Whitefly

Pests of Field Crops in Southern Africa

WHITEFLY

(Hemiptera, Homoptera: Aleyrodidae)

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Text Box: The tiny, white, moth-like creatures known as whiteflies, which are familiar to gardeners and flower growers, cotton and vegetable producers, are the adult forms of minute sap-sucking bugs. The eggs are tiny and elongated (about 0.2 mm long), and are laid on young foliage to which they are attached by stalks. These hatch into flattened, oval and scale-like nymphs which bear functional legs in the first instar only. They are almost translucent white with a narrow, waxy fringe around the body, and reach about 0.8 mm in length. This larval stage, usually found densely spread on the undersides of leaves, is often wrongly assumed to be the egg stage. The nymphs give rise to four-winged adults, which are dusted with a waxy white powder. They are about 1 mm long with a wingspan of not more than 3 mm. They are easily disturbed on the plants and are noticeable when they fly off, literally in clouds, where the infestation is heavy. Bemisia tabaci is probably the most common species found in field crops at lower altitudes, although its taxonomy has become somewhat complicated. Trialeurodes vaporariarum is likely to be more common in glasshouse and sheltered garden situations, and highveld areas. In the last few years, however, mixed populations of both species have become common in the middle and higher altitudes particularly on field crops such as cotton. TOP

Text Box: B. tabaci has been recorded on cotton, tobacco, tomato, sunflower, soyabeans, mulberry and cassava in Zimbabwe, but is known to include potato, beans, cucurbits, sweet potato, cape gooseberry, safflower, cowpea, brassicas and others amongst its hosts. Trialeurodes spp. have been recorded on cotton, haricot beans, tomato, cucurbits and various ornamentals such as Calceolaria, Verbena and Fuchsia. In addition, whitefly populations are also maintained on a variety of weed hosts such as Lantana, Physalis and Ipomoea. TOP

Text Box: Whiteflies damage plants by sucking the sap from young leaves, and causing general debilitation. The nymphs do most of the damage, often causing yellowing and sometimes wilting and shedding of the leaves. Experiments have shown that heavy infestations on cotton can be responsible for such effects as early cessation of flowering, shedding of leaves and immature bolls and early signs of senescence. They are also thought to cause premature boll opening. Continuous feeding by all the feeding stages of this pest results in the production of copious amounts of honeydew (a sugary excretion of the insects). This coats the leaves, and can cause a serious quality problem in cotton lint (stickiness) and also encourages the growth of sooty moulds with the usual effects of unsightliness and possibly reduced photosynthesis of the leaves. B. tabaci is in addition responsible for the transmission of various debilitating plant viruses, such as tobacco leaf curl and cassava mosaic. It is of some concern that this species appears to have expanded its host range in recent years. TOP

Text Box: Whiteflies are usually found on the undersides of leaves and they prefer sheltered places such as dense foliage. They can reproduce sexually or by parthenogenesis (development of unfertilised eggs when males are rare). The rate of egg-laying increases markedly in temperatures above 30oC and slows down below 25oC. This partly explains their abundance in greenhouse crops. The eggs are laid on the undersides of leaves in small groups (B. tabaci) or sometimes in semi-circles (T.vaporariarum). They hatch in roughly a week in the warm weather, and the first instar nymphs are mobile for a short while before settling down to feed. The subsequent two instars are immobile, and they feed copiously with their mouthparts inserted into minor plant veins (except when moulting). They secrete a waxy covering over themselves to protect against desiccation, which also has the effect of protecting them from water-based insecticides. The fourth instar, still immobile, ceases to feed and soon the adult whitefly emerges from a T-shaped split in its back. Emergence tends to be in the early morning, but is temperature dependent. Within about four hours the adults are capable of flight. Until this time, they feed and attain a waxy coating. Within the first day, mating takes place, but as mentioned, is not necessary for egg laying to occur. Mated females lay both males and females, while unmated females lay only female offspring. The whole cycle from egg to adult can take as little as three weeks in the warmer months, but is probably closer to four weeks on average, and several generations exist in a year. Adults live for only three to seven days. TOP

Text Box: Control of this pest is usually by insecticides, with the concomitant problems of environmental pollution, destruction of non-target species, cost and the development of resistance. In fact, the latter has become a world-wide problem, often making whiteflies very difficult to control. It is true to say also, that in many cases, whiteflies would not reach serious pest levels if it weren’t for the misuse or overuse of insecticides for other pests. Certainly resistance exists in the region to organo-phoshate and synthetic pyrethroid chemicals already. Registered insecticides vary from crop to crop, but for cotton, chlorfenvinphos, demeton-s-methyl, dimethoate, pirimiphos-methyl, and synthetic pyrethroids have been used; for brassicas and ornamentals, dichlorvos has been used; and for ornamentals, synthetic pyrethroids have also been used. In addition, an "insecticidal soap" formulation has been registered for use on vegetable crops where chemical residues on the produce are unacceptable. It has little effect on adults, and must be applied to the undersides of lower leaves where the larvae are found. Some control may even be achieved by using ordinary detergent in weak solution to target the nymphs. Since whiteflies (more particularly B. tabaci) have become a serious pest in cotton crops around the world, a good deal of research has been conducted into their control by the use of insect growth regulators (IGRs), which are aimed at the juvenile stages rather than the flying adults. These products basically work by disrupting the moulting procedure, although some of them have slightly different modes of action. Two examples are buprofezin and pyriproxifen. Good results can be obtained and in order to avoid development of resistance, a system of “windows of application” (which restricts usage to certain periods only) should be followed. Further research has been carried out into the use of plant oils, to which it is believed the bugs would not readily become resistant due to the physical mode of action (suffocation). The main problem with oils, as with detergents or any chemical that works chiefly by contact action, is that good cover and even spread is required on the under surface of the leaves, where the nymphs are to be found. These products have the added advantage over earlier insecticides that they are much less harsh on the environment and on beneficial insects. There is a danger, however, that IGRs may have an effect on some natural enemies such as ladybirds, which can play an important part in the control of other pests such as cotton aphids. In many whitefly situations, one finds that there is a considerable controlling influence by tiny parasitic wasps. One of the common wasp species causes the nymphs to become blackened, and often 50% or more on a single leaf can be affected. However, the levels of parasitism at first may not prevent the whitefly population from reaching serious levels. In addition to the wasps, there are a number of predators of the whitefly larvae, some of them fairly specialist, such as various mite species and the smaller species of ladybirds, and others which are general predators such as various small predacious bugs and lacewing larvae. These tiny natural enemies are, however, very susceptible to insecticides, which should therefore be chosen with care and applied only if strictly necessary. TOP

Whitefly adults on cotton.

Whitefly nymphs on cotton.

Whitefly life cycle.