A beekeeping friend wondered what could be eating big holes in her lettuce leaves. I asked whether they were very nice ROUND holes……why, yes indeed, they were, said she!
AHA! Leafcutter bees (Megachile), no doubt!
These delightful, small bees use their sharp mandibles to quickly cut perfect circles from leaves and petals to make their nests.
Like their smaller relatives, mason bees, in the same family (Megachilidae), leafcutter females build their nests in hollow stems, twigs, or other narrow cavities in wood, and occasionally in underground tunnels.
They make a tube of leafy or “petally” circles to line the nest; it will contain a linear series of about 9 cells, each holding a single egg atop a mound of pollen gathered as provision for the larva-to-be.
The female eggs are laid first, filling the cells at the bottom end of the tube. Male eggs are placed toward the entrance so that they will emerge in the spring before any females are ready and able to exit from nests in the vicinity. In this position, they are more vulnerable than are the females to predation by long-beaked birds that poke into the nest entrance holes seeking fatty morsels to feed their own young.
Once each tube nest is filled to capacity, the leaf-cutting mother bee closes the end with another perfect green disk, in contrast to the rough mud plugs employed by mason bees to close their nest entrances. When the larvae are fully grown, they pass the winter inside their cocoons within the sealed brood chambers.
Along the way, “Ms. Megachile” serves as an excellent pollinator as she gathers copious amounts of nourishing protein-food for her brood. Watch her visiting a Shirley poppy, if you get the chance: she lands directly on the receptive stigma at the center, dropping a bit of pollen onto its sticky surface before she dives into the surrounding sea of anthers and paddles through it, loading the underside of her abdomen with fresh pollen packed onto the scopa basket of hairs, specialized for that purpose.
In our garden, the leafcutters are numerous and quite active throughout the summer months. Unlike mason bees, which typically produce only a single generation of offspring each year, leafcutter bees can continue with a second generation in a single year, if warm summer temperatures continue into fall: that is, mature leafcutter bees emerge from the first round of nests instead of overwintering, and their own offspring will remain in their sequestered cocoons through the cold months, until the following spring.
If your own back yard is lacking Megachile, cocoons are available for purchase from various sources, just as one can buy mason bees locally or online, and the resulting population of leafcutters will readily occupy a drilled or tube-filled nest box you provide with openings of appropriate diameter.
There are many different species and sizes of Megachile. Most are bigger than Osmia mason bees, smaller than honey bees, so a 4-mm drilled hole or tube will work nicely for summer leafcutters, should you wish to invite them into your garden.
That is, if you can put up with a few holes in your lettuce ……etc.!
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