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June 15, 2006 Cottony Maple Scale Insects Feasting on Trees A large number of overwintering insect scales are present on many hardwood trees, such as maple, ash, elm, popular, boxelder and honeylocust. One that has been especially abundant and conspicuous is the soft scale insect known as the cottony maple scale. Trees infested by cottony maple scale are maple, boxelder, honeylocust, linden, poplar, basswood and elm. “However, the silver maple tree is its favorite host in North Dakota,” according to Janet Knodel, North Dakota State University Extension Service entomologist. “In the overwintering stage, the insect is oval, flat and pale to dark brown without obvious legs, antennae or wings. Later, a white cottony egg sac two to three times the length of the scale is produced in the spring. It gives the appearance of cotton balls being strung from twigs.” Knodel says she is receiving more than 10 calls a day about the problem. Overwintering females complete development in June and lay eggs through late summer. Each cottony white egg mass contains 1,000 to 1,500 eggs. Eggs hatch into crawlers in late June through July. The crawlers are flat, oval, brown insects with two distinct eyes, short antennae, tiny legs and are microscopic, or about the size of a period on this page. They migrate to the underside of leaves and insert their piercing-sucking mouthparts along the midrib and then withdraw sap from the tree’s vascular cells. They spend the remainder of the summer feeding on leaves. Males mature in late summer, emerge as tiny, winged gnatlike insects, mate with immature females and then die. Just before leaf drop, mated females move back to the branches and twigs and reinsert their mouthparts for overwintering. There is one generation a year. The insect feeding causes twig dieback. Severe infestations can kill major limbs and occasionally the entire tree, especially if the tree is stressed (drought). “Soft scale insects secrete sticky honeydew, which causes problems when cars or picnic tables are underneath scale-infested trees,” Knodel says. “Another negative of honeydew production is that it attracts ants and wasps to the area and also coats the leaves, which promotes the growth of black, sooty mold fungi.” Dormant oils can be applied before bud break to control the overwintering scale. However, many maples are sensitive to these oil sprays. Summer control of scale insects needs to be targeted at the crawlers (immature stage) that usually hatch from eggs in late June through July. The crawlers do not move very far from their hatching site. “The egg hatch occurs for a short period of time, so timing is crucial for effective control,” Knodel says. “To monitor for crawlers, place a white piece of paper underneath the branch and vigorously tap the branch to dislodge crawlers onto the paper, or wrap the branches with a wide, double-sided sticky tape near the active insects. Inspect the tape often for crawlers. Once the crawlers settle and insert their coiled mouthparts into the plant, which usually takes one to three days, it will be difficult to dislodge them. “Summer sprays of insecticidal soap or conventional insecticides should be targeted at crawlers and can be very effective in reducing the population,” she adds. “Unfortunately, conventional insecticides, such as orthene, sevin or malathion, also will kill the scale insect’s natural enemies and ultimately could make the infestation worse.” An alternative that is more friendly to beneficial insects (parasitic wasps and predators) is a systemic insecticide with the active ingredient imidacloprid. Some of the trade names are Merit and Bayer Advanced Tree and Shrub. It can be applied as a soil drench near the base of the tree. Depending on the size of the tree, it may take the insecticide a week for a small tree and up to three months for a large tree to reach areas where the insects are feeding. Other insects controlled by imidacloprid are adelgids, aphids, borers, lacebugs, leaf beetles, leafhoppers, leafminers, psyllids, thrips and sawfly larvae. Knodel advises that users of a systemic insecticide always read and follow the directions on the label and follow the recommended safety precautions. ### Source:
Janet Knodel, (701) 231-7581, janet.knodel@ndsu.edu
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