Mason bees are very effective pollinators. Just two or three females can pollinate a mature apple tree. Mason bees will also work in cool or rainy weather when honeybees are more likely to take the day off. At the end of the season, mason bee holes/tubes should be cleaned and the cocoons harvested for the following season. * Lots of information on-line* House is build from cedar wood and measures 15.5" high X 12" deep X 7.25" wide. Each house had two eye hooks to hang from an exterior wall or fence. The nesting blocks, two per house, are made of pine wood and measure 4" square and 6.25" deep. - each block has 5 rows of 4, 3/8" holes, totaling 20 holes per block - the holes are 3/8" in diameter and can be used either alone or if desired the owner can use a paper mason bee tube (not supplied) for ease of cleaning and harvesting. - the blocks are taped together for use and can be easily separated for cleaning at the end of the season.