Witch Hazel & Leaf Gall Aphids
Hanamelis virginiana, or witch hazel, is a shrubby plant that is known for the healing liquid extracted from the bark and young stems. Conical-shaped galls can often be found on the plant’s leaves. These are caused by aphids, which use the witch hazel plant to begin a complicated life cycle that is continued on nearby birch trees. The galls are usually harmless to the witch hazel plant, but can be unsightly if infestation is heavy. Does this Spark an idea?
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Witch Hazel Leaf Galls
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The conical galls formed by the witch hazel leaf gall aphid are typically 1/2 to 3/4 inch long and are formed on the twigs and upper surface of the plant’s leaves. They are green and may have reddish tips. Galls are formed by a single aphid and contain eggs that overwinter on the plant, hatching in early spring. Although the galls are present on the leaves throughout the growing season, they are usually empty by midsummer, the aphids having migrated to nearby birch trees. These galls do not harm their witch hazel host, and natural enemies typically provide enough control to prevent major infestations.
Witch Hazel Gall Aphid Life Cycle
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Hamamelistes spinosus, more commonly known as witch hazel gall aphid, lays eggs on witch hazel plant leaves in June and July. These eggs overwinter on the leaves and hatch the following spring. The newly hatched larvae feed on the buds of the witch hazel plant. If there are no buds formed at the time of hatching, the aphids die. This feeding is what causes the plant to form spiny galls. Within these galls the aphids lay their eggs, and a second generation of winged aphids develops. The second generation flies to nearby birch trees once hatched and produces a scale-like generation resembling whiteflies that overwinters on the birch. The following spring, the larvae feed on the birch leaves, producing winged aphids that migrate back to the witch hazel plant, as well as wingless aphids, called accessory females, that remain on the birch tree to mate and lay eggs. This produces an additional generation of winged aphids that later fly back to the witch hazel plant. Their winged counterparts that have already migrated to the witch hazel plant produce yet another generation of wingless males and females and the cycle begins again. This life cycle takes two full years and requires both the witch hazel plant and the birch tree to complete.
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Damage
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The galls formed on the witch hazel plant by aphids can be ugly, causing leaves to curl, and sometimes result in defoliation, but they are usually not harmful to the plant’s health. On birch trees, the aphids do not form galls, but their feeding can cause premature defoliation, and the production of honeydew can cause sooty mold to form.
Control
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By the time leaf galls are noticed on a witch hazel plant, it is usually too late for control. Gall formation can be prevented the following spring with the use of horticultural oils. On birch trees, pesticides sprayed in early spring or application of insecticidal soap can offer some control of the migrating insects, but this should be done every year before bud break as long as witch hazel plants are present in the landscape. The ideal control for witch hazel leaf gall aphids is the presence of natural predators such as the lady beetle.
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