Ch 1: Whiteflies

Whiteflies

Credit:Biovision-Infonet

Scientific Name: 

Aleurodicus disperses, Aleurothrixus floccosus, Aleyrodes proletella, Bemisia tabaci, Trialeurodes vaporariorumOrder / Family: 

Homoptera: AleyrodidaeType of

Pest: Insect

Host Plants: Beans, Cabbage/Kale, Brassicas, Cassava, Citrus plants, Cotton, Cucumber, Eggplant, Green gram, Mango, Okra, Papaya, Peppers, Pigeon pea, Pumpkin, Sesame, Sweet potato, Tomato, Watermelon

1. Geographical Distribution in Africa

2. General Information on Pest and Damage

Damage

Whiteflies cause direct damage to plants by sucking plant sap and removing plant nutrients, thereby weakening the plants. Damage may be more severe when plants are under water stress. In addition, they often produce large quantity of honeydew that leads to the growth of sooty mould on the lower leaves, blocking or reducing the photosynthetic capacity of the plants. The honeydew also contaminates the marketable part of the plant, reducing its market value or making it outright unsaleable. Infested plants may wilt; turn yellow in colour, become stunted or die when whitefly infestations are severe or of long duration. 

Whiteflies are also serious indirect pests as vectors of virus diseases. Bemisia tabaci transmits serious virus diseases on cassava, cotton, tobacco, tomato, beans, chillies, and sweet potatoes. Whitefly transmitted viruses are among the most serious virus diseases on plants; Virus infection often results in total crop losses. This whitefly is the vector of a range of leaf curl disease-inducing virus, in Eastern and Southern Africa, including Tomato Yellow Leaf Curl Virus, the African Cassava Mosaic Virus, Cowpea Mild Mottle Virus, Watermelon Chlorotic Stunt Virus among others.

The African Cassava Mosaic Virus is one of the most important factors limiting cassava production in Africa. In sweet potatoes B. tabaci transmits the Sweet potato Chlorotic Stunt virus, which together with the aphid-transmitted Sweet potato Feathery Mottle Virus causes the Sweet potato virus Disease, the most important disease constraint to sweet potato production in Sub-Saharan Africa (Legg et al., 2003).

Major species of whiteflies in Africa:

• The greenhouse whitefly (Trialeurodes vaporariorum)
• The tobacco whitefly or sweet potato whitefly (Bemisia tabaci)
• The spiralling whitefly (Aleurodicus dispersus)
• The citrus woolly whitefly (Aleurothrixus floccosus)
• The cabbage whitefly (Aleyrodes proletella)

Host range

The tobacco whitefly (Bemisia tabaci) and the greenhouse whitefly (Trialeurodes vaporariorum) attack a very wide range of wild and cultivated plants. Bemisia tabaci is the dominating whitefly in the region. Its host range includes cotton, tobacco, vegetables (tomatoes, eggplant, okra, bell peppers, cucurbits, etc.), legumes (beans, soybeans, cowpeas and groundnut), tuber and root crops (sweet potato, cassava, potato) among others. The host range of Trialeurodes vaporariourm is similar to the one for Bemisia tabaci, but the former usually occurs at higher altitudes and cooler climates than B.tabaci. Trialeurodes vaporariorum attacks many plants grown under protected conditions (greenhouses) in temperate countries, the most severely affected crops are aubergine, cucumber, beans, sweet peppers, tomatoes and a large number of ornamentals. The status of this whitefly in field grown crops in the region is not clear. 

The cabbage whitefly (Aleyrodes proletella) is a pest of Brassicas but rarely reaches levels that require intervention. 

The citrus woolly whitefly (Aleurothrixus floccosus) is found mainly on citrus plants, but also attacks coffee (arabica), guava, eggplant, aubergine, mango, and several wild plants. 

The spiralling whitefly (Aleurodicus dispersus) feed on many plants. In West Africa, it has been observed causing damage on many food crops, including cassava, soybean, pigeon pea, citrus, papaya and others. This whitefly has also been recently found in East Africa. 
 Symptoms

Feeding of whiteflies causes yellowing of infested leaves. Whiteflies excrete honeydew, a clear, sugary liquid. This honeydew covers the lower leaves and supports the growth of black sooty mould, which may coat the entire plant. Where plant viruses are transmitted plants show the typical symptoms of the virus diseases. Presence of whiteflies can also be recognised by a cloud of tiny whiteflies flying up when the plants are shaken. The whiteflies resettle soon on the plants. 

Affected plant stages

Seedling, vegetative growing and flowering stage 

Affected plant parts

Leaves. 

Symptoms by affected plant part

Leaves: honeydew or sooty mould.

3. Biology and Ecology of Whiteflies

Eggs are tiny (about 0.2 mm long) and pear-shaped. They stand upright on the leaves, being anchored at the broad end by a short stalk inserted into the leaf. They are laid usually in arcs or circles, on the undersides of young leaves. Eggs are whitish in colour when first laid, but gradually turn brown. Some whiteflies deposit large quantity of wax around the eggs in the form of a loose spiral like a fingerprint. Hatching occurs after 5 to 10 days at 30degC depending on species, temperature and humidity.

On hatching, the first instar or crawler is flat, oval, very small (barely visible even with a hand lens) greenish-white in colour. It is the only mobile immature stage. It moves to a suitable feeding location on the lower leaf surface where it settles. It moults, loosing the legs and antennae, and cannot move throughout the remaining immature stages. They pass through two additional feeding stages, known as nymphs. The nymphs are usually oval or oval-elongate in shape, and are simple in appearance like small scale insects. Nymphs of many species produce waxy secretions around the margins and the dorsal surface of their body. 

The last (fourth) immature stage is known as puparium. In this stage the metamorphosis to adult occurs. The red eyes of the adult developing inside are visible through the skin (integument). As the other larval instars it is greenish in colour and is scale-like, but becomes more bulky shortly before the adult emerge. They are usually found on mature leaves. The adult emerges about 6 days after pupation. It usually emerges through a T-shaped split in the dorsal surface of the pupal case.

The total nymphal (immature) period last 2 to 4 weeks depending to temperature. Large populations may develop within 3 weeks under optimum conditions, and the lower leaf surfaces may be almost covered by immature stages. 

Adults are small (1 to 3 mm long), with two pairs of wings that are held roof-like over the body. They resemble very small moths. Their body is pale yellow. The body and wings are covered with a powdery, waxy coating. Whiteflies are mostly white, but can also be yellowish and some species have dark or mottled wings. They have sucking mouthparts. They are often found clustered in groups on the underside of young leaves and readily fly away when disturbed. A female may live for 60 days; life of the male is generally much shorter (9 to 17 days). 

Means of movement and dispersal

Whiteflies adults do not fly very efficiently, but once airborne can be transported long distances by the wind. All stages of the pest, but particularly the immature stages (which are small and easily overlooked) are likely to be carried on plant materials.