I was given a house for bees, but the literature seems to say it is inadequate because the length of tubes is only 3 inches long and should be 7 inches minimum. Is this correct? Why do the tubes need to be so deep?
The majority of bee species nest in the ground, either using existing cavities or building their own nesting tunnels. All members of the Andrenidae and Halictidae families (mining bees and sweat bees) are in this group.
The second most common nesting habitat is wood tunnels or plant stems. Most wood nesting bees rely on existing tunnels left behind by beetle larvae and other insects. Very few species can actually chew into the wood and excavate their own nests; a well-known example is the carpenter bee (Xylocopa virginica). Those nesting in plant stems usually seek out pithy stems into which they can chew their own channels, or they use stems that are hollow to begin with (small carpenter bees of the genus Ceratina).
Nesting tubes in nature or in a provided “bee hotel” need to have a minimum depth to be functional, and that depth depends on the size of the bee species using it. The diameter of a tube usually defines which species are attracted to it, with larger diameters attracting larger species. The larger diameters also need to be deeper, so that the female bee will have enough room to install a sufficient number of egg cells. If the number of individual egg cells is limited to only a few, the balance between male and female bees could become disrupted. The imbalance occurs because the sex of bees, wasp, and ants is determined by the number of chromosomes. Fertilized eggs are diploid and result in females, while eggs that are not fertilized result in male bees. Bees deposit fertilized eggs deeper in the nest tunnel, which will result in female bees (diploid). Non-fertilized eggs are placed closer towards the entrance of the tunnel, which will result in male bees. Apparently, this is a way to protect the more precious female bees from predation. Because the male bees are closer to the tube opening, they will hatch before the female bees. Again, this will expose them to danger from predators before the female bees even begin to hatch.
In the attached image, one can see nesting cells with larger pollen masses deeper in the tubes, and smaller food provisions for the male bees closer to the entrance. These caring mother bees also included empty cells near the entrance as additional protection against predators. To sum it up, the nesting tubes need to be long enough to have space for both fertilized (female) and unfertilized (male) eggs to be deposited.
Therefore, narrower tubes, which will attract smaller species, can be shorter, allowing for smaller nesting cells (as short as 2 inches in my observations), but larger diameter tubes are useless if they are too short, say less than 3 inches.
There are numerous small and very small species of bees that nest in small holes in wood or in narrow nesting tubes. Often it is possible to say to which group a bee nest belongs by looking at the material used to divide the nest cells and to close the entrance tunnel when all cells are completed.
In the genus Osmia, known as mason bees (and including the orchard bees), most species are in the range of 10 to 15mm, but there are some that are as small as 5mm long. These bees use mud and sometimes chewed plant material for their nest divisions.
Among the related leafcutter bees (Megachile) species range from 21 mm to only 5 mm long, with a diameter of approximately 2 to 3 mm (1/8 inch) for these smaller species. As the name suggests, these bees cut leaves and flower petals as wrapping for their egg cells.
In the same family (Megachilidae) are the resin bees (Hoplitis and Heriades), who seal their nest with plant resins. I have seen resin on most of the narrowest nesting tunnels in my bee hotels.
The tiny yellow-faced bees belong to the family of the Colletidae and make their own cellophane-like substance as nesting material. This material is produced from glands on their body, and is astonishingly strong, waterproof, and has anti-fungal properties. Yellow-faced bees are hairless and resemble tiny wasps.
~Thomas Berger, Green Art, Kittery, ME
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