There are many intricate relationships between different species of plants and animals yet to be discovered. The interrelationship of sourwood (Oxydendrum arboreum), lettered sphinx moth (Deidami inscripton), and black bear (Ursus americanus) was first noted in the scientific literature just last year. Sourwood is a small tree, seldom growing to over 6o feet in height, that lives in oak forests and woodlands with acidic soils. It is the sole species in its genus and a member of the blueberry and azalea family. The leaves have a sour taste and can be chewed but shouldn’t be swallowed because they are mildly toxic with a high amount of oxalates. Scientists were studying the occurrence of a major defoliation event of sourwood trees near Unicoi, Tennessee a few years ago. Here, sourwood trees along with dogwood, summer grape, Virginia creeper, and greenbrier form the understory of a forest composed of red maple, black gum, northern red oak, pitch pine, Virginia pine, chestnut oak, scarlet oak, and striped maple. They found the sourwood trees were being defoliated by larva of the lettered sphinx moth.
A sourwood tree in fall foliage.
The lettered sphinx moth.
The larva of the lettered sphinx moth feeds upon grape, Virginia creeper, and peppervine; but just recently was discovered to have a preference for sourwood over all those plants in the Vitis family.
The lettered sphinx moth is the only species in its genus that lives north of Mexico. Lettered sphinx moth larva were known to feed upon the leaves of plants in the grape family which also includes Virginia creeper and peppervine. Lepidopterists refer to these plants as “host species.” However, when scientists discovered sphinx moth larva defoliating sourwood they conducted an experiment–they put sphinx moth larva in terrariums and offered them grape leaves and sourwood leaves. The sphinx moth larva preferred the sourwood leaves. This suggests sphinx moth larva will choose sourwood leaves wherever the ranges of sourwood and species in the grape family overlap.
Scientists hypothesize the oxalates ingested from the sourwood accumulates in the caterpillar, and the toxicity discourages avian predators. Nevertheless, bears are able to eat the caterpillars. The authors of the below referenced study found evidence bears were consuming large quantities of sphinx moth caterpillars during the defoliation outbreak. They saw stem breakage, claw marks on limbs, and bear scat filled with caterpillar remains all around the sourwood trees. Moth larva provides lots of protein and fat, and the partially digested plant material in their guts likely contains beneficial vitamins for the bears. The bear scat in turn helps fertilize the soil around the sourwood trees.
Black bear feeding on forest tent caterpillars. Caterpillars are nice fatty snacks for the bruins.
The interrelationship between sourwood, sphinx moths, and bears probably began during the Pleistocene or perhaps earlier; but it wasn’t noticed or recorded by people until last year. There are countless other examples like this, yet to be discovered.
Levy, Foster; David Wagner and Elaine Walker
“Deidamia inscripton (Lettered Sphinx Moth) Caterpillars feeding on Oxydendrum arboretum (Sourwood) and their Predation by Black Bears in Northeastern Tennessee”
Southeastern Naturalist 15 (3) 2016