White Fly

Whiteflies are sap-sucking insects that are common in vegetable and ornamental plantings. They produce sticky honeydew making the leaves sticky, promoting the growth of sooty mould and, if left untreated, killing the leaf. When the biological control of whitefly becomes disrupted outbreaks often occur, which are very difficult to treat.

Whiteflies usually occur in groups on the underside of leaves. They derive their name from the white wax covering the adults’ wings and body. Adults are tiny insects with yellowish bodies and whitish wings. Plants and leaves that have large amounts of honey dew and sooty mould are indicative of the presence of whitefly infestations. Whitefly can spread several strains of plant viruses and the diseased plants are often stunted and discoloured. Greenhouse crops like tomatoes, cucumbers, eggplants and poinsettias are most severely infested.

Whiteflies develop rapidly in warm weather. Populations can build up quickly in situations where natural enemies are destroyed and environmental conditions are optimal. Most whiteflies, especially the most common species’, the greenhouse whitefly (Trialeurodes vaporariorum) and silver leaf or tobacco whitefly (Bemisia tabaci), have a wide range of host species.

Whiteflies draw phloem sap. Large population infestation can cause leaves to turn yellow, appear dry, or fall off plants. Like aphids, whiteflies excrete honeydew, so leaves become sticky or covered with black sooty mould. The honeydew promotes the growth of sooty mould, which can affect the activities of natural enemies that may control whiteflies and other pests. Feeding by the immature silver leaf whitefly, Bemisia tabaci, can cause plant distortion, discoloration, or silvering of leaves. Whiteflies cannot be controlled easily with any available pesticides. The best approach is to prevent glitches from developing in your garden or greenhouse. In many situations, commercially available enemies provide adequate control of whiteflies.

Whiteflies have many natural enemies, and eruptions regularly occur when these natural enemies have been disturbed or destroyed by pesticides, dust build up, or other factors. Whiteflies have a number of naturally occurring parasites that can be very important in controlling some species. Encarsia formosa and Eretmocerus eremicus are parasitoids that are commercially available for release in greenhouse situations.
The degree of natural parasitization can be evaluated in plants by checking for colour change of the whitefly nymphs after 10-20 days, depending on temperature. Those that have been parasitized will be black (Encarsia) or yellow (Eretmocerus) and have round or oval exit holes and those from which a healthy adult whitefly emerged will have a jagged exit hole. Another important aspect of whitefly management is letting the natural enemies continue to reproduce in the plant system and not disrupt it by using pesticides.
Some of the other methods of purging include hand-removal of leaves severely infested with the non-mobile nymphs and pupal stages. Water syringing may also be useful in extricating adults. Small, hand-held, battery-operated vacuum cleaner can also be used for vacuuming adults off leaves, in extreme situations. Most often, the use of sticky traps and tape are used in greenhouse environments.
Aluminium foil or reflective plastic mulches can repel whiteflies, especially away from small plants. Mulches have been shown to deter the transmission of viruses in commercial vegetable crops.

Encarsia Formosa is a species of black and yellow wasp which is used to control GHWF (Greenhouse White Fly) and is considered an effective biological control agent of GHWF on a variety of greenhouse vegetable crops, like tomatoes. Encarsia females lay eggs directly into the immobile whitefly scales which remain white and develop normally until the Encarsia pupates, turning the scale black. Encarsia Formosa reproduces offspring which are females without mating. Females require protein in their diet which is acquired by feeding on whitefly nymphs; this is called host feeding. Host feeding is very minimal compared to parasitisation in whitefly control.

Eretmocerus eremicus species is another wasp that can be found parasitizing whiteflies. Eretmocerus eremicus is lemon yellow in colour with green eyes and females have clubbed antennae. Eretmocerus has a 99.1% mortality rate enabling it to prevent an increase in SLW and GHWF numbers for most of the growing season. Success can be further enhanced by making use of a professional greenhouse scout and using the advice of an IPM specialist.