Eastern tent caterpillars have begun hatching in Central Kentucky--and their population numbers are trending up.

"Populations of the Eastern tent caterpillar have increased noticeably over the past three years," said Lee Townsend, PhD, University of Kentucky College of Agriculture entomologist. "While infestations of the magnitude seen during the 2001-2002 mare reproductive loss syndrome outbreak are not anticipated, it is clear that Eastern tent caterpillar populations are on the upswing and could be heavier than normal in some areas. Assessments and management decisions can be made in a few weeks as the silvery, baseball-sized tents start to show up on branches."

According to Townsend, egg hatch is following a historically normal pattern so far this year in Central Kentucky. While daily temperatures will determine the development rate of the caterpillars, there is no way to predict areas where caterpillar numbers will be higher or lower. Entomologists will be closely monitoring caterpillar development over the next two to three weeks.

"Eastern tent caterpillars are early spring insects and can cope with the erratic weather patterns that can occur in March and April. Development, including egg hatch, occurs when the temperature is above 37 degrees Fahrenheit. At 50 degrees, it takes about a month for all eggs to hatch. Warmer conditions will promote hatch over a shorter period of time and give a more uniform population," he said.

According to entomologists, small caterpillars will soon move to feed on the leaves that have begun appearing in trees and will build tents at branch and limb forks. The caterpillars then will begin moving from branches to large limb angles within the trunk. Entomologists anticipate full-grown larvae by the third week of April. From the end of April to the beginning of May, caterpillars will likely leave the trees where they've eaten the available foliage and search for additional food to complete their development.

Once the caterpillars have reached these dispersing stages, controlling them becomes much more difficult, Townsend said. If needed, control should target caterpillars while they are gathered together in the trees.

However Townsend cautions against spraying too early.

"Poor early control in previous research has resulted in a recommendation against very early sprays against small caterpillars," he said.

Controlling Eastern tent caterpillars is vital to area horse farms, as UK research has strongly linked the caterpillars with outbreaks of mare reproductive loss syndrome (MRLS), which can cause late-term foal losses, early-term fetal losses, and weak foals.

During the 2001-2002 MRLS outbreak, an estimated 30% of the 2001-2002 Thoroughbred foal crop was lost, and the state suffered an economic cost of approximately $336 million due to losses suffered in all breeds of horses.

UK researchers conducted epidemiological and field studies which demonstrated that MRLS was associated with unprecedented populations of Eastern tent caterpillars on Kentucky horse farms. Studies since the 2001-2002 outbreak subsequently have revealed that horses inadvertently will eat the caterpillars and the caterpillar hairs embed into the lining of the alimentary tract. Once that protective barrier is breached, normal alimentary tract bacteria may gain access to and reproduce in sites with reduced immunity, such as the fetus and placenta. Fetal death from these alimentary tract bacteria is the hallmark of MRLS.

UK entomologists recommend that unless horse farm managers have been aggressive in managing Eastern tent caterpillars, or removing host trees, they should keep pregnant mares out of pastures bordered by cherry trees or other hosts for the next several weeks.

For a fact sheet about Eastern tent caterpillars, as well as periodic updates, visit ca.uky.edu/equine.  

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