My last post discussed ways to provide habitat for ground-nesting bees, which comprise about 70 percent of California's native bee fauna. And the other 30 percent? They are referred to as cavity-nesting, since they lay their eggs above ground in twigs, abandoned beetle galleries, and other tree cavities. Carpenter bees (Xylocopa spp.) and mason bees (Osmia spp.) are two of the common cavity-nesting bees occurring in urban gardens. As with the ground-nesters, these bees also provide important pollination services. While lifecycles vary by bee species, the general pattern is that eggs are laid in spring and summer with adult bees emerging the next year.
In wild areas where dead or dying trees are not removed, natural habitat for these bees exists. In urban gardens, however, we need to provide the cavities. The good news is that these bees are often flexible in their choice of nesting locations; studies have found that diversity of cavity-nesting bees can be high in urban areas due to the availability of alternate nest sites like wood fences or gaps in masonry walls.
Nesting material can be provided with a bee block, which is simply a piece of wood with correctly-sized holes drilled in it. Don't use pressure treated lumber, which may be toxic to bees. Holes should be between 3/16 inch and 5/16 inch in diameter (bees will use the hole that matches their body size), and four to six inches deep. The back of the block should be solid. Above-ground nests should face east to southeast and be protected from the afternoon sun. They should also be securely attached so they do not move in the wind.
To prevent disease, blocks should only be used for one year unless they are lined with paper straws that fit the diameter of the hole. Straws are removed yearly, thereby cleaning the cavity. Be sure to purchase straws specifically for bees, as regular paper straws do not work (an internet search for “Mason bee inserts” will turn up many sources). Straws can also be removed for inspection to see if they are being used.
Another option is bamboo, either plant stakes or purchased nest inserts. These are preferable to cutting your own, since purchased inserts are cut at the node to ensure they are closed at the back. At the Haven you'll see these in PVC tubes in one of our bee condos.
Finally, perennials with hollow stems can be pruned to leave six to eight inches of hollow old growth when spent flowers are removed. An example is sedum ‘Autumn Joy', which also provides a great late-season nectar source. Other garden options include leaving tree stumps and cut branches. Haven visitors will notice that we used the prunings from our orchard trees to line many of our secondary garden paths. I will post updates throughout the summer on the use of the garden's varied above-ground nests.