Japanese maples are ornamental landscape trees native to Japan, China and Korea. The stoutly trunked, grey-barked trees range from dwarf specimens to trees as high as 20 feet. Their delicately textured foliage, in summer shades of green or red, brings orange, purple, yellow or scarlet to the fall garden. The leaves' colours remain eye-catching even in shade. Japanese maple's foliage, however, is susceptible to sometimes-disfiguring aphid attacks.
Aphids' pear-shaped, soft bodies are up to 1/8 inch long. These pests are commonly yellow-green or green. Less frequent colours include, white, black, brown or red. Aphids have long antennae and legs. Their wings develop only when a large aphid population on one plant requires some to move to another. Aphids will congregate in solid masses on the undersides of your Japanese maple's foliage, where they suck fluids with their piercing mouth parts. These insects often cling to plants even when disturbed.
Signs of Infestation
Aphids are most likely to attack Japanese maples' newly emerged spring leaves. Infestations are possible, however, whenever your tree is actively growing. Leaves with aphids may curl or wrinkle. If the insects appear in spring, your tree's flowers might be deformed. The classic symptom of aphid infestation is a sticky, clear waste product called honeydew. The aphids secrete it onto leaves, branches and the ground around the tree. Ants streaming up your Japanese maples indicate the presence of honeydew and aphids.
Aphids in small numbers may cause cosmetic damage to your Japanese maple's foliage. Because colourful foliage is this tree's major ornamental feature, even minor damage is troublesome. A greater problem is that the honeydew that aphids produce attracts a fungus called sooty mould. Its fungal spores coat the honeydew on the tree with black, powdery mould. Like ants, the spores feed on honeydew. Heavy sooty mould may block sunlight, interfere with the leaves' photosynthesis and weaken your tree.
Stay ahead of aphid infestations with a weekly check when your Japanese maple is actively growing. Use a 10X magnifying lens and sheet of white paper to examine the lower surfaces of the leaves. Hold the paper beneath a branch. Tap the branch sharply to shake the insects free. If only a few aphids land on the paper, the infestation is minor.
Natural Aphid Control
Introduce aphid's natural predators -- lady birds, lacewings and parasitic wasps -- into your landscape. They'll control the pests naturally unless ants are present. Ants protect the aphids supplying their honeydew by killing the pest's predators. Wrapping the maple's trunk with a band of sticky material or spreading ant bait on the surrounding soil keeps them away.
Manage severe aphid infestations with insecticides. Apply a high-pressure spray of neem oil or insecticidal soap, saturating both surfaces of the tree's foliage. These insecticides destroy existing infestations, but don't prevent future ones. You'll need to repeat the applications whenever a weekly check uncovers new aphid populations.