Silverleaf whitefly
Bemisia tabaci ()
Sternorrhyncha: Aleyrodidae
Silverleaf whiteflies are distributed worldwide in the tropics and subtropics.
Why are whiteflies difficult to control?
- they are very small and difficult to detect
- some biotypes have developed multiple resistances against insecticides
Host plants and damage
Most non-B-biotypes of B. tabaci have a narrow plant host range. However, the B-biotype is reported as being polyphagus.
Whiteflies are found on many important vegetables, such as Lycopersicon esculentum (tomato), Lactuca sativa (lettuce), Cucumis sativus (cucumber), Solanum tuberosum (potato), and others.
The feeding of adults and nymphs causes chlorotic spots to appear on leaf surfaces. Depending of the level of infestation chlorotic spots may increase in size until the entire leaf is yellow. The honeydew produced by feeding of the nymphs covers the leaf surface.
B. tabaci is a vector of several plant viruses in the genera Geminivirus, Nepovirus, Closterovirus, and Potyvirus.
Morphology & biology
Eggs are laid usually in circular groups on the undersides of leaves. They are white in color when laid and later turn brown. Eggs are pear shaped and about 0.2 mm long. Hatching occurs depending on temperature and humidity after 5-9 days.
The first instar is flat, oval and scale-like and is the larval stage, which is mobile and able to move to suitable feeding locations on the lower leaf surface where it moults and becomes sessile. The first three nymphal stages last 2-4 days each. The fourth nymphal stage is often called a puparium and is approximately 0.7 mm long. The puparium lasts for about 6 days.
Adults start copulating 12-20 hours after emergence. A female lives up to 60 days, whereas males live only 9-17 days. Adults do not fly effectively, but can be transported long distances by wind.
Monitoring
Numerous chlorotic spots on leaves of infested plants indicate the presence of whitefly. Close observation of the undersides of leaves will show the tiny yellow/white larval scales. When infestations are severe you will see adults flying after shaking the plant.
Pest management
Biological control
Conservation of natural enemies is important in field crops where feeding damage is the cause of losses, rather than virus transmission.
The genus Encarsia has 22 species recorded as important parasitoids on Bemisia sp. Other parasitoids attacking Bemisia belong to the genus Eretmocerus. There are also numerous records of generalist predators. However, the combined impact of these natural enemies is insufficient to prevent virus transmission.
Chemical Control
Several biotypes of B. tabaci are reported to have resistance against insecticides. It appears that no single control measure can be used on a long-term basis against this pest. The integration of a number of different control agents must be used to achieve effective control.