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Reddish scale insects that are circular, flattened and about 1.5-2 mm in diameter, may be present on the stems, leaves and fruit of citrus and other host plants.
Citrus leaves as well as the leaves of some other host plants show characteristic yellow spots around each reproducing female. This may be followed by leaf drop and defoliation, accompanied by the dying back of twigs and eventually large branches (Bedford, 1998). Maturing citrus fruit can become completely encrusted with all stages of red scale. Such fruit start to dry out and fall off the tree (Bedford, 1998).
The entire trunk can become heavily infested with red scale, especially in the case of young citrus trees. This often leads to a severe set back of the tree, branches and even the tree dying (Bedford, 1998).
Due to the variable regulations around (de-)registration of pesticides, we are for the moment not including any specific chemical control recommendations. For further information, we recommend you visit the following resources:
A. aurantii has been the most notorious pest on citrus in many citrus producing countries in the world. The literature on this pest is very extensive, most of the research work having been conducted in California, USA. Excessive populations can build up very rapidly and, as the saliva injected during feeding is exceptionally toxic to the plant tissues, considerable damage can result, including the shedding of leaves and fruit, the dying back of even large branches and the death of young trees (Bedford, 1998). This pest has often been very difficult to control. Oil sprays and fumigation with hydrocyanic acid gas were the recognized methods of control for many years. During recent years there has been an increasing effort to maintain red scale under biological control especially in California, Australia, Israel and South Africa (Bedford, 1998).
According to Outspan International, no more than six scale insects of 1 mm diameter or larger (of any species) are allowed per average-sized orange for Export Grade (count 88), or 10 insects per grapefruit (count 40). For lemons and soft citrus, the figures are five and four scale insects on counts 113 and calibre 3, respectively, which are allowed per average-sized fruit for Export Grade. As a population density of only 50 females per 100 fruit results in culls of nearly 1% (1% of crop rejection for export in the packhouse), any increase above this level soon entails commercial losses. When red scale infestations are exceptionally heavy, cull can reach 100% at a scale density of 7700 per 100 fruit (Bedford, 1971).
In cases of severe infestations, developing scales form prominent pits on young fruit which are still evident when the fruit matures. Another culling factor results from dead scales which are still attached to the fruit because corrective sprays were not applied when necessary. Severe infestations can also reduce the final crop and the following year's crop by defoliation and dieback (Bedford, 1998).
Damage to the Tree
Heavy infestations of red scale are very toxic to the leaves, twigs, branches and fruit of citrus trees, most probably due to the action of the salivary juice. The red scale feeds by inserting its mouthparts deep into the plant tissue and sucking the parenchyma cell sap (Bedford, 1998).
As a severe infestation increases, the leaves show characteristic yellow spots around each reproducing female, followed by leaf drop and defoliation, accompanied by the dying back of twigs and eventually large branches. Maturing fruit can become completely encrusted with all stages of red scale and start to dry out and fall off. Even the entire trunk can become heavily infested, especially in the case of young citrus trees. Newly planted trees may be severely set back, or even killed if the branches die back. Furthermore, neglected heavy infestations lower the production of an orchard (Bedford, 1998).