There was a recent discussion about managing pith/stem nesting bees on the Beemonitoring Yahoo group, established by Sam Droege (USGS-Patuxent Wildlife Research Center).
By now we know that of the 5,000 native bees in the country (approximately, some have not been identified yet), 30% build their nest in hollow tunnels. This behavior can occur inside the soft pithy centers of some twigs, and in soft above ground rotting logs and stumps. For example, box elder (Acer negundo), elderberry (Sambucus canadensis), and cane berries are preferred plants by native bees.
Native bees like Megachile mendica have been observed nesting in vertical tomato stakes, typically one to two burrows in a multi-burrow nest. This bee prefers to nest in isolation, helping to avoid certain parasites. An example of this type of guarding behavior can be seen against Melittobia digitata, a tiny parasitic wasp which preys on Megachile mendica. This parasitic wasp moves through thin cracks that might be between burrows if the bee nest is a type of wood conglomerate (like commercially sold “bee hotels”); then a Melittobia infestation possibly will occur destroying all nests. Nesting for this bee species is safer in burrows or places with solid walls.
Check out the “Building a Bee Box” article so you can provide safe nesting sites for native bees!
Native solitary bees use a variety of plant stems provided that they are stable enough to resist the winter, wind and other harsh weather conditions. These stems need to have a pithy or hollow center approximately the diameter of a juice box straw or larger. Any new plants with pith or stem structures will not have bees in them, and you can safely cut them in Autumn or in early Spring before the first flight (when bees emerge in the first days of Spring) and they will be actively looking for nesting locations.
Bees build their nest in dead plant stems over the Summer, the next generation would overwinter in the same stem from where they came. Some bee species are multivoltine, meaning that two or more generations will come out along the year, and some are univoltine, one generation per year.
This means that if you have a series of plant stems older from the year before, chances are you would have one or several generations of different native bee species living and growing in your garden!
If you have forbs and shrubs in your garden, they are perfect for nesting construction. Really small stems or stems that are completely woody will not be appealing for native solitary bees to construct their nests.
Leaving some of the current year’s growth standing surely would provide nesting opportunities for next year’s bees, you just have to cut the tops off. Bees need the stem to be exposed and will not fill out a complete stem with chambers. You can cut as low as eight inches. If you leave a variety and combination of plant stems- sizes & heights- in your garden, you would have different native bees species.
We need to refocus our thinking when cutting-burning-planting in our gardens. For Pollinator Gardens, it’s important to remember: Plant stems need to stay where they are until next Spring, Summer and Fall, and then the following Spring and Summer. The second Summer bees will build their nests, then the next Summer new bees will emerge.
And remember there is plenty of room for creativity!