That white stuff on your citrus tree could be an infestation of cottony cushion scale, citrus mealybug or spider mites. The timing and the placement of the white marks can help you determine not only the type of insect, but also how to mitigate the problem effectively.
If silver-coloured marks and an overall scratchy texture cover the foliage of your citrus tree, spider mites may be the culprit. Another sign to look for is excessive leaf drop, distortion and blistering, which can mean a more widespread infestation. White, cottony masses on the actual citrus fruit can mean citrus mealybug or cottony cushion scale. The white markings are near the citrus fruit's stem, and no other major tree symptoms occur.
If citrus mealybugs or cottony cushion scale are causing the white marks on your citrus tree's fruit, the first outbreak will typically occur in the summer and all the way through harvest time, with future outbreaks hitting in the summers. If the white marks on your citrus tree's foliage appear in the summer or fall, spider mites are to blame. In the case of spider mites, white marks will occur as early as spring during future outbreaks.
To identify the tiny spider mite, you will need a hand lens. Under magnification, spider mites resemble miniature spiders. Citrus mealybug and cottony cushion scale, on the other hand, are visible with the naked eye, usually seen on leaves and twigs, and covered with a waxy substance. Female cottony cushion scale are 4.5mm in length with a rusty red-coloured body and black antennae, while males are 3mm, reddish-purple in colour and have blue wings. Citrus mealybugs also vary in appearance depending on sex; females are 3mm, oval and wingless, while males are smaller with two wings and long, white waxy tails.
Treatment and Prevention
Both cottony cushion scale and citrus mealybug infestations do not require treatment because the outbreak is rarely serious. You should control spider mites, however, to prevent leaf loss. Sprays that can aid in spider mite treatment include specially formulated spider mite pesticides called miticides or acaricides. If treating a houseplant, horticultural oils and soaps may be used. The Colorado State University Extension website recommends keeping the citrus tree adequately watered to prevent future attacks.