to the sources listed on this page together with the links provided your best resource is the University of California Cooperative Extension office in your county. They have a number of publications available either free of charge or for a nominal cost.
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