White cottony scale maple insects are small, flat, oval brown insects that commonly occur on boxelder, honeylocust, linden, poplar, basswood, elm and maple trees. The species Pulvinaria innumerabilis, cottony maple scale produces a white cotton-like egg sac two to three times the length of the body in spring and summer. Trees infested with cottony scale appear to have cotton balls strung from their twigs.
A mature female cottony scale insect is 1/8 inch long without obvious legs, antennae or wings. They begin as crawlers, microscopic insects about the size of a period printed on a page. They are sometimes mistaken for the winged adult woolly aphid which is a flying insect that also inhabits trees. The white cottony egg sac is comprised of waxy threads. It contains over 1,500 eggs.
Maple scales produce large amounts of liquid waste called "honeydew" that is shiny and sticky. The honeydew attracts black sooty mould fungus that grows to cover the branches and trunk of the tree. It also attracts bees, wasps and ants.
Female scale insects overwinter on the tree, complete their growth by June and lay eggs through the end of summer. Eggs hatch in late June through July and migrate to the underside of tree leaves. They insert their piercing-sucking mouthparts into the tree and withdraw sap from its vascular cells. Just before leaf drop in the fall, the white cottony scale insects move to twigs and branches to spend the winter. Males mature in late summer, mate with females and then die. The females mature over winter to begin the cycle again in spring.
The three-day time period between hatching and attaching to the tree is the optimum time to interrupt their reproduction cycle. You can detect the crawler stage by holding a white piece of paper underneath the site of infestation and vigorously shaking the tree. If they have already completed their crawling stage and reinserted into the tree they cannot be shaken loose. Dormant oil spray applied during this short time-frame is effective in stopping the scale from overwintering in the tree.
Cottony maple scale insects thrive on unhealthy, stressed trees. Following a regular soil fertility program and watering schedule promotes tree health and reduces insect infestation. Excessive use of synthetic fertiliser increases the incidence of scale insects, according to Lee Townsend of the University of Kentucky College of Agriculture. Healthy trees attract the natural predators of scale that keep its population under control. The black lady beetle is its main natural predator, eating the cottony egg sacs as they develop. The maple scale's egg sacs look tattered and loose when lady beetles are present.