The cotoneaster plant (sometimes referred to as a catoni aster shrub or hedge) is closely related to the rose, and the multiple species of this plant offer different coloring and sizes for variety in the garden. Regardless of the species, these plants grow as shrubs, generally reaching around 5 feet tall with colorful blooms. These plants grow best in full sun and rarely require supplemental watering, but they can suffer infestation from some pests that can cause damage to the plant.
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Aphids are a common pest to find on many trees, shrubs and flowers in the home garden. These tiny bugs are often hard to spot because of their size, and in small quantities, they cause little to no damage to cotoneaster. However, large infestations can cause drying, curling, yellowing and falling leaves. These pests also deposit honeydew on the leaves, which attracts mold and mildew. Aphids do their worst in warm but not hot temperatures, generally between 60 and 80 degrees Fahrenheit. You can treat them with an aphid-specific pesticide in early spring, or simply prune away any areas of the cotoneaster that exhibit damage from the bugs.
Cotoneaster can be infested by both armored and soft scales. These bugs, ranging from 1/8 to ¼ inch, are often mistaken for mold or fungus, since they don't really look like bugs. Like aphids, they feed on the juices of the plant, and in small quantities have little to no effect. They also secrete honeydew, which can lead to larger problems. In general, scale populations are controlled by natural predators, including beetles and wasps. If you notice a heavily infested area of your cotoneaster, prune it away to prevent the scales from spreading. Some horticultural oils are approved to serve as pesticides for scales; read and follow all product instructions and warnings on your chosen oil.
There are two types of mites that can infest cotoneaster: eriophyid and spider mites. The first type of mites include gall mites and blister mites, named for the damage they cause on leaves and twigs. You can identify them because they look like orange or yellow felt gathered on the undersides of leaves. These mites cause swells and blisters that distort the plant and cause defoliation, but they cannot seriously harm the plant. There is no chemical control for eriophyid mites, so keep your cotoneaster healthy to help it combat damage. Spider mites grow at most to 1/20 inch, and look like moving dots on your plant. These mites cause defoliation and loss of water as they feed. Regular watering will take care of these pests; use horticultural oil if necessary.
Flathead borers are metallic blue, black or green bugs that tunnel into the bark of the cotoneaster to lay their larvae. The burrowing can kill whole branches or limbs, but the cotoneaster can fight off an infection and avoid serious harm as long as it is not damaged by pruning, sunburn or other causes. The cotoneaster web worm, a type of caterpillar, can devour full leaves and weave webs that lead to damage and defoliation. Trees that are properly pruned and cared for will resist this damage; remove infected areas of the cotoneaster.