More on birch bugs
It seems the larvae prefer dining on the back side of birch leaves, but many are found on the front too. Later, they drop gossamer threads and it seems to be their purpose to catch the wind and blow to another tree, thus spreading the infestation among many many birches. However my reading indicates that they drop to the ground and spin small, ribbed, fawn cocoons on the underside of sticks, leaves, or stones. The insect spends the winter in the pupal stage within the cocoons, and moths emerge in early July to lay eggs on tree leaves.
The eggs hatch in late July or early August and the tiny caterpillars bore into the leaves and feed between the two surfaces. About 10 days later, they come out and molt under a small white web. After molting, they feed on the lower surface of leaves between the veins; this gives the leaves a “skeletonized” appearance .
I’m not sure the long term impact that Birch Skeletonizer bugs have, but I’ll be looking into it. Here are the bugs on leafs with the browning that is evident across the landscape shown on this tree.
Looks pretty with the sunlight shining through the threads, but the parachuting caterpillars are lowering themselves to the ground to prepare to overwinter.
Natural Resources Canada report that such outbreaks occur roughly every ten years, with the visual impact peaking after a five year period and then returning to a more benign level of existence virtually unnoticable.