The greenhouse whitefly (Trialeurodes vaporariorum) is one of the major pests of vegetable and ornamental crops in greenhouses throughout the world. The greenhouse whitefly (Trialeurodes vaporariorum) originally comes from tropical and subtropical America, probably Brazil or Mexico. Hundreds of plant species from different families serve as suitable hosts for this whitefly. It occurs on many crops, including aubergine, beans, cucumber, sweet pepper, tomato, roses, gerbera and poinsettia.
The greenhouse whitefly (Trialeurodes vaporariorum) 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 white, oval case encircled by a ring of erect waxy rods. This is called the pupa stadium and these pupae are found on the oldest leaves.
The adult emerges from the pupa via a T-shaped fissure. The adult whiteflies can usually be found in the top of the plant and on the underside of young leaves, where they deposit their eggs. When shaking infested plants, adults will first fly, then return to the underside of the leaves. The adult greenhouse whitefly (Trialeurodes vaporariorum) 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 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.
- Viruses can be transmitted.
- The consumption of plant sap and secreting of honeydew by whiteflies reduce the aesthetic value of crops. This is particularly important in ornamentals.