Eastern and Forest Tent Caterpillars Feeding; Timing Right for
Horse Farms to Implement Control Strategies
LEXINGTON, Ky. (April 5, 2007) – Experts report that eastern
tent caterpillars and forest tent caterpillars are developing
normally this spring with egg hatching complete and populations
feeding, making this an optimal time for area horse farms to
assess caterpillar activity and implement control strategies
while the larvae are still small and most susceptible.
“There is no indication of widespread high populations of the
eastern tent caterpillar this year”, said Lee Townsend,
University of Kentucky College of Agriculture entomologist.
There are, however, abundant populations of the caterpillar in
some localized areas.
Townsend added that this week’s freezing nighttime temperatures
won’t adversely affect populations of the eastern tent
“The eastern tent caterpillar is an early spring insect so it
typically faces erratic weather patterns like abnormal warm or
cold spells” he explained. “It is well-protected by its ‘hairy’
body insulation and aggregating (gathering) with other
caterpillars in the tents.”
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
During 2001, when MRLS hit Central Kentucky particularly hard,
an estimated 30 percent of the 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.
Epidemiological and field studies conducted by UK researchers
demonstrated that MRLS was associated with unprecedented
populations of eastern tent caterpillars, wild black cherry
trees and waterfowl on horse farms in Kentucky. A series of
studies over the next five years has subsequently revealed that
horses will inadvertently eat the caterpillars and that 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.
Now is an optimal time for controlling populations because the
caterpillars are still gathered together in the trees, Townsend
said. Currently, small caterpillars are moving to feed on the
leaves that have been appearing in trees and have built tents at
branch and limb forks.
“Initial growth of the caterpillars will be slow, but over the
next two to three weeks caterpillars in limb nests will begin to
move to the main truck branches and join together in a smaller
number of large tents on individual trees. This aggregation
behavior can be used advantageously to manage the caterpillars
by destroying or treating accessible populations,” he said.
After a few weeks, the caterpillars will leave the trees where
they’ve eaten the available foliage and search for food to
complete their development. Once the caterpillars have reached
these dispersing stages, controlling them becomes much more
difficult, Townsend said.
Townsend offered the following recommendations for controlling
populations. “Foliar sprays for caterpillar control can be made
during this time period. Spray residues of products based on
Bacillus thuringiensis (Bt) must be eaten by small caterpillars
to be effective; there is no contact effect. Consequently,
applications should be made to as much of the canopy as is
feasible, especially the foliage around active nests. Direct
application to nests will not provide any control. Bt residues
on foliage can be broken down by sunlight in three to four days
so it is important to assess control and re-treat if necessary.
Effectiveness of Bt decreases as caterpillar size increases,” he
“Foliar sprays with products such as bifenthrin (Talstar) or
carbaryl (Sevin) have both stomach and contact activity so they
can be effective when sprayed on foliage or tents. The residual
life of carbaryl is about a week; that of bifenthrin is at least
two to three weeks. Another option is to inject trees with
either bidrin (Inject-A-Cide “B” or 2 percent Abacide).
Regardless of the treatment used, it is important to revisit the
sites in about five days to assess caterpillar activity,” he
Additionally, experts recommend that horse farms scout for the
telltale white tents in cherry and crabapple trees and err on
the side of caution by keeping mares away from any caterpillar
Townsend also explained that forest tent caterpillars have been
at outbreak levels along the Ohio River for the past two years
and appear abundant in the Boone County area. The forest tent
caterpillar is similar to the eastern tent caterpillar and
Townsend didn’t rule out the possibility that it could cause
similar problems in horses as the eastern tent caterpillar does.
He added that the forest tent caterpillar does not seem to be
active in Central Kentucky.
Contact: Lee Townsend, 859-257-7455
Holly Wiemers, 859-257-4883
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Kentucky Cooperative Extension Service
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