Infestations usually occur on the trunk and main limbs of the tree. If these become more severe then twigs, leaves and fruit are also affected. This results in yellow spotting developing on the underside of leaves, which prematurely drop off, dieback of twigs and eventually the killing of branches. Heavily infested bark becomes dark and dull, appears tight and finally splits, allowing fungi to further attack the tree.
Symptoms are caused by adults of the Citrus Snow Scale (Unaspis citri). The egg is ovoid, bright orange in color and approximately 0.3 mm in length. The adult female scale is 1.5 to 2.3 mm in length and its small, dark scales are often mistaken for dirt on fruit. Females insert their mouthparts into the tree and never move again, whilst eating and reproducing in the same spot. Their armor is shaped like an oyster shell, and is brownish purple to black, with a grey border. Male armored scales are also immobile until maturity. Immature male scale armor is white with parallel sides and three longitudinal sections, one central and two marginal ridges. U. citri secrete a protective coating composed of wax and cast skins of prior instars, which creates its armor. The armor will remain on fruit long after the insect is dead, disfiguring the fruit.
The parasitic wasp Aphytis lingnanensis is effective in helping to control Citrus Snow Scale if it is already established within the grove. Use lime sulfur (polysulfide sulfur) or wettable sulfur, then leave at least 30 days between spraying lime sulfur and sprays of oil. However, lime sulfur could adversely affect Aphytis lingnanensis. The beetle Chilocorus circumdatus has also shown to be a successful biological control agent. White oil, soap and horticultural oil sprays work by blocking the breathing holes of insects causing suffocation and death. Spray the undersides of leaves, the oils must contact the insects. A second application of soap or oils may be necessary after 3-4 weeks. The effectiveness of biological control agents may be reduced with the application of chemical control agents.
Always consider an integrated approach with preventive measures together with biological treatments if available. Malathion 50% is useful against Citrus Snow Scales, spray the underside of leaves. Synthetic pyrethroid insecticides are also effective against the active nymphs. Malathion and synthetic pyrethroids are likely to kill natural enemies, therefore should be avoided if possible.