While a cold winter delayed hatching for some counties, others are just now starting to reach the threshold.
By Jordan Strickler
LEXINGTON, Ky. (March 20, 2025) — The Eastern Tent Caterpillar (ETCs) gained notoriety during the early 2000s when its accidental ingestion by pregnant mares was linked to Mare Reproductive Loss Syndrome (MRLS). From 1999 to 2001, MRLS caused an estimated 3,500 foal losses, including abortions, stillbirths and neonatal deaths. Central Kentucky alone lost around 30%[EJ1] [SJ2] of its 2001-2002 Thoroughbred foal crop, resulting in an estimated state economic impact of $336 million across all horse breeds—making tracking and managing the pests significant for horse owners, even to this day.
Due to a colder winter and slower accumulation of growing-degree days (GDD) this year, experts at the University of Kentucky Martin-Gatton College of Agriculture, Food and Environment anticipate a delayed hatch for ETCs.
“ETCs begin to hatch after enough warm days accumulate,” said Jonathan Larson, assistant extension entomology professor in the Department of Entomology. “We’ve seen a slower approach to that number this winter, which gives horse owners more time to monitor and prepare.”
These native insects typically begin hatching when 90-100 GDD have accumulated. As of March 17, representative counties such as McCracken (99 GDD), Barren (116 GDD), Fayette (82 GDD) are approaching or ticking over that threshold, while others, such as Harlan (28 GDD) remain well below that threshold. Horse owners and farm managers should begin vigilance for small caterpillars, as ETC infestations pose a serious risk to pregnant mares. Locally, managers can look for forsythia shrubs that have started to bloom as an indicator of ETC hatch.
Identifying and managing Eastern Tent Caterpillars
Eastern tent caterpillars are commonly observed on wild cherry, apple and crabapple trees, although they may also infest hawthorn, maple, peach, pear and plum. True to their name, these caterpillars construct compact silk “tents” in the forks of branches. The tightly woven nests serve as shelters, helping to protect the growing larvae from predators and harsh environmental conditions.
Over the winter, ETCs remain dormant as egg masses wrapped around pencil-thin twigs, waiting for the warmer days of early spring to hatch. Once they emerge, the larvae feed primarily at dawn, dusk or overnight and can grow to a length of 2 to 2.5 inches. As resources become scarce—or when it is time to pupate—these caterpillars often leave their host trees, wandering along fence lines and into pastures in search of new food sources or protected pupation sites.
In pregnant mares, ETC hair cuticles can lodge in the lining of the digestive tract, creating an entry point for bacteria. The resulting infection may harm both the fetus and placenta, posing a significant threat to equine health and foal development.
“If farm managers notice high numbers of nests, they should relocate pregnant mares from areas near wild cherry trees to reduce the risk of exposure,” Larson said. “The greatest danger occurs when mature caterpillars leave the trees to pupate and transform into moths.”
Preventive Strategies for Horse Owners and Farm Managers
Horse owners and farm managers can employ several preventive strategies to mitigate risks. First, monitoring wild cherry and other host trees is important, as it requires carefully inspecting them for egg masses and developing tents. Early detection allows for timely intervention before the caterpillars spread.
When infestations are discovered, removing affected branches may be necessary. Smaller tents can be wound onto a stick, while larger ones should be pruned. Burning tents with an open flame is not advisable, as this can easily damage the tree.
With insecticide use, early intervention with the organic pesticide Bacillus thuringiensis (Bt) can help target young caterpillars. For taller trees or extensive infestations, professional arborists may recommend trunk injections with labeled products such as Tree-äge (emamectin benzoate), Inject-A-Cide B (Bidrin), Abacide 2 (abamectin) or Lepitect (acephate).
Finally, it is essential to stay informed by tracking the local growing-degree days and consulting extension resources. By doing so, horse owners and farm managers can anticipate when hatching and peak caterpillar activity will likely occur, thereby reducing potential risks.
“ETC activity can vary annually due to climatic conditions, predators and disease,” Larson said. “However, understanding their life cycle and habitat preferences allows us to address potential risks preemptively. Awareness is our best defense. By tracking the hatching and development of these caterpillars, we can implement targeted interventions to protect our trees and, crucially, our horses.”
For more detailed guidance on identifying and removing egg masses, download the UK entomology publication “Checking Eastern Tent Caterpillar Egg Masses” at https://entomology.ca.uky.edu/ef449.
###
Writer: Jordan Strickler, jstrickler@uky.edu
The Martin-Gatton College of Agriculture, Food and Environment is an Equal Opportunity Organization with respect to education and employment and authorization to provide research, education information and other services to individuals and institutions that provide equal opportunities for qualified persons in all aspects of institutional operations and do not discriminate on the basis of race, color, national origin, ethnic origin, religion, creed, age, physical or mental disability, veteran status, uniformed service, political belief, sex, sexual orientation, gender identity, gender expression, pregnancy, marital status, genetic information or social or economic status.
Contacts: Jonathan Larson, jonathan.larson@uky.edu
Media Requests: C.E. Huffman, ce.huffman@uky.edu