RAPID CITY, S.D. (AP) — A recent aerial survey shows the 18-year battle to eradicate the mountain pine beetle from the Black Hills of western South Dakota is slowing the pest's progress.

The survey was conducted by the U.S. Forest Service, South Dakota's Agriculture Department and Wyoming's State Forestry Division. It measured trees that were infected by pine beetles that left trees in the fall of 2012 and spread to new timber last year.

The Rapid City Journal reports that the survey indicates beetles infested just 34,000 new acres in 2013, compared to as many as 67,000 acres as recently as 2011.

Black Hills National Forest supervisor Craig Bobzien says there are encouraging infestation declines in some areas due to measures such as tree thinning, but officials need to stay vigilant.

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