Photo 3. Adult sweet potato whitefly, Bemisia tabaci, and skins of the fourth stage (puparium) also present.
Sweetpotato whitefly, tobacco whitefly, silverleaf whitefly
Bemisia tabaci. There are a number of closely related strains (or biotypes) that appear the same as the local strains, but can only be identified by molecular techniques. The two most important are the silverleaf, MEAM1 or B biotype, and the Q or Mediterranean (MED) biotype. The B biotype is also referred to as a different species, Bemisia argentifolii, and more species may be named in the future from within this group. Both spread many viruses, and are resistant to many insecticides.
Worldwide. Previously, tropical and sub-tropical countries, but now distributed widely as the whitefly has become a glasshouse pest, although parts of Europe are still whitefly-free.
Surveys in the late 1990s, recorded four biotypes in Oceania:
Originally, Bemisia tabaci had a narrow host range in the tropics and sub-tropics, feeding on cassava, cotton, sweetpotato, tobacco and tomato. But with the appearance of the silverleaf B biotype in the 1980s, the host range widened to more than 500 species, including many vegetables - beans (and other food legumes, including peanuts), capsicum, cucurbits (especially watermelon and squash), lettuce, papaya - ornamentals, poinsettia in particular, and a large number of weeds. The Q biotype has a similar wide host range.
Damage occurs in several ways: (i) the whiteflies have piercing mouthparts, and suck sap from the leaves removing nutrients, turning the leaves yellow, causing them to fall early and generally weakening the plants; (ii) whiteflies produce honeydew as they feed on plant sap, and this is excreted in large amounts onto the foliage. It contains sugary substances favoured by sooty mould fungi, and the leaves and stems turn black, reducing sunlight (see Fact Sheet no. 51); (iii) the B and Q biotypes produce toxic reactions in zucchini, squash, melon and tomato, spoiling their appearance; and (iv) whiteflies are very efficient at spreading viruses that cause severe diseases.
More than a hundred virus diseases are spread by this whitefly, among which begomoviruses (previously called geminiviruses) are some of the most important, causing, for example, African cassava mosaic virus, Cotton leaf curl virus, and Tomato yellow leaf curl virus. The latter is one of the most important tomato diseases worldwide, although not yet recorded from Pacific islands.
Eggs are laid in small groups on the underside of leaves; they are white at first, later brown (Photo 1). The first stage nymphs have legs and antennae, and are mobile; they are known as 'crawlers'. Later, they settle down, become pale yellow-green, flat, and scale-like, and pass through three more stages, the last is called a 'puparium' - the hardened skin of the third stage (Photo 2). When the adults emerge, they are a little over 1 mm long, yellow, with white waxy wings (Photos 3&4). The life cycle is completed in about 18-28 days, depending on the temperature, with the females living about two weeks, and laying up to 300 eggs.
The silverleaf B biotype is a major concern. It develops faster than the sweet potato strain, produces a greater number of off-spring, has a wider host range, is responsible for more sooty mould development, and induces a silverleaf disorder in tomato and cucurbits, squash in particular.
Both the B and Q biotypes have developed resistance to many insecticides.
Spread over short distances occurs with the movement of the crawlers, and the flight of adults. Over long distances, spread is by wind, and on plants distributed in the international trade in ornamental plants and cut flowers.
In general, local strains of Bemisia tabaci are a minor threat to crop production. They have been managed successfully by a combination of control measures, including the use of insecticides. However, in recent years, new biotypes have arisen which have become major pests. They impact on crop production by (i) breeding faster; (ii) spreading more viruses; (iii) infesting more plant species; (iv) rapidly developing resistance to insecticides; and (v) in some cases, causing toxic reactions in plants, or promoting sooty moulds, reducing growth or effecting quality.
On cotton, the whitefly honeydew makes makes it difficult to gin, i.e., separate the fibres from teh seed.
The effect of viruses is especially acute, with lost yields of cotton amounting to many millions of dollars annually in India and Pakistan, and in tomato production worldwide. Epidemics of African cassava mosaic virus and, more lately, Cassava brown streak virus, where yield losses of up to 25% have been reported in East African countries, continue to cause crop failures, and threaten food security.
Look for plants (especially at the margins of fields) with sooty moulds and/or silver leaf symptoms (on tomatoes and cucurbits). Look for whiteflies taking flight when foliage is shaken and immediately settling on the undersurfaces of the leaves. Look for insects with brilliant white wings, held tent-like over the body, but slightly apart when at rest so that the yellow body can be seen between them. Look on the undersides of the leaves for the stationary nymphs and puparia.
Natural predators include lacewing larvae (see Fact Sheet No. 270), big-eyed bugs (Geocoris spp.) (see Fact Sheet no. 370), ladybird beetles (see Fact Sheet No. 83), the larvae of syrphid (hoverflies) flies (see Fact Sheet No. 84), and predatory mites. They attack the immature stages of whiteflies. There are parasitoids, too, tiny wasps that are natural biological controls of whitefly populations. Species of Encarsia and Eretmocerus are those most successful, and some are commercially available for use in greenhouses. Eretmocerus attack both the sweet potato and silverleaf B biotypes, and are effective at higher temperatures.
Biosecurity authorities should take note of the risk that this whitefly presents to crop production, especially the silverleaf B and Q biotypes, consider pathways for their introduction, and ensure that regulations are adequate to prevent them becoming established. Assessments of the risks of introduction of this whitefly should include its ability to spread large numbers of plant viruses, many of which are extremely damaging to their hosts. To reduce this risk, authorities might consider the need for area-freedom of the more important whitefly biotypes and the viruses present where consignments originate.
Broadspectrum insecticides such as pyrethroids (e.g., deltamethim, lambda cyhalothrin, bifenthrin), neonicotinoids (e.g., imidacloprid) and organophosphates (e.g., chlorpyriphos) should be avoided, as repeated use promotes the development of resistant populations of whiteflies, and will also destroy natural enemies. Resistance to insecticides is a particularly serious problem in the control of the silverleaf B and Q biotypes, which have become resistant to a wide range of insecticides.
If insecticides are required, do the following:
AUTHOR Grahame Jackson
Information from Tobacco whitefly (Bemisia tabaci), and Silverleaf whitefly (Bemisia tabaci (MEAM1)). Plantwise Technical Factsheets, CABI; and from Q biotype Bemisia tabaci species complex. Department of Agriculture and Fisheries. Queensland Government. (https://www.daf.qld.gov.au/plants/field-crops-and-pastures/broadacre-field-crops/integrated-pest-management/a-z-insect-pest-list/whitefly-overview/q-biotype-bemisia-tabaci-species-complex). De Barro PJ et al. (1998) Distribution and identity of biotypes of Bemisia tabaci (Gennadius) (Hemiptera: Aleyrodidae) in member countries of the Secretariat of the Pacific Community. Australian Entomological Society 37, 193-287. Photo 1 Lesley Ingram, Bugwood.org. Photo 2 Pest and Diseases Image Library, Bugwood.org. Photo 3 Scott Bauer, USDA Agricultural Research Service. Photo 4 Richard Markham, ACIAR, Camberra.
Produced with support from the Australian Centre for International Agricultural Research under project PC/2010/090: Strengthening integrated crop management research in the Pacific Islands in support of sustainable intensification of high-value crop production, implemented by the University of Queensland and the Secretariat of the Pacific Community.
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