The recent problems caused by whiteflies are addressed by developing
alternative control strategies based on integrated pest management (IPM), with
emphasis on biological control using indigenous and naturalised natural enemies
(parasitoids and predators).
The two major whitefly pests in South Africa are B-type Bemisia
tabaci (Gennadius) and Trialeurodes vaporariorum (Westwood). Both
species are serious pests and vectors of plant virus diseases of a number of
field and greenhouse crops worldwide.
Bemisia tabaci is the vector of a newly discovered tomato yellow
leaf curl-like disease, which has been designated Tomato curly stunt virus
(ToCSV-SA). Trialeurodes vaporariorum is a major pest on greenhouse
crops. Following a global trend, whiteflies in South Africa have developed
resistance to many chemical pesticides, including organophosphates, carbamates
and pyrethroids.
The recent problems caused by whiteflies are addressed by developing
alternative control strategies based on integrated pest management (IPM), with
emphasis on biological control using indigenous and naturalised natural enemies
(parasitoids and predators). The control of B. tabaci as a vector of
the new virus disease is being approached in an integrated fashion,
investigating the use of a range of different control measures.
General information
Whiteflies are pests of a number of field crops, such as cotton, tobacco and
sunflowers and also attack greenhouse crops and ornamental plants.
Whiteflies are tiny insects, measuring between 0.6 and 2 mm in length. They
are easily recognized because of their white appearance caused by a waxy powder
on wings and body. However, not all species are white; a few are dark grey or
black in appearance and are thus referred to as blackflies.
Whiteflies are not flies as their name implies but are related to aphids,
mealybugs and scale-insects. In common with other members of their group they
have piercing-sucking mouthparts which they use for feeding on plant sap. Both
adults and nymphs damage plants by extracting sap from the leaves, which can
result in severe yield reduction. When occurring in high densities, whiteflies
cause leaves to turn yellow, wither and drop off, eventually leading to plant
death. These insects also damage plants by producing honeydew, which provides a
substrate for fungi that reduce the respiration and photosynthesis of leaves. In
addition, whiteflies are transmitters of several viral diseases to plants.
The eggs of whiteflies are usually laid on the underside of plant leaves.
Whiteflies have two mobile stages, the adult and the first-instar nymph, which
is also known as a "crawler". Newly emerged crawlers wander, normally on the
same leaf where the eggs were laid, before they attach themselves to the leaf
with their mouthparts and become sessile. The three subsequent nymphal stages
are immobile and often covered with waxy excretions. The fourth and last nymphal
stage is commonly though erroneously referred to as a "pupa". During this stage
a tin-like pupal case is formed by wax excretions of species-specific structure,
and the nymph transforms into the adult. Unlike in most groups of insects, it is
the "pupae" and not the adults that are usually used for species identification.
Whiteflies can have several generations per year.
A number of natural enemies attack whiteflies including parasitic wasps,
predatory beetles and mites, and fungi. Some of them have been used successfully
in biological control. An example for South Africa is the control of the spiny
blackfly (Aleurocanthus spiniferus) on citrus by a tiny species of
parasitic wasp.
A preliminary survey of whiteflies revealed that the South African fauna
comprises about 20 species. This figure is rather small compared with other
African countries, such as Congo and Chad, whose whitefly fauna has been more
extensively studied, and more species are expected to occur in South Africa.
Contact: Dr. Kerstin Krüger
