Calico scale – a serious pest of landscape trees

Adult female calico scales are easy to see on the trunk of this infested tree. They are large, about 1/4 inch in diameter, and distinctive: dark brown and white. It’s as if they are asking for attention, and they should get it!

Prying up a few of these large females should reveal hundreds of small eggs that will hatch into active crawlers soon after the females die. The crawlers will move to feed on sap from expanded leaves during the summer. This scale is most vulnerable now and the opportunity should not be missed. Once dispersed into foliage, control is much less successful. The crawlers will not return to bark until autumn, just before leaf fall. There, they will spend the winter and mature. There is one generation each year.

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Insecticide applications, timed to coincide with emergence of young crawlers, will break the cycle of development and help to reduce further plant stress.  The ideal time to treat is when about 75% of the globular adult females have turned brown, which should be soon.

Several tree and shrub insecticides can provide good control. Read the product label to be sure scales and your tree species are listed. Thorough coverage of infested twigs, branches and adjoining leaves is important. Eggs hatch over a period of several weeks so a second application 2 to 3 weeks after the first may provide more complete control. Horticultural oil, insecticidal soaps, and tree injections of systemic insecticides have not been very effective against this scale insect.

Calico scales infest honeylocust, hawthorn, hackberry, sweet gum, yellowwood, dogwood, flowering crabapple, and sugar and Norway maples. During part of their life, they cover the trunk and branches, feeding on the phloem tissue. Like other soft scales, they excrete large quantities of nutrient rich liquid waste (“honeydew”) that drops onto leaves and branches.  Soon, the deposits are covered with sooty mold fungus.  The sweet liquid also will attract bees, wasps, and flies.

Infestations usually don’t kill trees but severely weaken them, making them highly vulnerable to wood borers, drought and other stresses.

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