Study Associates Caterpillars and Reproductive Losses

A recently completed collaborative effort at the University of Kentucky (UK) associated the Eastern tent caterpillar and its frass (excrement) and mare reproductive loss syndrome (MRLS). This is only the first step in looking at the caterpillar/MRLS connection.

The caterpillar project was planned in late 2001 because there was a need to have the project ready to go as soon as the caterpillars in Kentucky started hatching this spring. The research was conducted on the university's Spindletop Farm in Lexington with pregnant mixed-breed mares leased from an eastern Kentucky farm.

Pasture plots of 20 by 200 feet were used for this study. The pens were separated by 20 feet. For 10 days, mares were put on the plots for six hours and allowed to graze. They were divided into three groups--one group had
caterpillars and their frass placed in the field each day; one group only had caterpillar frass put in the field each day; the third group was kept away from caterpillars and frass as much as possible, but handled and sampled the same way as the other two groups.

Each day, the plots were reduced by 20 feet, and more caterpillars and/or frass was added. This means the concentration of exposure increased each day and by Day 8, the horses were in a 20- by 40-foot pen. Researchers put 10,000 or more caterpillars on the plot per day. (Little-known fact: It takes about 900 newly hatched caterpillars to make one pound, but one pound of mature caterpillars is only about 250 caterpillars.)

Bruce Webb, PhD, a professor of entomology in UK's College of Agriculture, oversaw the research project. He said the College of Agricultural Engineering designed "traps" from PVC plastic pipe cut in half (nearly a mile of pipe). Most caterpillars were not able to crawl around the C-shaped curve. An added precaution to this barrier to keep caterpillars in pens was the addition of TangleFoot (a sticky substance) on the top of the pipe to prevent their escape.

Webb said he wanted to do this study because the survey done by the university last year implicated Eastern tent caterpillars. He thought this basic research would either point to or rule out an association between the caterpillars and MRLS. Webb's initial thoughts were that caterpillars were not involved in the syndrome, but the results of the experiment changed his mind.

"We needed to try and reproduce the syndrome," said Webb. "There are only five to six weeks of caterpillars each year (in Kentucky). We had people working seven days a week on the farm with the mares and in the laboratory."

Assisting Webb in caterpillar collection and storage were Claire Collins, who will be starting her PhD work in the fall, and Walter Barney, who earned an entomology degree in 1988 and came back to UK to work on this study.

Webb felt it was important to use "Kentucky-bred" caterpillars in this research to most closely mimic the natural environment. Therefore, he had students collecting caterpillars from along roadsides, from nurseries, and from farms. He had about 160 pounds of caterpillars left in the laboratory prior to finishing his initial research project. (See Article Quick Find #3465 at for details on this research.)

The experiment showed a statistically significant correlation between early fetal loss and caterpillar frass. Webb and his assistants have collected almost 50 pounds of frass and stored it for future study.

What They Learned

"I think it is the caterpillars that are actively feeding and producing frass that are the problem," said Webb. "So we need to control caterpillars early while they're still eating."

What's Next?

Webb repeated this study of correlating caterpillars and their frass with MRLS while there were still caterpillars in the right stages of development. However, since it is the feeding stages of the caterpillar that Webb thinks are the most problematic to horses, he wanted to use frass rather than caterpillar exposure.

He also wants to irradiate the frass and kill any biologic agents it contains to see if there is a toxin or biologic agent (bacteria, fungus, virus, etc.) that is causing the problem.

Webb said excess spring rain probably contributed to the lower loss rates. He thinks the rain "drives" frass into the soil.

The timeline of Eastern tent caterpillar growth and movement and MRLS occurrences in 2001 was as follows:

  • March 31: Eastern tent caterpillar eggs start to hatch.
  • April 21 through mid-May: Caterpillars start wandering.
  • April 25: By this time, black cherry trees were defoliated.
  • April 28-May 23: Foal loss numbers rose.

"We've probably had MRLS around at low levels for a long time," said Webb, "but maybe now we don't have to live with it."

Update on 2002 Early Fetal Loss in Central KY

Ultrasound examinations undertaken on mares over 42 days of gestation by veterinarians specializing in equine reproductive practice in central Kentucky provided the following information:

Of 3,089 mares examined between April 30 and May 17 of this year, 2,783 (90%) remained pregnant as of May 22. The normal rate of loss over this time period would range from 1-2%. The figures are derived from members of two equine practices in Lexington serving a large number of horse farms in the area.

Data collected by 20 veterinarians of Hagyard-Davidson-McGee Associates from mares greater than 45 days pregnant examined between May 6 and May 17 indicate 1,848 (88%) of 2,103 mares remained pregnant. Similar figures from the Rood and Riddle Equine Hospital of mares greater than 42 days pregnant examined between April 30 and May 17 indicate 935 (95%) of 986 mares remained pregnant.

These figures provide a "snapshot" of information gathered over a specific time period during the early part of May. More comprehensive data will become available as the breeding season progresses.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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