By: Rachel Melvin, University of Maryland Extension-Queen Anne’s County Horticulture Educator & Master Gardener Coordinator
Spring has sprung right into the heat of May without a second glance. As the temperatures warm and the flowers grow, so do the insects. Not only do we get bombarded with mosquitoes upon stepping out of our homes but our trees, flowering perennials and vegetable plants are subject to the same misfortune. As the Horticulture Educator & Master Gardener Coordinator for the University of Maryland Queen Anne’s County, I get my fair share of questions on anything from, insects to plants to rodents. But this week the hot topic in the area has been; what’s wrong with my river birch?
The common variety of birch best suited for Maryland’s climate is the river birch, Betula nigra. Additionally, nearly all the cultivars of river birch for the landscape is the ‘Heritage’ variety. This cultivar has a majestic exfoliating bark in colors ranging from beige to salmon, is tolerant of heat and drought, and is resistant to many insects and diseases. However, the common pest on birch, particularly river birch right now, is the Spiny Witch-hazel Gall Aphid Hamamelistes spinosus. It has a complicated life cycle in that it alternates between two hosts: birch (Betula) and witch hazel (Hamamelis spp.). Overwintering eggs are laid on witch-hazel in June and July. These eggs hatch the following spring and the new aphid nymphs crawl to the flower buds to feed. Feed on the flower buds induces the plant to form a spiny gall. A second generation of winged aphids develop inside the gall, but then leaves and moves on to the birch. These winged aphids give birth to scale-like females that settle and hibernate on the birch until the following spring. As the birch leaf buds open, the scale-like aphids move from the bark to the leaves and begin feeding. The growth and reproduction of the aphids is rapid, and the leaves soon develop characteristic “cor¬rugations.” The corrugations on the undersides of the leaves fill with aphids and a white granular material. Winged aphids develop on the birch leaves then seek witch-hazel on which to lay eggs and complete the life cycle. This activity takes place before the end of June. In addition to the characteristic corrugations of the leaves, this aphid can cause a variety of issues from premature leaf drop to dead twigs and branches.
Control of this pest can be best left to nature. Examine the undersides of the leaves for beneficial insects such as ladybird beetles and their larvae, predaceous midge larvae, Syrphid fly larvae, lacewing eggs and larvae, and parasite activity. Descriptions of these beneficial insects may be found in Home and Garden Mimeo #HG 62, IPM: A Common Sense Approach to Managing Problems in Your Landscape. Any combination of these predators and parasites may give sufficient control without having to spray with an insecticide. If damage is heavy, spray tree with a registered systemic insecticide. Coverage of the lower surfaces of the leaves is critical, as the aphids are fairly protected in the corrugated folds of the leaves.
For more information contact Rachel Melvin, Horticulture Educator with University of Maryland Extension-Queen Anne’s County at (410)758-0166 or at firstname.lastname@example.org . Also check out the University of Maryland Extension Home and Garden Information Center http://www.hgic.umd.edu Home and Garden Mimeo #HG58, for more tips on growing great birch trees.
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Ladybird Beetle Eggs: The female ladybird beetle will lay her eggs on plants where she knows aphids, scale or mealybug colonies are present. This supplies the ladybird beetle larva with a fresh supply of food. A single lady beetle may eat as many as 5,000 aphids in its lifetime.
Spiny Witch-hazel Gall Aphid Hamamelistes spinosus: Heavy infestation of Spiny Witch-hazel gall aphids found on a birch tree in Centreville. Aphids produce a sticky secretion called Honeydew which will attract ants.
UME HGIC Fact Sheet HG58