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How can I provide nests for wood-nesting bees?

Answer: Commercially produced bee blocks, consisting of a wood block drilled with a series of dead-end holes, are now widely available. These types of bee nests were initially developed in the 1960s by alfalfa seed producers in the western United States to attract and manage large numbers of the non-native alfalfa leafcutter bee (Megachile rotundata).More recently they have been modified to manage the blue orchard bee (Osmia lignaria), a bee that is active only in the spring and will not pollinate later-flowering fruits and vegetables. Consequently, all of the nest tunnels are a uniform size and depth, which may be either too large or too small for many other species. Nest blocks with a greater diversity of hole sizes and depths are necessary to attract a variety of bees that are active throughout the year. Under the best circumstances these nests can attract large numbers of cavity-nesting bees and boost their local populations. However, because these nests concentrate bee populations in unnaturally large numbers in a small space, they can become infested with parasites and disease spores after several seasons. Without regular sanitation or the phasing out of nest materials, these parasites and diseases threaten long-term pollinator health wherever they are used. Because contaminated nest blocks left unattended in the landscape continue to attract wild bees from the surrounding area, they have the potential to do harm. Only with proper management can these nests maintain healthy bee populations indefinitely.Wooden blocksUse preservative-free dimensional lumber to construct wooden nest blocks. A four-by-four is appropriate for blocks with smaller diameter holes. A four-by-six works for blocks with larger diameter holes. In one side, drill a series of nest holes of appropriate sizes and depths. Nesting holes should be between 3⁄32 and 3⁄8 inch in diameter. Holes of 1⁄4 inch or less in diameter should be 3 to 5 inches deep. Holes larger than 1⁄4 inch should be 5 to 6 inches deep. The female bee controls the gender of her off spring and usually finishes the nest with a few male brood cells. A deeper hole ensures space for more female brood.The holes should be about 3⁄4 inch from center to center and no closer than that to the edges of the block. Attach a backing board if you drill all the way through your block, because bees will not use a drilled hole that is open at both ends. With smaller diameter drill bits, you may not be able to achieve the 3-inch minimum recommended depth. If that is the case, simply drill as deep as you can; bees that use holes of smaller diameters will often nest successfully in ones that aren’t as deep.Bees may avoid a rough interior, so your holes should be perpendicular to the wood’s grain and drilled with a sharp bit. You can buy paper straws to line the holes, although it may be hard to find straws that fit all diameters. One solution is to wrap your own paper straws out of parchment or newspaper using dowels of various diameters that match the inside diameters of your drilled holes. Paint the outer tips of the straws black to help attract bees.The exterior of the block can be any color, although there is some anecdotal evidence that bees are most attracted to dark blocks, which can be achieved by lightly charring the front surface with a propane torch. Whatever the color, bees are likely to use the block as long as the holes are of appropriate diameters and depths, and hung in an appropriate location. As a final step, you can attach an overhanging roof to provide additional shelter from the rain.Colonization is often more successful when blocks are attached to a large visible landmark such as a building. The actual height from the ground does not much matter, although if the nest is too low (less than a few feet), rain splash may dampen it and vegetation may cover it. Nest blocks should be hung in a protected location where they receive strong indirect sunlight. Direct sunshine in the morning will help bees warm themselves up to flight temperature, so you may wish to place nests facing east, allowing the morning sun to fall on the entrance holes. Direct sunlight later in the day can be detrimental, causing eggs or developing brood to overheat and die.Since these types of nests mimic trees frequented by woodpeckers, do not be surprised if one finds your nest. To protect against damage, you might want to store your nests in an unheated building at the end of the season. Alternatively, you can protect nests over the winter by surrounding them with hardware cloth. Be sure to remove the cloth before nesting resumes because hardware cloth can disorient nesting bees and damage their wings.Stem bundlesIn addition to wooden blocks, artificial nests can be constructed with bundles of reed, teasel, cup plant or bamboo cut so that a natural node forms the inner wall of the tunnel. Cut each stem below the nodes (usually indicated by a ridge) to create a handful of tubes, each with one open end. Strap the tubes together into a tight bundle with wire, string or tape, making certain that the closed ends of the stems are all at the same end of the bundle. A variation on this is to tightly pack the stems ? open ends out ? into a tin can, paper milk carton, square plastic buckets or short section of PVC pipe. The bundles should be placed in a sheltered location (such as the side of a barn or garden shed) with the stems horizontal to the ground.To learn more, consult the ATTRA publication Alternative Pollinators: Native Bees, which provides information and resources on how to plan for, protect and create habitat for native bees in agricultural settings.