River birch is a tough and adaptable native tree that is also a popular landscape plant. There a three common indicators however that can indicate something wrong is going on with your tree.
1) If you notice some of the leaves on your river birch have become thickened, distorted, or discolored in the last few weeks don’t be alarmed. While caused by a type of Aphid, these symptoms are not a sign of serious damage and can even be an indication of good things to come to your yard.
2) If you’ve noticed leaves that look like they’ve been chewed on or caterpillar-like insects on your river birch; these are most likely larvae of the Dusky Birch Sawfly. These 1 inch long, yellow-green bodied, black headed larvae, feed in groups along the leaf margins, eating their way to the center vein. Adult sawflies lay eggs on birch leaves in early spring. The larvae that emerge to feed upon the birch leaves until fall, when they spin cocoons and drop to the ground to spend the winter.
3) If your birch tree exhibits yellowing leaves with sparse foliage at the top of the tree, branch or twig die-back and increased side growth on lower branches; you may have a Bronze Birch Borer which is the larvae of an iridescent olive brown beetle. You may also find D shaped holes and ridges on the tree truck and the branches. For about 6 weeks in the summer, the adult beetles lay their eggs in the bark crevices, usually near a wound. The larvae that hatch and bore into the wood of the tree just beneath the bark. The feeding and tunneling of the larvae stop the flow of water by cutting the conducting vessels in the tree thereby causing twig die-back. Weak, young or newly transplanted trees are most affected and may die.
Following are Mr. Burger’s thoughts on what to do in each case:
APHIDS – A common spring pest, aphids are small soft bodied insects sometimes referred to as plant lice. Size wise, most aphids are little larger than a grain of rice and come in many different kinds and colors. Like many pests, most aphid species are adapted to feed on a narrow range of plants and will not spread to everything in your yard.
Aphids feed on plant sap with straw like mouth parts. On ornamental plants, their feeding can cause leaves and stems to become twisted or distorted, but rarely leads to serious damage. As they feed aphids secrete honeydew, a sticky sweet substance that can attract ants and wasps. Black sooty mold, a harmless fungus, often grows on the honeydew, causing aphid infested plants to appear dirty or sooty. Black sooty mold does not damage plants and can be washed off with water or horticultural oil.
The specific type of aphid that feeds on river birch foliage is known as the spiny witch-hazel gall aphid. In the spring this aphid feeds on river birch leaves. In early summer it moves on to witch hazel foliage, but will not infest any other plant. When spiny witch-hazel gall aphids feed on river birch the leaves become crinkled and discolored, with pink corrugated ridges. If you look on the underside of affected leaves you will see the aphids tucked into the ridges.
River birch trees will shed most of the leaves these aphids feed upon, but they quickly grow new leaves and no lasting harm is done to infested trees. This type of damage is often referred to as cosmetic, since it causes no serious injury, and does not require treatment. Even better, the aphids that cause this damage attract hundreds of ladybugs and other beneficial insects to your yard that will persist long after the aphids have gone.
Aphid populations build up very quickly and it usually takes a week or two for ladybugs to find them so be patient if at first your aphid infested plants lack ladybugs. Each ladybug will eat hundreds of aphids so it does not take many to wipe out even a sizable aphid population.
Control of Aphids – If you just can’t wait for Mother Nature; aphids can be controlled with the following insecticidal sprays: Malathion, Thuricide, Fertilome Triple Action Plus, Spinosad, Insecticidal Soap or Bayer 3 in 1.
Dusky Birch Sawfly – Although this insect looks like a caterpillar, it’s actually the larva of a wasp relative, the sawfly. While this insect can cause a lot of foliar damage, they only exist for a short period of time and the tree will drop and regrow new leaves. Control of the sawfly “caterpillar” – the larvae are a common food favorite for local birds so they can help or the caterpillars can be controlled by simply hand picking them off or by spray washing the larvae off the tree with water or in the case of large numbers, can be controlled with the following insecticidal sprays: Malathion, Thuricide, Fertilome Triple Action Plus, Spinosad or Fertilome Systemic Drench.
Bronze Birch Borer – When birch leaves yellow in the spring/summer and branch or twig dieback first occur; it’s important to cut out and destroy all dead and dying branches. If the tree is small enough to reach the top safely; spray the tree and two more times in two week intervals with an insecticide that contains permethrin like Hy-Yield’s 38 Plus, to kill the young Birch Borer larvae before they burrow into the wood. If the tree is too large, you may want to call in a certified arborist to spray your tree. Maintain good tree vigor by watering and fertilizing regularly.
We hope you find this information helpful and can use it to keep your Birch tree looking great for many years to come.
Partial contents of this post were written by Charlotte Glen and information taken from “The Ortho Problem Solver, 7th edition” book.