By: Anthony C. Arnoldi, Board-Certified Master Arborist WI-0102B
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. Even when pointed out, they are looked at in disbelief—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 explosive reproductive abilities.
The young scales hatch from eggs in spring and move towards new plant growth (they are called crawlers in this stage – the only time they can move). They then settle down to insert their mouthparts into the plant tissue. At this time, they lose their legs and antennae and form the hard shell (scale) over themselves, never moving again. 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, or the patio furniture below. This promotes the growth of sooty mold fungi that can blacken 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 are directed at the vulnerable crawlers in spring or summer. It is important to obtain an accurate diagnosis of the specific scale to know exactly which material to apply and when. Dormant oil is another useful treatment to smother overwintering eggs and adults and can assist in further reduction or prevention.
Any woody plant has potential scale enemies. Recently, we have seen the heaviest attack on many ash, maple, honeylocust, mugo pine, yew, and magnolia. Magnolia scale, in particular, is a large, heavily damaging scale that must be controlled when discovered.
When sections of your plant go off-color or weaken, scale may be the culprit. Notify your Wachtel Certified Arborist so an appropriate control program can be prescribed.
© Copyright 2009 – Wachtel Tree Science & Service, Inc.