In the past (more than 20 years ago), paraformaldehyde pellets were recommended for use in tap holes, and some out-of-date sugaring manuals still suggest their use. Research shows that the use of paraformaldehyde pellets can damage the trees, so they are now illegal. Similarly, the use of denatured alcohol as a disinfectant for tap holes is also illegal.
The purpose of the pellet or alcohol was to help keep the tree from healing up too fast in the spring and allowed a longer sap flow, which increased the years production. This practice was made illegal in the 1980’s because of potential tree damage and the basic principal that the use of formaldehyde at any stage in a food process just does not make sense. Maple farms are government inspected (without notice) on a regular basis and as far as we know no one uses formaldehyde any more. Government and private agencies on both sides of the border keep a watchful eye just in case.