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Obscure scale is a key pest of oak, Quercus spp. in Northern United states. This armored scale insect attacks a wide variety of other woody host plants such as beech, Fagus spp.; dogwood, Cornus spp; hickory, Carya spp.; maple, Acer spp.; and willow, Salix spp.
This insect is not a pest of these trees when theyâ€™re growing in the forest.The femaleâ€™s waxy covering is flat, about 3 mm in diameter, dirty gray on the top side, with a black cap that is slightly off center. The underside of the waxy covering is black except for a white, silk-like coating in the center.
Life cycle:On pin oak in Maryland, eggs are laid from the end of June through the beginning of September. Crawlers emerge from the first of July through mid-September; however, crawler activity peaks from mid to late July. Crawlers settle, begin feeding, and form their protective cover within an hour. Males will feed and grow until the following April at which time they discontinue feeding and enter a pre-pupal and pupal stage.
Adult males will emerge from the pupal stage throughout the second half of May, mate with adult females, and then die.The female feeds throughout her entire life until she lays eggs that following summer. Approximately two weeks prior to crawler emergence, adult females will add a small flap to their cover through which crawlers may exit. There is one generation per year. The scaleâ€™s development on white oak occurs one month later than that on pin oak.
Damage:This species removes plant fluid from twigs and branches with its piercing-sucking mouthparts, causing infested host plants to drop leaves prematurely and dieback. Numerous sunken areas appear on the bark where this pest has been feeding, producing a roughened appearance. Infested branches appear to be sprinkled with wood ashes or small pieces of silver glitter. Severely infested trees may have three to four layers of the waxy coverings of this pest on the bark. In some cases the trunk of small host trees or branches will be disfigured due to a severe infestation.
Heavily infested trees will have large numbers of scales on twigs and branches. Scales may also be found on exposed roots and on the trunk of young trees. Scale insects feed on plant sap with their long thread-like mouthparts (stylets), which are several times longer than the insect itself. The continual drain of sap from the scaleâ€™s feeding and the disruption of the photosynthetic and respiratory functions of the bark due to encrustation weaken the infested tree. Infestations seldom kill the tree directly, but can cause extensive die-back of twigs and branches which decreases the treeâ€™s aesthetic value. Weakened trees also are more susceptible to secondary infestation by other insects and diseases, which can kill.
Control:Three factors make reduction of this pest a difficult task. First, egg laying occurs over a relatively long period of time, resulting in an extended crawler period. Second, crawlers often settle beneath old clusters of waxy covers from previous generations and, therefore, are protected from exposure to contact insecticides. Third, the waxy cover over a developing life stage of this armored scale provides protection against insecticide penetration.Monitor three- to four-year old twigs, especially on pin oak, Q. palustris, for the presence of the gray waxy covers of this scale insect. Early detection of an infested tree is critical for effective management.
Prune heavily infested twigs and branches to reduce the spread of this pest. Effective scale insect management should target the crawler stage. Infested oaks belonging to the red oak group should be treated in late July; oaks in the white oak group should be sprayed during mid-August. During these times crawlers are most abundant and parasitoid adults are least active. To reduce heavy crawler populations, two applications should be made according to insecticide label directions 14 days apart. There are four species of wasp parasitoids, one mite predator, and three species of lady beetles that feed on this armored scale insect. The use of short residual insecticides will conserve these natural enemies.
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