We like carpenter bees, and want to encourage them to live in our yard in Northern California. We are planting lots of native flowering plants that should attract them, and have started to see a few (mostly Xylocopa californica). But our house is in a neighborhood without a lot of habitat where carpenter bees would nest, and we’d like to try to be the habitat we want to see.
Looking online for tips on how to encourage carpenter bees, I mostly find pages explaining how to exterminate them. :(
So I have a few questions for anyone with experience in this:
Do carpenter bee houses work? These seem to be basically a tree round hung on a fence or tree with holes drilled in parallel to the grain.
Are there design specifications (type of wood, diameter of hole, placement, etc.) that are more likely to work if I decide to make one of these?
Is there another approach (beyond the pollinator-friendly yard) to attracting and housing carpenter bees that I should know about?
We have Xylocopa sonorina in my part of California. The only time I ever found nests they were in a large log lying on the ground. The bees must be capable of making their own holes in it? In addition to native plants there are several non-native plants they seem to like: wisteria, senna/cassia, European sages.
I’ve seen a few ‘bee hotels’ that were made with a range of different diameters of bamboo bundled together with an open front and a covered top, sides, and back to keep the weather out (something similar to the picture below). They had a pretty active population of bees living in them, too, so they seemed to work. They were placed in a flower garden as well, so there was a food source nearby. That would probably be the biggest determining factor in whether it would be used or not.
We’ve had one of those a few years, in our flower garden, and only one mason bee has ever used it. Perhaps the microhabitat is wrong. Carpenter bees that I’ve seen in the wild nest in holes bored in solid wood, and I’m not sure tubes like this would suit them.
The ones that I saw occupied were in Vicksburg, Mississippi while I was there on a temporary assignment for work, so no guarantee that the same approach would work here (different species, different climate), but the bees I observed were carpenter bees (Xylocopa virginica) at least, not mason bees (unless all the IDs were wrong, of course), so it’s at least a proof of concept.
Carpenter bees will be looking for soft (old but not necessarily rotten) wood or pith-filled stems to nest in. They make fairly long tunnels parallel to the grain of the wood and use the dust from excavating the tunnels to construct the walls between the cells.
I have a balcony (thus no good source of dead wood) and have found that shelf fungi are well received by the ones we have here in Europe. But Californian Xylocopas may be accustomed to somewhat different habitats.
There’s an older article on California carpenter bees that includes a discussion of nest substrates (you should be able to find the full text by googling, but I don’t have a direct link): Paul D. Hurd, Jr. (1955). The Carpenter Bees of California. Bulletin of the California Insect Survey, 4, no 2.
I doubt a bee hotel would be their first choice of nesting site – they are designed for smaller bees that nest in existing cavities rather than chewing their own, so neither the size of the tubes nor the substrate would be what Xylocopas are looking for. Sometimes males will sleep in tubes if they are spacious enough, and around here, where adults of both sexes overwinter, they may use them as overwintering spots. But a well-constructed bee hotel should not normally have tubes that are so large; most cavity-nesting bees prefer holes that are closer to the diameter of their bodies (usually between 2 mm and 8 mm depending on the size of the bee). If your nest aid is not getting much use, you might check what local bees would be expected to use it and what their requirements are. Also: some bees are reluctant to use tubes that aren’t sealed at the back, so if the tubes aren’t pushed firmly against the back wall and well-secured, this may be one reason it isn’t getting much use. Most cavity nesters also prefer that bamboo pith has already been removed. Unfortunately a lot of commercial bee hotels are often unsuitable for the bees they are intended to help.
Our bee researcher explained, that if you drill holes, you should make sure the edges are smooth so you don’t tear their wings.
If the bee is chewing her own hole those surfaces, bite by bite, must be satin smooth and silky.
