Adults and nymphs of Dialeurodes citri colonizes the underside of the leaves of citrus. Whiteflies can be observed flying in clouds when trees are disturbed. A closer look at the lower side of leaves also shows flat, elliptical, scale-like nymphs, closely fastened there. Heavily infested tissues are often covered with sticky and transparent honeydew, that can drop onto the upper surface of underlying leaves. Black sooty mold grow on this sugary substance and the foliage may acquire an almost completely black appearance in case of heavy infestations. In some cases, an intermittent pattern of white-and black patches is visible, corresponding to the accumulation of whiteflies and sooty mold respectively.
The symptoms are caused by adults and nymphs of the whitefly Dialeurodes citri. It is a polyphagous insect, about 1.5 mm in length, with 2 pairs of uniformly white wings and a yellow mealy body covered with white wax. Females insert their stalked, yellow eggs into the lower leaf side, preferably in the inner, shaded part of the trees. After hatching, the flat, transparent nymphs start to suck the sap of leaf tissues, weakening the plant. However, the worst damage is actually caused through the production of honeydew by the nymphs. This attract opportunistic sooty mold fungi that cover the surface of leaves and fruits, disrupting photosynthesis and gas exchange. This can lead to leaf drop, reduced fruit size and quality as well as yield reduction. The threshold of development for the whitefly is 11.3°C. Citrus is the main host but the insect can also infest a broad array of alternative hosts.
Biological treatment with the parasitic wasp Encarsia lahorensis and several ladybird beetles (for example Clistostethus arcuatus, Delphastus pusillus and several species of Serangium) were used successfully against the citrus whitefly. The use of this insects is compatible with careful application of insecticides as required for integrated control of pest management and good field practices. Predators include several mirid bugs, lacewings, mites, ants, and a species of thrips Aleurodothrips fasciapennis. Some parasitic fungus can also parasitize the nymphs of Dialeurodes citri and are considered a potential treatment to control populations of Dialeurodes citri. Otherwise, the sooty mold can be washed away with soaps before marketing.
Always consider an integrated approach with preventive measures together with biological treatments if available. Chemical treatments should be considered only for very heavy infestations, in order to retain beneficial insects in the environment. Insect growth regulators that mimic or inhibit the insect hormones, have been used with some success to reduce populations of Dialeurodes citri. Control of sooty mold with copper solutions should be considered against its possible negative effect on beneficial fungi. Spray oil can also be used to remove sooty mold on fruit and leaves.
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