If you see tiny dark nubs on your citrus or olive twigs, your tree may be infested with black scale.
Black scale (Saissetia oleae) build a soft, thin, way covering over themselves as protection. Unlike armored scale insects, this protective covering cannot be removed from soft scale insects. Black scale is a type of soft scale that produces a lot of honeydew. Honeydew is a nice way of saying sugary bug poop. Honeydew provides the perfect growing medium for sooty mold. It also attracts ants, which can carry diseases.
Black scale damage
All scale insects are sap-suckers. They use piercing mouthparts to access vascular tissue within the host plant. This feeding weakens the host plant, leading to fruit drop, wilting, twig dieback, stunting, and overall loss of vigor. In this weakened state, citrus trees are more susceptible to damage by drought, temperature extremes, other insects, and disease. Heavy infestations of black scale can kill part or all of a tree.
Black scale lifecycle
Each May and June, here in California, and again in October and November, female black scale insects lay as many as 2,000 eggs, without ever mating! These eggs hatch out into crawlers, which crawl around on the host plant, looking for a nice place to set up shop, usually on leaves. These crawlers go through several instars, or stages. In the second instar, they develop a ridge on their back. This ridge expands into an “H” shape. After they molt a second, time, they head for twigs, and begin growing in earnest. This is where their bodies become more circular and their protective cover turns a leathery mottled gray. As each of these offspring approaches reproductive age, the cover gets darker and harder, and the “H” often disappears.
How to control black scale
Black scale insects tend to be a serious problem every 5 to 10 years. Normally, local parasitic wasps and other predators keep this pest under control. Dust and the use of broad spectrum pesticides can interfere with these natural controls. In years of heavy infestation, specific pesticides may be needed. Before using a pesticide against scale insects, be sure to look closely. If you see tiny holes in the scales, there is no need to spray. Those holes are from scale parasites which have already done their job.
You can also reduce scale populations by hand. Simply grab a wet, soapy rag and start wiping them off. While you’re at it, you can help your tree increase photosynthesis by wiping off the sooty mold, too.
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