Common name: Cottony maple scale
Scientific name: Pulvinaria innumerabilis
Hosts: The primary hosts of cottony maple scale are silver maple (Acer saccharinum) and boxelder (Acer negundo). It can also infest other maples (Acer), white ash (Fraxinus americana), hackberry (Celtis), dogwood (Cornus), beech (Fagus), apple (Malus), oak (Quercus), linden (Tilia), elm (Ulmus), black locust (Robinia), and honey locust (Gleditsia). This insect is native to North America.
Scale insects are unique and look quite different from other insects. In their juvenile growth stage, they are referred to as crawlers. As crawlers, they are highly mobile, six-legged, have no protective cover, and are very small (<1/32 inch). At maturity, scale insects are immobile, have a covering over their body and are large enough to see without a magnifying glass.
Scale insects fall into two categories, soft scales and armored scales. Cottony maple scale is a soft scale. The soft scales are usually larger than armored scales. They cover themselves with waxy secretions. Most soft scales overwinter as immature females. In most years, this insect is considered a minor pest. Every few years, the population rises dramatically and does more damage. Natural predators often rise in a reaction to the increased population and, over time, they reduce the pest population significantly.
All scale insects have sucking mouthparts and feed on sap from the tree. Soft scale, like cottony maple scale, feed directly in the tissues that move sap (phloem). Scale can remove large quantities of sap, stressing the host tree. Trees can usually tolerate small populations of scale. The extensive feeding by a larger population often leads to yellowing of leaves and dieback of twigs. Over time, an untreated population of scale may lead to the decline of the tree.
Soft scale excrete their waste as honeydew. Honeydew is sticky and will coat plant parts and often drip onto surfaces under the tree. A black fungus called sooty mold will grow on the honeydew but does little actual damage to the plant. The sticky honeydew and black sooty mold are often noticed before the insects are seen. The honeydew may also draw other insects like ants and wasps to the tree.
In most years, cottony maple scale populations are small and the host tree can tolerate the feeding. The pest may go unnoticed except for the honeydew under the tree. Damage to the host tree is caused when heavy populations of cottony maple scale withdraw sap from the plant, resulting in branch and twig dieback. Leaves may turn yellow and drop prematurely.
Cottony maple scale produces one generation per year. The insect overwinters as immature females on the bark of twigs and branches. They are small, brown, ⅛-inch long, with a flattened body. By late spring, the female scale has matured and begins to lay hundreds of eggs within a white frothy mass. The egg mass is several times greater than the overwintering scale, and resembles a piece of popcorn. The eggs hatch into pale, yellow-green crawlers in mid-summer, about the time when bottlebrush buckeye is in flower. They migrate to the underside of leaves, feeding along the veins and midrib, withdrawing sap from them. In late summer, mature, winged males mate with immature females. The males lack feeding mouthparts and die a few days after mating. Before leaves begin to drop in fall, the immature females migrate to the twigs where they attach themselves for overwintering.
It may be possible to manage small populations of scale by hand. If the insect is limited to a small branch or two, pruning out those branches may be enough. Small populations can also be carefully wiped off of branches.
Serious outbreaks of the cottony maple scale may last for two to three years, but their numbers become greatly reduced when natural predators, including a number of wasps and fly parasites, are present. The major predator of the immature scale is the twice-stabbed lady beetle. The adult beetle is black with a red spot on each wing. The immature lady beetle resembles a mealy bug and can be found buried in the cottony egg mass. Both adult and immature beetles feed on eggs and nymphs of the scale. When beneficial insects are present, avoid spraying insecticides.
Scale insects are very vulnerable in the crawler stage when the young are looking for a place to feed. Adult scales are usually protected from chemicals because of their waxy coating. Registered sprays applied before the crawlers are present will have little effect on population control. Timing of the application is critical. Chemical sprays should be used at the time of crawler emergence (mid-summer about the time when bottlebrush buckeye is in flower).
When sprayed on the tree, insecticidal soaps and horticultural oils can be effective against the crawler stage, but usually have no effect on the adult scale. These products require thorough coverage and must come in contact with the crawlers to kill them. These products may also need multiple applications since they have no residual activity. Check the label of the product you use for the timing of additional sprays. Dormant oil applied to overwintering nymphs in early spring can also be effective. This treatment should be applied before buds open in the spring. Insecticidal soaps and horticultural oils are good choices as they have a limited impact on beneficial insects.
Other insecticide sprays are available to both home gardeners and professionals, for the treatment of crawlers. Systemic insecticides can be injected into the tree by a certified arborist several weeks before the crawlers become active. These products will kill the crawlers but take time to move through the tree, so they must be applied early enough to take effect at the appropriate time.
The pesticide information presented in this publication is current with federal and state regulations. The user is responsible for determining that the intended use is consistent with the label of the product being used. The information given here is for educational purposes only. Reference to commercial products or trade names is made with the understanding that no discrimination is intended and no endorsement made by The Morton Arboretum.
For more information, contact The Morton Arboretum Plant Clinic (630-719-2424 or firstname.lastname@example.org).