Depending on species, Leaf-cutting bees (Family Megachilidae) use a variety of things to make nest plugs including mud, resin, plant fuzz, and as their name implies, leaves. Many of our leaf-cutting bees chew leaves into a pulp as they build their nest plugs. Because they are made from chewed vegetation they can have a fibrous characteristic. If you inspect them closely, they are made of bits, like a puree of spinach that hasn’t been strained. They often use leaf pulp alone, with no other ingredients, but some species may incorporate other materials (wood, dirt and even pebbles).
Newly made chewed vegetation plugs will be green: light green, bright green, or dark green, but green. Just as leaves on trees lose their green and turn colors in the autumn, the chewed vegetation plugs will become less green over time. Once they dry they may look darker, appearing brown or blackish, or they may fade to a sickly olive-tan. Chewed vegetation plugs also have a habit of shrinking and splitting away from the tunnel walls over time. This is the norm for this sort of plug. The fibrous characteristics of these plugs may become exaggerated as the plugs dry. One of the reasons we ask you to check your blocks every two weeks is because these plugs morph over time. It’s easier to see “green” before it fades.
This week we will focus on two of the earlier nesting leaf-cutting bees that use chewed vegetation, the mason bees: Osmia and Hoplitis. Later in the summer we will revisit chewed vegetation plugs and the bee-sties that create those later in the summer.
Osmia spp. – Mason Bees
Osmia are our earliest spring bees. These squat bees overwinter as adults (still in their cocoons) and are ready to pop as soon as the spring warms. While most our Osmia are metallic blue, we do have a few metallic green and black species. We find the Osmia to be locally common. When you have them, you may have a lot of them. They only have one generation per year, so you won’t see any emergence from these nests until next spring. There are many species that nest in the Front Range. Most will use the medium sized holes, although some will nest in small or large holes, especially if the medium holes fill up. These typically use only chewed leaves, although at least one species will add wood chips from the nest block itself in the nest plug. Perhaps this makes it more camouflaged? Depending on species, some plugs are flush with the entrance, but many are recessed 1/4 to even 1 inch deep. These bees visit early spring flowers, and a few specialize on Asteraceae.
Hoplitis spp. – Mason Bees
Hoplitis are typically the next leaf-cutting bee to appear on the scene, beginning to nest in late spring or early summer. Closely related to Osmia, Hoplitis are longer and thinner. They vary greatly in color; we have both metallic green and black species that nest here. They tend to be more common in the foothills. They nest mostly in the medium size holes, but like other bees, may use the small or larger tunnels if the medium holes are full. The plugs are totally cool in that they cement little pebbles in place using chewed leaves, creating cobblestone plugs. These are typically flush or very near the block entrance.