Garden Tips: There’s a large-scale attack on your trees. Here’s how to fight back

Scale insects are sneaky little devils that often go unnoticed by gardeners because they blend in so well with their surroundings. Scale typically start out as tiny (1-2 millimiters in size), mobile “crawlers” or “nymphs.” These crawlers emerge from eggs produced by female scale beneath their protective outer covering or “scale.”

Scale crawlers have antennae and legs that allow them to wander about looking for a good location for feeding. Once they find a suitable spot on a tree or shrub twig, they settle down to feed, staying in this same spot for the rest of their life. As they start to feed, a protective scale or shell starts to develop over their body

There are two main types of scale – soft scale and armored scale. Soft scale are generally more noticeable because they are larger, up to a quarter inch long, and have a humped or helmet-shaped scale that is smooth and waxy. Their scales are part of their body and serve as their outer body wall.

Soft scale feed on plant sap by inserting their piercing mouth or stylet into the host tissue. They tend to take in more sap than they actually need, so they secrete the excess as sugary honeydew. When a soft scale infestation is severe, leaves and objects below a tree can become coated with the sticky honeydew excrement.

Most armored scale are smaller than soft scale, not any larger than an eighth of an inch in diameter. Once their crawlers settle down, they secrete a flattened hard scale over their bodies. However, their scale is just a covering, leaving their body intact if the scale is removed. Armored scale feed by tapping into individual cells and sucking out the contents. Unlike soft scale, they do not overindulge and do not secrete honeydew.

The approach to controlling scale infestations varies with the type of scale, its life cycle, and the severity of an infestation. When it comes to soft scale, light infestations will usually not cause significant harm to a tree or shrub. If detected, the infestation should be monitored to make sure the numerous natural enemies of scale insects are keeping them in check. These enemies include lady beetles, lacewings, predatory mites and parasitic wasps.

A severe infestation of soft scale often indicates that the attacked tree or shrub is stressed. To protect against damaging soft scale infestations, maintain healthy, vigorous trees with adequate watering, mulching, and fertilization.

Because they are so small, armored scales are particularly insidious. An infestation of tiny armored scales, like San Jose scale, can build up without notice until twigs and branches start dying. Check susceptible plants periodically to make sure there is not a scale problem.

If needed, pesticides can be used to help manage severe scale infestations. Because soft scale feed on plant sap, soil applied systemic pesticides can be helpful in their control. For both soft and armored scale, horticultural oils applied when the crawlers are present and before they form their scale coverings can also provide control. Also, dormant oils applied in late winter at the dormant or delayed dormant stage will control certain kinds of scale by smothering them.

The WSU Master Gardeners tell me that they are coming across numerous heavy scale infestations on local trees and shrubs, so you may want to check your plants.

On maple, linden, ash, elm, hawthorn, and locust, look for cottony maple scale; on elm look for European elm scale; on spruce look for spruce bud scale; and on maple, elm, birch, ash, and linden look for lecanium scale. Look close for armored scales, specifically oystershell scale, San Jose scale, pine needle scale, and juniper scale. For photographs and specifics of control for each type, go to http://hortsense.cahnrs.wsu.edu.

This story was originally published May 19, 2018 at 5:32 AM with the headline "Garden Tips: There’s a large-scale attack on your trees. Here’s how to fight back."