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Cottony Maple Scale

May 18, 2005

We are receiving reports of silver maple trees dripping in the Kankakee County area and have seen some specimens of cottony maple scale from northern Illinois. This insect occurs in the northern half of Illinois. with its southern boundary being at or slightly north of I-72. It attacks primarily silver maple and sumac but is also found in much lower numbers on other maples, honey locust, black walnut, and linden. This scale can cause some dieback of branches on silver maple but is typically not numerous enough on other hosts to cause obvious damage. It’s important to realize that little can be done right now about the problem.

At this time, the scale are reddish brown, about 1/4 inch diameter, roundish bumps on the twigs and branches. They are feeding on the plant sap, extracting much of the nitrogen and water, and excreting the remaining concentrated sap or light syrup solution known as honeydew. In very heavy infestations, one can stand under the tree and feel a light “rain” of honeydew. The honeydew is sticky and covers tree branches, sidewalks, and cars under the tree. A black sooty mold grows on the honeydew, turning branches and other objects a dull black.

During the first half of June, each female produces a white, spherical egg mass up to 3/8 inch in diameter. On heavily infested trees, the branches appear to be covered with popcorn. These eggs hatch into crawlers around mid-July, when Queen Anne’s lace, Daucus carota, and elderberry, Sambucus canadensis, are in bloom. These tan crawlers, first-stage nymphs, are very tiny but are easily seen as specks moving on the twigs and leaves.

The crawler is the stage of the life cycle that disperses to other hosts. They orient themselves with their posteriors facing into the wind and raise their abdomens and hind legs up into the air in an effort to be blown off of the plant. Updrafts may carry the crawlers for miles, with some eventually landing on suitable hosts. The crawlers also climb onto birds’ feet and other insects for a free flight to another host.

After a couple of weeks of roaming, the crawlers settle down onto the undersides of leaves and molt to the next nymphal stage. These young nymphs appear as elongate-oval brown bumps 1/16 to 1/8 inch long. They retain their legs, and so they can crawl back onto the twigs by late-season leaf fall. Back on the twigs, they molt to a legless stage for the winter.

This is a native insect that has an important natural enemy. The twice-stabbed lady beetle adult is a black, round beetle about 1/8 inch in diameter. There is a large red spot on each of the two wing covers, giving the insect its name. They lay eggs that hatch into grayish white, fuzzy larvae that look much like the cottony maple scale egg mass. If you prod one of these larvae, it will move very slowly; but, of course, scale egg masses don’t move at all. Turning a larva over will reveal a yellowish underside, with short legs and small head easily seen with a hand lens. Both the larvae and adults of the twice-stabbed lady beetle eat all stages of the cottony maple scale—eggs, crawlers, older nymphs, adults.

These beetles build up in numbers in response to the increase in scale population. However, like all successful predators, the prey increases in numbers before they do. If they reproduced too fast, the lady beetles would eat all of the prey too fast and starve to death before their larvae reached adulthood when they could reproduce. As a result, cottony maple scale usually becomes very noticeable for about 3 years until the lady beetles build up to wipe them out. After the third year, the adult twice-stabbed lady beetles are very numerous crawling around on trees, houses, and other upright objects looking for food. Most of them starve to death, causing a crash in the lady beetle population. This allows the cottony maple scale gradually to increase in numbers to become a noticeable pest about 7 to 8 years later.

This infestation may be less than 3 years because of the presence of the multicolored Asian lady beetle, the insect we love to hate. This is the nineteen-spotted, orange to red, lady beetle that comes into our houses in huge numbers. This arboreal lady beetle will likely feed on the cottony maple scale as well.

Control recommendations are to spray the cottony maple scale during the winter with a dormant oil spray, but do not apply it to sugar or Japanese maple. At that time, the twice-stabbed lady beetle adults will be under loose bark and not harmed by the spray. Crawler sprays of acephate (Orthene), befenthrin (Talstar), cyfluthrin (Tempo), insecticidal soap, malathion, or summer oil can also be applied in July when the crawlers are active. If twice-stabbed lady beetle larvae or adults are numerous, they will probably eat more scale than the spray will kill, so do not spray. Typically, the only way that cottony maple scale is a problem for more than 3 years is when poorly timed crawler sprays provide little control of the scale but kill off the lady beetles. Do not apply acephate to red or sugar maple.

Author: Phil Nixon


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