Answer: Hmmm.. without personally inspecting the plant, I can only offer a few educated guesses. First, cotoneaster webworm. The cotoneaster webworm over winters in the masses of dead leaves on the shrubs. In the spring they develop into adults, which eat for a short period of time on cotoneasters and then lay their eggs. In late July and early August, the eggs hatch and the larvae will begin devouring the leaves. The larvae is dark brown in color and up to an inch in length. At the first sign of feeding, apply a chemical treatment. Use Diazinon, Malathion or Sevin. Other insecticides labeled for leaf feeding caterpillars such as Tempo can also be used. The active ingredient in Tempo is available in Bayer Advanced Garden Multi-Insect Killer. When using insecticide, mix and apply according to label directions. If webbing has already been produced, it must be thoroughly wetted in order to achieve control of the webworm.
Or, it could be a spider mite problem. Adult mites are reddish brown or pale tan and 1/150 to 1/50 inch long. They are more easily visible through a hand lens. Mites lay their eggs on leaves and buds or at the base of plants. Spider mites turn cotoneaster leaves silvery or yellow, giving them a parched appearance. The leaves may curl and become covered with fine webbing, especially on undersides, and fruit is deformed. These pests are more common in some regions than others. They favor hot, dry locations. Contol them by spraying infested plants early in the morning with a forceful jet of cold water. Repeat the water spray daily for three days. If that doesn't do the job, spray the mites with insecticidal soap every three to five days for two weeks. Dormant oil spray will destroy many overwintering mites. Spray with 1 part flowable sulfur fungicide in 10 parts of "superior" type dormant oil before growth begins in spring.
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