QuestionHi, I live in Homer Glen, IL I have (2) 35 foot river birch trees in the back yard and (1) out in front of my house. Problem: it seems that the leaves are turning yellow (this usually happens) but this time the leaves have some kind of raised little black bumps on them? When the leaves are dried they look like a web that you can see thru? My husband thinks I nut's but these little black bumps are falling all over and I think I can hear something falling from the trees? Please help.
Thank you,
Diane Sarosiek
PS. the two river birch trees in the yard are over 25 years old and the one triver birch out front is about 22 years old.
AnswerSounds like birch leafminer. The birch leafminer, Fenusa pusilla (Lepeletier), is a small sawfly native of Europe that was first detected in Connecticut in 1923. It has since spread throughout northeastern North America.
Damage to birch trees is done by the larval stage as it feeds between the upper and lower layers of the leaves. This feeding produces large blotch mines in the leaves. Often, several larvae will completely mine a single leaf. Multiple generations and high populations can cause damage to almost every leaf by mid-summer. The mines turn brown and will cause the tree to look dead with wilted brown leaves. This forces the tree to refoliate and reduces its ability to produce food for growth. This damage may also make the trees more susceptible to bronze birch borer attack. Though the adult birch leafminers are related to wasps, they do not have a sting.
Mature larvae overwinter in the soil under host trees. When the soil warms in the spring these prepupae pupate, usually in April, to transform into the adult stage. Within a few weeks the adult sawflies dig out from their earthen cells and fly to the newly expanding foliage of birches. The small black adults prefer to mate and oviposit on the upper leaves, especially in sunny areas. Mated females use their needle-like ovipositor to punch a hole in the leaf and lay eggs. Often the female withdraws the ovipositor without laying an egg. The damaged spot may turn brown as the leaf expands and hardens. The eggs are often visible in the tissues between the major leaf veins. They may appear as small raised spots on the leaf surface. Within 7 to 10 days the eggs hatch into tiny flattened larvae with wide front segments. These larvae produce a blotch mine that may join with others. The old mines may contain considerable amounts of dark fecal pellets. The larvae mature in 14 to 20 days and are about 1/4 inch long. By this time the larva have the diagnostic small black square marks on their lower surface. Mature larvae cut a hole in the leaf epidermis and drop to the ground. Here they dig one to two inches into the soil to form a pupation chamber. Birch leafminers usually produce two to three generations a summer.
Birch leafminers prefer sunny areas but will attack susceptible trees almost anywhere. Birches tolerate leafminers best when they are planted in shady, cool, moist areas. Since birch leafminer attacks may weaken the trees, making them more susceptible to borer attack, good fertility and horticultural care are needed. When chemical controls are needed, control of the first generation will generally eliminate any need for additional applications.
Systemic Insecticides for Larval Control Systemic insecticides, especially easily translocated materials, can be applied to the foliage,or by soil injection. Attempt to control the first generation of larvae when the mines or eggs are first apparent. Systemic insecticides registered for control are: acephate (Orthene), and a newer insecticide called Bayer Advanced.
This insect will not kill the tree but can reduce the amount of growth. I would try the systemic insecticide treatment now either spraying the foliage or soil application. Most of the insects may have completed the life cycle but this will reduce the population next year. Next year watch the tree and when the first signs of damage appear then spray. This will kill the population as they infest the tree.
