Climate change appears to be affecting the maple sugar industry. Sugar maples have been subject to stress by acid rain, asian longhorn beetles, and pear thrips. In 2011 the deer herd had grown too large for habitat, and many resorted to eating bark to survive the winter, destroying trees in the process. In addition, the sugar maples need a certain period of cold to produce sap for maple syrup. The time to tap these trees has shrunk to one week in some years. The tree may be replaced by the more aggressive Norway maples, in effect forcing the sugar maples to "migrate" north to Canada.