Despite a colder start towards spring, warmer days have pushed most of Kentucky into a hatch for this native pest.
By Holly Wiemers
Published on March 16, 2026
LEXINGTON, Ky.—
The first eastern tent caterpillars (ETC) of the season are now emerging. Initial sightings have been recently reported in Fayette County and University of Kentucky Martin-Gatton College of Agriculture, Food and Environment entomologists say the hatch is now occurring statewide. The normal time for egg hatch is when the forsythia bloom starts and when the first Eastern redbud blooms emerge.
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% 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.
Temperature heavily impacts astern tent caterpillars’ development.
“The eastern tent caterpillar is one of our early riser pests,” said Jonathan Larson, UK Extension entomologist in the Department of Entomology. “They overwinter as eggs and begin hatching around 100 growing degree days, a measurement of development for insects that reflects the high and low temperature of any given day.”
Protecting trees and horses
ETC’s preferred food plants are wild cherry, apple and crabapple, but may also appear on hawthorn, maple, cherry, peach, pear and plum trees. As the caterpillars mature to 2 – 2.5 inches and become hairy, they may leave their host trees searching for places to spin their cocoons or additional food sources if their original tree is depleted. This often leads them along fences and into pastures.
“Right now, caterpillars are small and their nests won’t be noticeable. Tents will become more obvious over the next 4-6 weeks though. 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 risk occurs when these mature caterpillars leave the trees to pupate and transform into moths. As they travel through the grass, they could be accidentally ingested”
ETCs are named for the compact silk-tent shelters they construct in the crooks and crotches of branches to defend against predators and parasitoids. This species is known for its tidy, nest-building compared to the fall webworm, which creates large, messy webs at branch tips. The ETC population varies annually due to climate, predators and diseases.
Despite not reaching the extreme levels seen during the MRLS outbreak, the presence of these caterpillars still raises concerns. Larson emphasizes the importance of notifying horse owners about the onset of caterpillar activity each year, allowing them to monitor and manage the risk of MRLS by checking their property for caterpillar webs.
As part of caterpillar management practices, pregnant mares should be kept away from infested trees, and preferred host trees should either be removed or not planted close to horse farm paddocks. In certain situations, using insecticides, such as the organic pesticide Bacillus Thuringiensis (Bt), to control the caterpillars in addition to those preventive measures may be necessary. Treating tall trees that are challenging to spray may also be necessary.
For the latter scenario, professional arborists will treat trees via trunk injection. Products labeled for ETC control include Tree-äge (emamectin benzoate), Inject-A-Cide B (Bidrin), Abacide 2 (abamectin) and Lepitect (acephate). Applicators should read and follow all label instructions. All four injectable products are labeled for use on horse farms.
“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.”
To track growing degree days for each county, visit the UKAg Weather Center . For more information about how to assess trees for egg masses, the UK Entomology publication, Checking Eastern Tent Caterpillar Egg Masses, is available.
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Media Contact:
Holly Wiemers
holly.wiemers@uky.edu