Scale insect infestations range from mildly annoying and unsightly to highly damaging to cherry trees. Scale insects not only drain nutrients from the tree but make it more susceptible to secondary fungal infections. Several species of scale insects affect cherry trees, and the unusual form the insects take makes them hard to identify and eliminate.
To the average observer, it is difficult to tell that scale insects are even insects. Although the larvae emerge as typical, mobile insects, they quickly settle into a location on the cherry tree and secrete a hard or waxy coating that makes them appear as little more than bumps on the cherry leaf or branch. Most scale insects lose their legs at this point and will remain in place for the rest of their lives, slowing draining sap from the tree.
Scale insects may be classified as armored or soft. Armored scales secrete a hard shell and remain immobile throughout their adult lives. Soft scales develop a waxy coating, and some species retain their legs and can crawl short distances. Both types affect cherry trees. According to the Virginia Cooperative Extension, armored scales affecting the cherry tree include the San Jose scale, white peach scale, greedy scale and scurfy scale, and soft scales include European fruit lecanium and cottony maple scale.
Female scale insects lay their eggs inside their protective coating. After one to three weeks, the eggs hatch as crawlers, mobile insects that move around the tree until they find a suitable place to feed, where they insert their mouthparts and begin developing their protective coating. Females molt twice before becoming immobile, legless parasites. Males molt four times and pupate, eventually developing a winged form without mouthparts. Armored scales that affect cherry trees have three generations per year, while soft scales have only one.
Scale insects have mouthparts that often extend six to eight times the length of their bodies. They insert these into the cherry tree and draw out sap, depriving the tree of water and nutrients. Scale insects that affect cherry trees primarily attach to twigs and branches, although the cottony maple scale begins its life on the leaves before migrating to the twigs. Scale-infected trees may lose twigs and branches, grow poorly, weaken and die if too heavily infested. Scale insects also secrete a sticky substance called honeydew that makes cherry leaves more prone to mildew infections.
The coating that covers adult scales protects them from most insecticides, making them difficult to treat. Beneficial insects, such as ladybugs, and horticultural oil treatments are effective against adult scales. Always check oils before spraying to be sure they are intended for use on cherry trees. Insecticides and insecticidal soaps are effective against crawlers, but timing is important since scale insects remain crawlers for only short periods of time. Contact your county extension office to find out when crawlers emerge in your area.