Anyone who has ever been near a birch tree in our coastal area this time of year will tell you the tree’s foliage is sticky, and it’s for a good reason. A close examination of the undersides of leaves will reveal thousands of aphids in a feeding frenzy literally sucking the sap out of the tree. Although birch trees are prized throughout our landscapes for their aesthetic appeal, they are also notorious for providing a banquet for aphids. Once aphids hatch on birch foliage their populations quickly explode. This time of year, when the weather is warm, aphids can develop from new born nymphs to reproducing adults in less than two weeks. Because each aphid can produce up to 80 offspring in a matter of a week, aphid populations can increase with great speed to the point of doubling every two days.

Large populations of aphids cause curling, yellowing and distortion of leaves and stunting of shoots. In addition to taking a toll on tree foliage, aphids also produce large amounts of a sugary liquid waste called “honeydew.” The honeydew that drops from these insects is what causes the foliage to be sticky and can spot the windows and finishes of cars parked under infested trees. A fungus called sooty mold can grow on honeydew deposits that accumulate on leaves and branches, turning them black. The appearance of sooty mold on birch trees may be the first time that an aphid infestation is noticed. The drops can attract other insects, such as ants, that will feed on the sticky deposits.

There are several options for controlling aphids. Summer oils kill aphids by suffocating the insects and /or disrupting their membranes. Be sure to read and follow label directions as oils can cause foliage burn if temperatures are high. Do not spray dormant oils during the growing season. There is no residual effect so additional applications may be necessary. Fatty acid salts or insecticidal soaps are also very good against aphids. As with summer oils, they work to disrupt cell membranes. They require direct contact with the insects and leave no residual effect.

It’s not only aphids that find the foliage of birch trees irresistible. Birch leafminers are prevalent in our coastal area as well. Although they are related to wasps, bees and ants they don’t have a constricted abdomen like the rest of their relatives. The adults are small black wasps which emerge in April and May and begin ovipositing eggs into newly developing birch leaves. The eggs hatch and the larvae begin to feed on leaf tissue. What sets this insect aside from most others is the fact that the larvae of the birch leafminer feeds on the mesophyll, the interior tissue of the leaves, leaving the outer epidermis intact. Having the epidermis intact provides a nice protective house for the leaf miner to feed safely. The upper surface of the leaf can appear transparent allowing the careful observer to actually see the leafminer at work. Areas where leafminers are active appear as blotchy brown leaves. Birch leafminers can produce several generations during the growing season. In severe infestations, trees may begin to totally defoliate prematurely.

Options for the control of Birch Leafminer are limited. Pinching the leaves of small trees to kill the miner will help to some extent. WSU currently lists both acephate (Orthene)* and imidaclopyrid (Merit)*as registered insecticides for the control of birch leafminer when used according to labeled directions. Although birch leafminers may result in brown foliage and partial early tree defoliation, trees will often produce new leaves to replace the damaged ones. The insect seldom causes tree mortality.

*To simplify the presentation of information, it is sometimes necessary to use trade names. No endorsement of products is intended nor is criticism of unnamed products implied.