Leaf-cutting Bees

In the fantastic world of bees and wasps we have a few species that are "social" insects. Their world is comprised of a colony of individuals all working together for the common good, with a queen that produces all the eggs and workers that do all the chores of nest building, food gathering, defense of the colony, and care of the immature stages as they are growing. We find this social structure in honeybees, bumblebees, yellowjackets, and paper wasps, and of course with ants, which are close relatives of wasps. We also find the social structure in all species of termites, with some slight differences in the makeup of their colonies. These social bees and wasps have the rather undesirable habit of protecting their precious colony, and will readily and aggressively go after any intruder they believe poses a threat to their queen and the larvae they are caring for. With their stinger they quickly inject venom into us to encourage us to move away from their hive. While these bees and wasps are otherwise quite beneficial for their role as pollinators or as predators of other insects, when they create their hives too near our homes and our recreation areas it increases the chances for that unhappy encounter.

However, most kinds of bees and wasps in North America are not such a problem for us, and these are the ones we call "solitary" bees or wasps. They do not create a hive with a queen and workers, they do not work together for a common cause, and they do not exhibit the aggressive behavior toward someone or some animal that ventures too close. Some of the common kinds of wasps that you find around your home and garden that are solitary wasps include the mud daubers, potter wasps, and spider wasps like the Tarantula Hawk. The bees include a wonderful variety of mason bees, andrenid bees, plasterer bees, mining bees, and carpenter bees, all of which are solitary kinds that pose very little threat of stinging to people or your pets. This article will focus on the Leaf-Cutting Bees, in the family Megachilidae, which are so common around our landscapes in the spring and summer, and which are the ones that leave those tell-tale leaves with the missing parts.

The stinging apparatus of a bee or a wasp, and many species are at least capable of stinging if they so choose, is the device that the female insect uses for egg laying - her ovipositor. Obviously only female bees will have this device, so for the solitary bees all the males are incapable of stinging. Some kinds put on a pretty good show, such as male carpenter bees, which will posture and threaten and dive at you in an effort to convince you that they could do some harm, but in reality they are essentially harmless. The females of the solitary bees can sting, but without that instinct to protect a colony they are highly unlikely to do so. You would have to directly threaten the bee itself to elicit such a response, such as trapping the female bee in your clothing or your hands, in which case she might sting only as a means for escaping. Because they pose so little threat to our health and happiness, there is little justification for killing solitary bees that you find around your home, and the benefit we get from them with their activity of pollinating plants should easily outweigh any of their negative aspects.

One entomologist allowed himself to be stung by various kinds of bees just to see what effect it had, and then ranked the pain level of each kind. Leaf-cutting bees he likened to "lightly brushing a thorn" (from Justin Schmidt, Tucson, AZ). On a scale of 1 to 4 he ranked the sting from leaf-cutting bees a Zero. Other solitary bee stings also ranked very low, while the stings from social bees ranked much higher, including the bumblebee, which he described as "like stepping on a hot nail". There simply is little threat to our health from the solitary bees.

Leaf-cutting bees are aptly named. The female does all the work in preparing a home for her offspring and gathering the food she will leave there for them. Bee larvae nearly always feed on a "bee bread" mixture of pollen, nectar, and other minor ingredients, and the female creates this food supply for her offspring. She constructs a narrow hollow tube by rolling together those sections of leaves that she so carefully gathered from selected plants in your yard, rapidly slicing off that section with her sharp jaws. Once the section is the proper length she will place the bee bread inside, place a single egg next to it, and seal up the chamber. She may continue to add chambers to this first one until the tube is several inches long, or she may elect to start a new one somewhere else. This is highly dependent on the place she has chosen to build that larval home, and if it happens to be in a hole in some wood there may not be a lot of room for additional chambers. If it happens to be in some longer hollow channel the number of chambers could be numerous.

We recently cleaned out a small fountain in our back yard, replaced the water with clean water to discourage mosquitoes, and started the pump. Our particular fountain has a concrete frog that "spits" the water out of its mouth, and the moment the first gush of water came through my wife gave a startled sound. Along with the water came about 5 inches of dark tubes of some sort, all of which landed in the bowl of the fountain. A close look revealed this to be the tubes from a leaf-cutting bee, which discovered that hollow area inside the frog in the week or two that it had not been used, and quickly built the homes for her offspring. I have a fondness for leaf-cutting bees, and placed these tubes inside some narrow PVC sprinkler piping and set that in a protected place in the hopes everyone inside will be okay. Once she has the construction completed and the food and eggs placed inside, the mother bee is done. She seals the chambers and never goes back to them again. The bee bread she has placed there is sufficient to get her offspring through their larval stages to ensure the next generation will be a healthy one.

Any protected hole or gap may be appropriate for the leaf tubes. They can be found under the edges of shingles on a roof, along the tracks of sliding windows, along fences, inside holes in wood, including the siding on homes. The bee does not create the hole herself, but she is opportunistic, and takes advantage of the holes she finds already there. I once was using a cordless drill, and a leaf-cutting bee was relentlessly following my movements with the drill in her effort to get into a hole in the drill housing that led to one of the screws. I really didn't want her using my drill for her nest purposes, so I discouraged it but did provide a piece of wood with a number of holes the same size drilled into it.

Leaf-cutting bees likely choose the plants they dismember based on some criteria of texture, softness, rigidity, etc. In my yard it commonly is roses and crepe myrtle, as well as a small red bud tree that hasn't done much growing in the 7 years I've owned it, perhaps due in part to the loss of foliage each summer from leaf-cutting bees. I am happy to sacrifice this one plant for the benefit of encouraging these little bees. Leaf-cutting bees are such efficient pollinators that they are kept for that purpose for the pollination of alfalfa crops. The leaf-cutters are much more effective on alfalfa than are honeybees, and beekeepers care for them and provide potential cavities for the leaf tubes by constructing panels made from paper tubes with the ends exposed. These can be positioned near alfalfa fields at just the right time to produce numbers of the bees for the pollination of this important crop.

The family Megachilidae is a large group of many species, only some of which are leaf-cutters. These gather the pollen needed for their larvae and pack it onto a structure on the bottom of their abdomen called a "scopa". On some of the species this scopa is brightly colored, and leads to one of the pet names for these bees as "jelly-belly bees". Some species are a bright, metallic green or dark blue over their entire body, while other kinds are black or brown with lighter stripes around the abdomen. They range is size from around the size of a honeybee to much smaller. Their flight is rapid and their activity in gathering the pollen is aggressive, with the bee more or less swimming around in the flower to gather the pollen and pack it onto her scopa. They will visit flowers frequently, increasing their value as pollinators.

I am often asked how to control leaf-cutting bees, and my answer generally will be that there is no need to do so, and to kill them may even be counter-productive to a healthy garden. If a particularly valuable plant exists that cannot afford to have the loss of parts of the leaves it might be covered with netting during the periods these bees are active. There are no pesticides labeled for use against leaf-cutting bees, and given their value probably never will be. Their nests do not pose any hazard to homes, as might honeybee hives made from wax and filled with honey. Their sting is of little consequence and very rarely is given to anyone. The damage to leaves is offset by the pollination of plants. Leaf-cutting bees are best left alone and enjoyed.