Issue 16, September 10, 2012

Scale Increase Due to Drought

There are a couple of scale species that become much more numerous after droughts and other tree-weakening events. European fruit lecanium scale becomes very common on stressed trees and remains in high numbers for several years until the tree recovers. It is common on maples, oaks, hazlenuts, crabapples, and many other tree species. Large infestations produce high amounts of honeydew such that it feels like it's raining under the tree.

European fruit lecanium scale on silver maple.

Mature lecanium scales appear similar to small army helmets or turtle shells. They are round, slightly-raised, brown domes about one-quarter inch in diameter. They produce pink crawlers that emerge in mid-June in central Illinois when Queen Anne's lace is blooming. Being soft scales, they are controlled with dormant oil sprays.

Scurfy scale becomes more common after droughts as well. It attacks many hosts including hawthorn, quince, crabapple, firethorn, mountain ash, horsechestnut, elm, hickory, maple, willow, and dogwood. It is grayish-white and about one-eighth inch long. When numerous, it coats trunks and branches, making them appear to have white bark. Purple crawlers hatch in spring when bridal wreath spirea, Spirea X van houttei, is in early bloom. Crawler sprays are most effective at that time. Being an armored scale that overwinters in the egg stage, it is not very susceptible to dormant oil sprays and does not produce honeydew. (Phil Nixon)

Phil Nixon

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