California Red Scale

California red scale is a citrus pest found throughout California, except in Coachella Valley, where an eradication program is in place. These insects may be tiny, but California red scale is a serious pest of citrus trees. And their range is expanding.

CA red scale colony on leaf surface (Victor Cegarra) CC BY-SA 4.0

Like other armored scale insects, California red scale (Aonidiella aurantii) have piercing, filamentous mouthparts that are inserted into stems, fruit, and leaves, and suck life-giving sap from your tree. These particular scale insects prefer lemons, limes, Valencia and Navel oranges.

California red scale and yellow scale are almost indistinguishable to the untrained eye. Yellow scale (Aonidiella citrina) is less of a problem than red scale and is rarely found on mature wood, whereas red scale can be found anywhere on the tree. The two pests behave in very similar ways, with the California red scale being a far more problematic pest.

Red scale lifecycle

You will probably never see a tiny, flying male red scale. They live for about 6 hours and have only one purpose. The females, however, attach themselves to your citrus trees, where they feed on your tree and give birth to 100 to 150 crawlers. Two or three crawlers are born every day to each female. These crawlers leave to find their own feeding site. They can also be blown to nearby trees by the wind, or move from place to place by catching a ride on a bird in a practice known as phoresy - though I don’t know if they do it on purpose. Once they settle on a new location, both males and females begin to grow a waxy dome over themselves. Male covers are more elongated, while female covers are more rounded. Females molt two more times, while males molt under their first dome four times before taking to the air.

Damage caused by California red scale

Chlorosis, twig and branch dieback, fruit loss, and, in severe cases, tree death can all result from California red scale infestations. This damage most commonly occurs at the end of summer, when trees are water stressed and scale populations are at their peak.

How to control California red scale

Scale insects are naturally protected from pesticides. And California red scale has developed a resistance to many insecticides, so, unless you are a commercial farmer or city government, you do not have access to chemicals powerful enough to kill off California red scale. [And would you really want to spray that stuff on your food?] Keeping your trees healthy with regular, deep summer irrigation will reduce water stress. And avoiding the use of broad spectrum insecticides will allow natural predators to do their thing against scale populations. Parasitic wasps and several varieties of lady beetles can provide significant control of scale insects. ​

​Because ants, dust, and poor air flow all make it more difficult for these beneficial predators to find and catch their prey, be sure to prune for good air flow, wrap tree trunks with sticky barriers to block ants, and give your trees an occasional rinse with the hose during the dustier parts of summer. In winter, apply dormant oils.
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The next time you go water your citrus trees, take a closer look to see if California red scale has made an appearance.