The tobacco whitefly (Bemisia tabaci) belongs to the family Aleyrodidae, and the sub-family Aleyrodinae. This insect was first identified on tobacco in Greece in 1889, hence the name tabaci. It has subsequently been found in most tropical and subtropical countries of the world. The original habitat was probably a tropical or subtropical area, possibly Pakistan.
The tobacco whitefly (Bemisia tabaci) has an enormous host range and has affected an extremely wide range of crops throughout the world. It causes damage in (sub-)tropical areas especially. The tobacco whitefly (Bemisia tabaci) is feared because of its high level of resistance to many insecticides and its propensity for the transmission of viruses.
Life cycle and appearance of Tobacco whitefly
The tobacco whitefly (Bemisia tabaci) goes through six stages, namely egg, first, second, third and fourth larval stage (often referred to as ‘pupa’ although strictly spoken this is not true) and adult. The larvae are found on the underside of young leaves and have an oval shape. The first instar larvae (crawlers) are mobile, whereas the rest of the larvae stadia remain flattened on the leaf. The fourth larva stadium develops into a yellow, almost circular so called pupa, in which the red eyes and white wings of the adult are already clearly visible. These pupae are found on the oldest leaves.
The adult emerges from the pupa via a T-shaped fissure. The adult whiteflies are often scattered over the entire plant where they deposit their eggs. This is why on one leaf all different developmental stages can be found together. When shaking infested plants, adults will first fly, then return to the underside of the leaves. The adult tobacco whitefly (Bemisia tabaci) has well-developed piercing-sucking mouthparts and begins to feed on plant sap very soon after emerging. The insect is covered with a white waxy substance.
The adult of the tobacco whitefly (Bemisia tabaci) closely resembles the greenhouse whitefly (Trialeurodes vaporariorum), but is slightly smaller and yellower. More distinctively, the wings of the tobacco whitefly (Bemisia tabaci) are held vertical and parallel along the body.
The larvae of whitefly need a lot of protein for growth, and thus consume a large quantity of plant sap. This contains a high proportion of sugar, and the excess is excreted as honeydew, with larger larvae expelling large quantities. The damage that whiteflies cause to a crop is the result of sucking out the sap from the plant leaves and secreting honeydew. This can have the following consequences:
- If the population is very large, feeding on plant sap can affect the physiology of the plant, as a result of which growth is retarded. In full sunlight, leaves can wilt and fall. Such leaf damage can in turn influence the development of fruit and lead to a reduction in yield.
- The honeydew deposited on fruit makes it sticky. Dirt adheres to the fruit, and the growth of sooty moulds (Cladosporiumspp.) is encouraged, making it unsuitable for sale. In serious cases the fruit will rot. Sooty moulds also develop on the leaves, reducing photosynthesis and transpiration.
- Bemisia tabaci is known for the transmission of viruses, including TYLCV in tomato.
- The consumption of plant sap and secreting of honeydew by whiteflies reduce the aesthetic value of crops. This is particularly important in ornamentals.
- The larvae inject enzymes into the plant that alter their normal physiological processes. In some host plants, this can cause damage, including irregular ripening in tomatoes and sweet peppers, yellowing of the flower stalks in gerbera, and serious yellowing of the leaves of French beans. Other symptoms include the appearance of chlorotic patches, yellowing, fruit and leaf fall and misshapen fruit.