Generally speaking the first most obvious sign of this pest is browning and wilting of the foliage on your birch tree. The larvae cause this discolouration and distortion by feeding that create tunnels within the individual leaves. These tunnels first appear as a greenish colour but quickly turn to a grey, then eventually a brown. These tunnels contain the waste of the larvae and are blackish in colour. This waste is known as frass. The larvae are flat to aid them in their tunneling habits and are a greenish yellow in colouration. They also have four black spots on their underside at the front. By holding the leaves up to a light the eggs and larvae will appear as transparent spots.
Birch leaf miner will create 3-4 generations per year. The adult will emerge from the soil in May and June and will deposit their eggs in slits created in the surface of a newly developed leaf. The larvae consume the tissue situated between the upper and lower leaf surface. They will eventually eat their way through to the surface of the leaf and fall to the ground to pupate. The adults emerge again and continue the cycle. They will over-winter below the tree and emerge again in the spring.
CONTROLLING THE PROBLEM:
An infestation of birch leaf miner generally speaking will not kill a healthy mature tree. By defoliating the host tree they can however weaken it, making it more susceptible to attack by other insects and diseases. If you are using insecticides to control this pest, please be sure to follow all directions on the label before and while applying. By controlling the initial generation of birch leaf miner you can go a long way to minimize the damage caused by additional generations. Use a systemic insecticide before the buds open to prevent this pest from egg laying. You can also apply insecticides as the new leaves emerge.