The traps are not meant to catch enough beetles to reduce their number. Rather, they are designed to detect the leading edge of an ash borer infestation. If officials know where the beetles are, and where they are going, they have a chance to slow them down.

Finding even one beetle in a trap will set off federal and state quarantines, as happened this summer when an emerald ash borer was found at the United States Military Academy here, 60 miles north of New York City. Knowing that West Point’s 16,000 acres were in the path of the ash borer onslaught, Robert G. MacKenzie, a forester who works for the military academy, had volunteered in May to put up Barney traps.

He hung 16 traps in healthy ash trees at the center of the academy’s grounds. Each side of the trap is corrugated plastic, 14 inches wide by 24 inches high and tinted a deep purple; inside is a chemical lure known to attract emerald ash borers. The panels are folded to form a prism and covered in a glue that ensnares the insects.

On July 13, Mr. MacKenzie lowered one trap and found a single insect that looked like an ash borer. Once the Forest Service confirmed the find, all of Orange County, N.Y., was put under federal quarantine, which prevents the interstate transportation of any ash wood or ash wood products. A state quarantine, which restricts some movement within New York, was added in August.

Only two other beetles were found at West Point, suggesting that it was early enough to try a preventive strategy. One option is to inject trees with insecticide, but that can cost several hundred dollars per tree, and is not practical for large stands.

Instead, West Point conservators have decided to cut down about 78 ash trees that are so close to buildings that they would cause serious damage if they become infested and fall. The countless ash trees in West Point’s woods will be monitored, but not cut down.