I was called in on a consultation with Nic Baumer, Allen County Agricultural and Natural Resources Extension Educator. His fairgrounds manager was concerned about the condition of a couple of trees on the main thoroughfare through the grounds. The initial identification of the trees was American sycamore (Platanus occidentalis). Nic sent several images of the trees and insects found under the dead patches of bark at the end of November 2024.
Nic’s images showed extensive injury to the tree trunks and large limbs. The bark in these areas was discolored, desiccated, cracked, and easily lifted off the trees. Under the bark were several cocoons (which means the insects were moths) with live, late-instar caterpillars, lots of frass pellets indicating that the caterpillars had been feeding under the bark, and tunnels generated by the caterpillars. I couldn’t identify the caterpillars from the images, nor could I find a reference to a caterpillar that tunnels under the bark of American sycamore. Thus, there was a need for a personal visit to the site for additional observations and collection of insect specimens.
The site visit occurred the first week of January 2024. The first change to come to light in the understanding of the diagnosis was the trees in question were not American sycamores, but rather London planetrees (Platanus × acerifolia). London planetree is a hybrid cross of American sycamore (P. occidentalis) and oriental plane tree (P. orientalis). Even though the identification of the tree species was slightly off, a hybrid tree rather than a pure species, it could still be important in the final diagnosis.
My initial thought was that the caterpillars were secondary to the problem and a disease was the primary pest impacting the health of the bark…I was wrong! Samples of the caterpillars under the bark were collected to bring indoors for closer examination and photographing.
Several images of the caterpillar’s head capsule, relatively hairless body, prolegs, and crochet (little hooks on the base of the prolegs) pattern on the base of the prolegs were taken. The caterpillars varied in body coloration ranging from off-white to dull green to pinkish purple. There were a few that were dark brown that may have been the result of being parasitized. A few braconid wasps emerged in a bag holding the caterpillars as well as some were infected by a fungus (probably a Beauveria sp.).
Ultimately, I had to consult other entomologists for help, Joe Boggs and Dave Shetlar. Dave Shetlar tentatively identified the moth as the American Plum Borer (Euzophera semifuneralis). He had encountered the American plum moth infesting young maple trees in nurseries in Ohio from an earlier diagnosis. With the tentative identification, a literature search could be conducted. The American plum borer is a native moth in the family Pyralidae. It is known as an orchard pest of apples, cherries, and plums (https://www.canr.msu.edu/ipm/diseases/american_plum_borer).
Clearwing borers also infest some of these fruit trees, but they can be distinguished from the American plum borer based on the color of the caterpillar and crochet pattern on the prolegs. Clearwing borer caterpillars tend to be white while American plum borer caterpillars are off-white to dull green to pinkish-purple. The crochets of the clearwing borers are arranged in two rows that are perpendicular to their bodies. American plum borer crochets are more numerous and arranged in a complete circle or oval on the base of the prolegs. Click on the link for a comparison diagram: https://ipm.ucanr.edu/PMG/E/I-LP-ESEM-KC.013.html The caterpillar’s crochets in this case were arranged in a circle or oval.
The American plum borer does take advantage of physical injury wounds and cankers and galls caused by disease (https://ecommons.cornell.edu/items/19307bcb-7c56-4f52-9718-e7f70adb40d8). Eggs are laid on and under the bark around the wounds. The caterpillars feed under the bark and in the phloem, cambium and into the new xylem tissue etching the surface of the wood. They also are known to infest several species of ornamental trees including London planetrees (Biddinger and Howitt, 1992). Reference to American plum borer significantly attacking London planetree can be found in Johnson and Lyon’s “Insects that Feed on Trees and Shrubs 2nd Edition, Revised” and fruit, nut and shade trees in Cranshaw and Shetlar’s “Garden Insects of North America 2nd Edition.”
The remaining caterpillars were held in a container at room temperature to collect moths once they emerged to further verify the identification. By the first week of February, moths began to appear in the container. The adult moths matched images of known American plum borer moths (https://bugguide.net/node/view/39393).
Thus, the supporting evidence that the culprit in this diagnosis was the American plum moth was strong. What was the recommendation for the trees in this scenario? The one tree was heavily damaged and beyond rescue treatments. It may simply need to be removed. Its partner was less heavily damaged and potentially could be rescued with some selective pruning to remove unhealthy grow and possibly an insecticide treatment to manage the remaining moth population.
Winter diagnostics can be a bit more challenging than summer diagnostics, but doable if we follow our standard procedures for conducting them, including proper plant identification, documenting the injury, collecting, examining, and photographing the discovered insects to share with other diagnosticians, knowing who to turn to for help, and holding immature insects until they complete their development for verification of identity using the adult insect.
References:
Biddinger, David J. and Howitt, Angus J. 1992. "The Food Plants and Distribution of the American Plum Borer (Lepidoptera: Pyralidae)," The Great Lakes Entomologist, vol 25 (3)
Cranshaw, W., & Shetlar, D. J. 2018. The Garden Insects of North America: The Ultimate Guide to Backyard Bugs (2nd ed.). Princeton University Press.
Johnson, W. T., & Lyon, H. H. 1991. Insects that Feed on Trees and Shrubs (2nd ed. revised). Cornell University Press.
