Sometimes by midsummer you can identify stands of paper birch from far away: they are the tan patches on otherwise green hillsides. Identifying the cause of the discoloration is more difficult – even up close. Most of the time, it’s one of the many insects called leafminers, whose tiny larvae feed between the top and bottom epidermal layers of a leaf. There must be a reason that birches are so prone to leafminer damage. Whatever it is, many leafminer species have evolved to exploit this vulnerability. There are at least a half-dozen common and widespread leafminers that specialize on birch, each having its preference for paper, yellow, gray, or another of the 10 birches native to this continent. Most of the birch leafminers that cause significant damage have been introduced from Europe.
An insect has to be quite small to succeed at mining leaves. Working conditions are cramped, and the leafminer larvae that live in and eat the palisade cells and spongy mesophyll that occupy the interior of a leaf are flattened, as well as being small. Sometimes a dozen or more larvae feed within a single leaf and consume all but the papery dead skin, eliminating all shades of green. A medium-sized birch tree can feed tens of thousands of leafminers.
Fenusa pusilla, called the birch leafminer, is a sawfly in the order Hymenoptera. It has spread relentlessly since its arrival from Europe sometime before 1923, when it was first noted in Connecticut. The adults emerge from their pupal cases in the ground just as new birch leaves are unfolding in May. Eggs are laid on opening buds and tender new leaves. When first hatched, the feeding of the tiny larvae results in small blotches on the leaf. As the larvae grow, so do the blotches, and if there is more than one miner per leaf, the mined areas coalesce into one large, tan patch that expands outward to the leaf margins. After 10 to 14 days of feeding, the larvae drop to the ground to pupate. It takes only about 35 days to go from an egg to an egg-laying adult, allowing for several generations each year. Fortunately, tender new foliage is required for egg laying, so later generations are not nearly as successful as the first.
In its native range, the birch leafminer is kept in check by 17 or more wasp species that lay eggs inside the leafminer larva. The wasp larva slowly consumes and kills its host. Two of these parasitoids, as they are called, have been imported and released, beginning in the 1970s, at various places across the U.S. and in Canada, and one of them, Lathrolestes nigricollis, has become established, is spreading on its own, and seems to be responsible for markedly decreasing leafminer damage in several different locations. At a release site in Massachusetts, 50 to 54 percent of birch leaves were mined before the wasps were released there in 1979. Only 3 percent had mines 15 years later.
A previous insect introduction, the late birch leaf edgeminer (Heterarthrus nemoratus), eats older leaves if they have not been damaged, and the signs of its feeding aren’t apparent until late July. It hitched a ride across the ocean in about 1905 and was a serious pest until about 1940, when, for mostly unknown reasons, populations subsided. Some native parasitoids and predators develop a taste for non-native insects over time, and perhaps this explains the decrease in late birch leaf edgeminers.
Damage caused by a more recent immigrant, the early birch leaf edgeminer (Messa nana), is now on the rise. It was first identified in Maine and New York in 1966, and, like the late birch leaf edgeminer, its mines start at the edge of the leaf and expand inward. Browned leaves show up early in July. As Messa nana expands its range, it is causing severe defoliation.
After a few decades, some measure of control – by introduced or native parasites or by unknown causes – seems to affect the population levels of these introduced leafminers. The most recent immigrants, having left their parasites and predators behind, are able to raise havoc inside the leaves of one kind of birch or another. Though Messa nana is now the bad kid on the block, at least in some places, there are still plenty of the more seasoned miscreants, contributing their mandibles to the job of turning birch leaves gray, tan, and brown from May to August.
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