Cottony maple scale is being found in DuPage, Livingston, and Grundy counties, on silver maple, sugar maple, honeylocust, green ash, and hawthorn. In some cases, the scales are quite numerous.

Egg sacks appear as white cottony masses emerging from beneath the brownish scale covers of females. The scale itself is about 1/4 inch, and the egg sack is about 1/2 inch in size. Egg sacks look like popcorn on the underside of small branches on the scale’s primary host (silver maple), other maples, alder, apple, beech, black locust, box elder, dogwood, elm, euonymus, honeylocust, linden, mulberry, oak, osage orange, peach, pear, plum, poplar, rose, sumac, and sycamore. Infested trees are rarely killed, but heavy infestations may cause dieback of individual branches and a general decline in plant vigor.

Cottony maple scale overwinters as immature females (nymphs) on twigs and branches. Female scales mature when plant growth resumes in the spring. In late spring and early summer, females produce conspicuous cottony egg masses that may contain more than 500 eggs. Eggs hatch into the crawler stage of the scale in late June and early July in central and northern Illinois. Crawlers are extremely small and appear as yellow-orange moving dots.

Crawlers, as their name implies, crawl to the underside of leaves, where they settle and feed on the sap. Male scales mature in late August and early September, when they mate with immature females and die. In late September, just before leaf drop, fertilized female nymphs migrate from the leaves to twigs and branches, where they overwinter. Only one generation of cottony maple scale occurs each year.

One symptom of heavy infestations of cottony maple scale is honeydew. After crawlers begin feeding on leaf sap they can create enormous amounts of honeydew. In heavy infestations, leaves may be stuck together, and the ground beneath the trees may be extremely sticky. Infestations of this magnitude can occur on silver maple but are less likely to occur on other hosts. Sooty mold is commonly found growing on the honeydew in mid- to late summer.

Cottony maple scale is a food source for many predaceous insects and parasites. The twice-stabbed ladybeetle, which feeds on soft-bodied insects, is commonly found dining on cottony maple scale crawlers. Cottony maple scale outbreaks often subside in two to three years, due to control by natural enemies. Unfortunately, the predators and parasites are susceptible to many insecticides used in the landscape and are often eliminated before they can adequately control the scale.

Before treating for crawlers, look for twice-stabbed lady beetles. At this time of year, beetles are usually in the larval stage. The full-grown larva is about 1/4 inch long, whitish to grayish, and covered with waxy filaments. It looks very similar to the egg mass of the cottony maple scale. A lady beetle larva will move, albeit slowly, when poked or prodded; a scale egg mass will not. Adult twice-stabbed lady beetles are about 1/8 inch long, roundish, and black, with two red spots on the back. Adults were present in mid-May; I photographed one in my backyard in central Illinois at that time.

Adults are usually present in July when the crawlers are present. It’s common for both larvae and adults to be present at the same time. If larvae and/or adults of this lady beetle are present in reasonable numbers, it’s best to avoid spraying for cottony maple scale crawlers. In severe infestations of cottony maple scale, chemical control may be desirable. Dormant-oil treatments control overwintering female nymphs on most hosts (except sugar and Japanese maple) after leaf drop in the fall and before budbreak in the spring. One spray during that period should be sufficient. Spray when the temperature will stay above freezing for 24 hours after the spray. During the dormant season, twice-stabbed lady beetles overwinter as adults under loose bark where they are not affected by the spray.

Other treatments should aim to control crawlers on foliage in July, when they first become active. Repeat treatments may be needed after ten days. Foliar applications of acephate (Orthene), malathion, or summer horticultural oils to the underside of the leaves should provide effective control of cottony maple scale. Do not use acephate on red or sugar maple.

(Other contributors to this article: Mary Pfeiffer, Grundy County Extension Unit; Mary Shier, Livingston County Extension Unit)