Horticulture/Whiteflies

The whiteflies, comprising only the family Aleyrodidae, are small hemipterans which typically feed on the underside of plant leaves. While feeding damage can cause economic losses, it is the ability of whiteflies to transmit or spread viruses that has had the widest impact on global food production. In the tropics and subtropics, whiteflies (Hemiptera: Aleyrodidae)have become one of the most serious crop protection problems. Economic losses are estimated in the hundreds of millions of dollars. While several species of whitefly cause crop losses through direct feeding, a species complex, or group of whiteflies in the genus Bemisia are important in the transmission of plant diseases. Bemisia tabaci and B. argentifolii, transmit African cassava mosaic, bean golden mosaic, bean dwarf mosaic, bean calico mosaic, tomato yellow leaf-curl, tomato mottle, and other Begomoviruses, in the Family: Geminiviridae. The world-wide spread of emerging biotypes, such as B. tabaci biotype B, also known as, 'B. argentifolii', and a new biotype Q, continue to cause severe crop losses which will likely continue to increase, resulting in higher pesticide use on many crops (tomatoes, beans, cassava, cotton, cucurbits, potatoes, sweet potatoes). Efforts to develop integrated pest management, IPM, systems aimed at environmentally friendly strategies to also reduce insecticide use will help re-establish the ecological equilibrium of predators, parasitoids, and microbial controls that were once in place. New crop varieties are also being developed with increased tolerance to the whiteflies, and to the whitefly-transmitted plant diseases. A major problem is the fact that the whiteflies and the viruses they carry can infect many different host plants, including agricultural crops and weeds. This is complicated by the difficulty in classifying and detecting new whitefly biotypes and Begomoviruses. Proper diagnosis of plant diseases depends on using sophisticated molecular techniques to detect and characterize the viruses and whiteflies which are present in a crop. A team of researchers, extension agents and growers working together are needed to follow disease development, using dynamic modeling, to understand the incidence of disease spread.

Symptoms and Signs
Whiteflies feed by tapping into the phloem of plants, which carries food down the plant to the roots. Plants have vascular tissues that carry water Xylem up the plant, where it combines with photosynthesis to produce the needed food for plant growth. The plants lose turgor and wilt, and may react to the whiteflies' toxic saliva. As whiteflies congregate in large numbers, they overwhelm plants quickly. It can get so bad that when you tap a leaf, a swarm of whiteflies will go airborne before re-settling under the leaf. They also excrete honeydew, which promotes ../Sooty Mold/ growth and can ruin a cotton crop with its stickiness.

In 1997 ../Tomato Yellow Leaf-Curl Begomovirus/, (TYLCV) was discovered in the USA, in Florida. This plant disease is the worst viral disease of tomato. The disease is transmitted by the whitefly, Bemisia argentifolii. The whitefly is also been shown to transmit almost all of the 60 known whitefly transmitted plant viral diseases.

Ecology
They have an unusually modified form of metamorphosis, in that the immature stages begin life as mobile individuals, but soon attach to the plant, and the stage before the adult is called a pupa (though it is not a true pupa in the sense of insects that undergo complete metamorphosis).

Control
Whitefly control is difficult. The greenhouse whitefly has developed resistance to many pesticides. The USDA recommends "an integrated program that focuses on prevention and relies on cultural and biological control methods when possible." They advise use of yellow sticky traps to monitor infestations and only selective use of insecticides.