Dave Bienemann, Municipal Arborist/Utility Forester, City of Hamilton, alerted me to two pests appearing on river birch (Betula nigra) in southwest Ohio. Dusky Birch Sawfly (Croesus latitarsus) larvae are munching on the leaves while Spiny Witchhazel Aphids (Hamamelistes spinosus) are producing raised ribs or "corrugations" on the upper leaf surface.
Both of these native insects have such a strong preference for river birch they have alternate common names that include a reference to the native tree. The dusky birch sawfly is sometimes called “river birch sawfly” and the spiny witchhazel aphid is sometimes called “river birch aphid.”
Ravaged Birch Leaves
Dusky birch sawfly larvae feed in aggregations that may number 10 - 20 individuals. All instars have shiny black head capsules and distinct black spots on their bodies. When disturbed, the larvae form their bodies into an "S" shape (S for sawfly?).
Early instar larvae are dark gray, middle instars are greenish-gray, and late instars are yellowish-green in color. Early instars consume the interveinal tissue leaving the mid-vein and main lateral veins; late instars consume entire leaves.
This sawfly has two generations per season in Ohio with the second generation typically causing the most damage owing to larger numbers of larvae compared to the first generation. Thus, targeting the first generation with suppression tactics will limit the overall damage this season.
Fortunately, it’s extremely rare for an entire tree canopy to be affected by this sawfly. The aggregations focus the defoliation on individual branches. Also, this sawfly has a history of widely swinging populations from year to year meaning that trees with high populations this season may host no sawflies next season.
The caterpillar-like dusky birch sawfly larvae are sometimes mistaken for caterpillars and targeted with the biological insecticide Bacillus thuringiensis (Bt). There are strains of Bt that will control moth caterpillars (order = Lepidoptera); however, sawflies belong to the order Hymenoptera (e.g., wasps, bees, and ants). Thus far, no Bt strains have been discovered that will kill hymenopteran insects.
The simple way to tell the difference between lepidopteran caterpillars and hymenopteran sawfly larvae is to count the number of abdominal prolegs; the fleshy "extra legs" that are lost during pupation. Counting from front to back, the first 3 pairs of legs just behind the head are the hardened "thoracic legs;" these legs will also be found in the same position on the adults. The following pairs of fleshy legs are the prolegs. The last pair of prolegs are called the “anal prolegs” and they won’t be counted.
Sawfly larvae have 6 or more pairs of abdominal prolegs; caterpillars have 5 or fewer pairs of prolegs. A handy way to remember this identification tip is to compare the number of prolegs to the number of fingers on your hand.
White-Splattered and Corrugated River Birch Leaves
The spiny witchhazel gall aphid is so named because it hijacks expanding witchhazel (Hamamelis spp.) flower buds to produce hollow, spiny, globular galls. The galls provide a home and food for the aphid’s progeny.
The galls have no impact on the overall health of their witchhazel host because they only affect flower buds rather than stem or leaf buds. Presumably, aphids that arise from the galls fly to a river birch to become river birch aphids, but more on that later.
The aphids produce three symptoms on river birch. The most obvious are raised ribs or "corrugations" on the upper leaf surface that matches deep furrows between the veins on the lower leaf surface where the aphids live. Occasionally, leaves become distorted curling over the aphid’s hideout. The affected leaves will eventually turn yellow and may prematurely fall off of the tree.
River birch aphids are prolific honeydew producers and the clear, sticky droplets rain down upon lower leaves, understory plants, slow-moving gardeners, etc. The aphids cover themselves in a waxy, white, flocculent material that sloughs off to become stuck in the honeydew making leaves look like they’ve been splattered with white paint. Of course, the honeydew may eventually become colonized with black sooty molds to present a nice contrast.
Although symptoms may be obvious, river birch aphids seldom produce enough damage to seriously affect the overall health of their host tree. Based on personal observations, it’s rare for more than 20% of the leaves to be infested which is far below a threshold that will cause harm to the overall health of an affected tree.
Additionally, as with the sawfly, it’s not usual for populations to rise and fall dramatically from year to year. The wide swings in population densities are driven by the “3-Ps”: predators, parasitoids, and pathogens.
This aphid is reported to require both birch and witchhazel as alternating hosts in its complex life cycle. However, I’m beginning to believe birch and witchhazel are alternate hosts with H. spinosus capable of completing its life cycle entirely on birch.
I can present no scientific evidence in support of my skepticism, only anecdotal evidence. It’s common to find significant populations of the aphids on river birch located many miles from witchhazel in southern Ohio. Conversely, I seldom find large numbers of galls on witchhazel in my part of the state. Indeed, despite yearly searches, I’ve only ever taken one group of photos of the galls on witchhazel. I believe the relationship between the two hosts and the aphid deserves a second look by researchers.