By: Anthony Arnoldi, Board-Certified Master Arborist
It has become apparent this year that an explosive increase in the number of several scale insect species has begun to damage many trees and shrubs.It has become apparent this year that an explosive increase in the number of several scale insect species has begun to damage many trees and shrubs.
Scale insects are very surprising to those who discover them for the first time. They are small, inconspicuous, and do not move. They can be very numerous, but because they look so much like warts or bumps on the bark or leaves, they are often missed – they do not look like they have any life!
Nevertheless, these are very important insects. They are some of the most damaging when their numbers get high, and they often do get high because of their remarkable reproductive abilities.
The young scales hatch from eggs and move towards new unoccupied positions on twigs, bark or leaves (they are called crawlers in this stage – the only time they can move). They soon settle down to insert their mouthparts into the plant tissue. At this time, they lose their legs and antennae and form either a waxy coating over them (Soft Scales) or form a hard shell (Armored Scales), never moving again. Once the coating or shell has formed, they are nearly impervious to sprayed control materials. Damage to the host plant is caused by sucking large quantities of sap.
Scales often produce large amounts of honeydew, a sticky, sugary feeding byproduct that they exude. These droplets can coat leaves, branches, cars, or the patio furniture below. Sooty mold fungus often grows on and blackens the honeydew-coated surfaces. The sooty mold is not very damaging itself, but is unsightly and often gives away the presence of the scale insects.
Control treatments vary with the species of scale, degree of infestation, and the time of year when discovered. Spray applications directed toward the vulnerable crawler stage are needed to cut off the next generation of scale. These sprays are quite exacting in timing and materials used. In addition, appropriate soil systemic insecticides may be needed to deal with the current generation of adult scales, if their feeding damage is too high. Dormant oil is another useful treatment to reduce overwintering eggs and adults and can assist in further reduction or prevention. Once populations are reduced, Dormant oil might be substituted for the crawler sprays. In any case, control of scale insects is not always easy, and their potential to make a comeback is high. This is why it is important to obtain an accurate diagnosis of the specific scale to know exactly which material to apply and when.
Any woody plant has potential scale enemies. This year, Lecanium scale has heavily attacked many ash, maple and honeylocusts. Fletcher scale has increased on many yews and arborvitae. Magnolia scale, in particular, is a large, heavily damaging scale increasing in numbers that must be controlled when discovered.
When sections of your tree or shrub go off-color or weaken, scale may be the culprit. Notify your Wachtel Certified Arborist so an appropriate control program can be prescribed for you.