Possible MRLS Solutions Focus on Caterpillar Control

Because of the proposed link of the Eastern tent caterpillar and Mare Reproductive Loss Syndrome, the Grayson/Jockey Club Research Foundation and the University of Kentucky held an informational session Friday to discuss controlling and eradicating caterpillars. The main presenter was Dan A. Potter, a professor in the entomology department at the university.

Potter was joined by several other University of Kentucky researchers, as well as private arborist Larry Hanks and UK colleague Leslie Foss. There were about 130 people in the audience for the educational session, which was held at Fasig-Tipton Kentucky sales pavilion in Lexington, Ky.

"My area of expertise is not at the caterpillar/horse interface," Potter said. "My usual role is working with management on insects that attack turf, grass, and trees."

Potter said that MRLS correlated in time and space with an outbreak "of plague proportions" of caterpillars in 2001. In a timeline that UK agronomist Jimmy Henning had created, he showed that egg hatch of the ETC began March 31; by April 25 the cherry trees were defoliated in Central Kentucky; from April 21-mid-May the caterpillars were wandering. The foal loss numbers were elevated from April 28-May 23. So, the wandering phase of the ETC coincided with MRLS.

"There is evidence for ETC as a risk factor in MRLS," said Potter. "This is not my work, but so far, we have evidence that ETC is a player in this problem. The farm surveys showed high correlation between ETC and MRLS."

The causal agent of MRLS is not known at this time, emphasized Potter. However, he said based on the research and surveys that have been done, exposure of pregnant mares to ETC should be minimized.

Because of this link, there is tremendous interest in control and eradication of these caterpillars in and around horse farms. Some research was done last year during the caterpillar season, and from those studies, Potter gave some recommendations.

First, controlling the caterpillars by controlling the egg masses prior to hatching is important. There are products that can be sprayed on the egg masses that have residue that can kill the larvae when they hatch. The egg masses are covered by several layers of what looks like bubble wrap, and thus far this seems to be impenetrable to the insecticides used. However, the caterpillars chew their way out of the egg masses, and if their first meal contains these insecticides, there is a good chance of killing them.

There are, however, only a limited number of products that can be used to kill caterpillars around livestock. And there are none that will kill the ETC that can be sprayed on pastures. Therefore, it is important to kill the caterpillars as early as possible before they leave the nest and start wandering.

In monitoring egg hatch on three horse farms last year, researchers found that caterpillars hatched over a three-week period. The first hatch of the spring last year was March 15; 50% were hatched by March 29, 90% by April 6, and 95% by April 14.

That the caterpillars hatch over a three-week period is important because it lets those trying to control them know that a short-acting insecticide won't work. Various tactics were tested, and those with the best results were insecticides spot-sprayed on egg masses to kill the larvae as they hatch, and insecticides injected into trees with multiple egg masses that killed the caterpillars as they ate the leaves from the trees.

The insecticide Talstar (Bifenthrin) is known to be less toxic to mammals and environment than other products that are sprayed onto trees. Talstar gave near 100% control within 24 hours of hatching. The insecticide Dipel (Bacillus thuringiensis or BT) was effective, but slower. It required three days for 95% control.

When repeated with older, larger caterpillars, Talstar gave 75% control after one day and nearly 100% control after two days. Dipel was slower, requiring five days to achieve 84% control.

These insecticides were also tested for residual effects. Cherry tree shoots were sprayed in the field, then harvested after one, three, and seven days. They then were challenged with mid-sized larvae to test the residual of each compound. Mortality was determined after 72 hours. Talstar was 100% effective at three days and seven days. Dipel was 100% was effective at three days and only 52% after seven days.

"There was no avoidance of fresh residues of any of the insecticides," said Potter. "Therefore, larvae will not 'bail off' treated trees."

Injecting Bidrin into the base of a tree also was found to be effective. This technique puts insecticide directly into the tree so there is no exposure to the applicator and no drift into the environment, said Potter. The technique calls for drilling small holes in base of a tree and inserting capsules charged with insecticide that look like corncob pipes. Then the tree "sucks up" the insecticide and passes it to the leaves. When all the insecticide is taken up by the tree, the capsules are removed and the tree heals.

In one study, injections of Inject-A-Cide "B", which contains Bidrin, were done by a professional arborist (a special license is required to handle these chemicals). Trees were climbed and 10 nests per tree were pruned out and were dissected. Researchers looked at every caterpillar in every nest and counted dead and live ones. Results showed that injection on April 16 gave 99% control of young larvae (second and third instars) within one week.
When trees were injected just one week later (larvae were larger, in the fourth and fifth instars), control dropped to 50%. This is a restricted use pesticide due to high acute oral and dermal toxicity of the active ingredient, said Potter. The applicator must remain at the site until uptake is finished and the capsules removed. This chemical is used in a lot of sites that are environmentally sensitive, he noted. However, "We are talking about thousands of trees (in this situation), so if we could work with something less toxic, it would be better."

That chemical possibly could be Abacide, which contains abamectin (which is kin to the equine dewormer ivermectin). Potter said it is reportedly longer-lasting than Bidrin, less intrinsically toxic, not a restricted use insecticide, and the applicator need not be certified or remain on site. This chemical has never been evaluated with Eastern tent caterpillars, said Potter, "but the gurus think it will work."

A field experiment will be conducted this spring using both of these chemicals. "We won't know if this works until egg hatch," said Potter.

Where Do we go from here?

"Right now it is a scorched earth policy," said Potter. "This is a stop-gap until they figure (the cause) out. We can't spray every cherry tree in Kentucky every year. That's not ecologically or financially feasible."

Update on 2003 Caterpillar Crop
Scouting indicates that area-wide, the recent outbreak of ETC is on the decline, said Potter. "Populations likely will be much lower in 2003 than in the past three years. But, there still are some sites with abundant egg masses still being found. However, I'm seeing 20 spent (last year's) egg masses for every live one."

He said that while he doesn't expect the number of caterpillars to be as large as the past two years, "Keep your guard up and be aggressive about minimizing exposure."

Options for 2003
1. Scout trees; spot-treat foliage with Talstar or Dipel when nests are first seen. Talstar is fast and works against any size ETC. Dipel needs to be repeated at least three times and will not control large ETC.
2. Micro-injection with Inject-A–Cide "B" (Bidrin) when nests are small.

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