A leafcutter bee is a fascinating example of a hard-working insect species that can cut perfect round holes out of leaves. These are also known as cutter bees and they cut segments of rose, lilac, and other shrub bushes to create nests. The holes are neat circles that on closer inspection are more crescent-shaped in nature. The holes don’t cause lasting damage to plants and some of the created patterns are fascinating.
In this article, we will take a closer look at leaf-cutter bees and why you want them in your garden.
Meet the Leafcutter Bee
The leafcutter bee is a solitary species that belongs to the Megachile genus in the Megachilidae bee family. The females use the leaf pieces to make cells in their nests and they are active from late spring through to late summer.
The typical nest site is a natural tunnel, such as a hollowed plant stem, rotting tree stump, and others. The tunnel tends to have a dry soil or rotted wood structure and the leaf pieces are placed to form stronger thimble structures.
These nests are then filled with a pollen and nectar mix to ensure that the female bee has plenty of food. When the female bee is ready, she lays one egg on the stored food and then caps this cell with more pieces of circular leaf. The female cutter bee will repeat this process up to thirty times to create multiple larval cells.
The female cutter bees are similar in size to a typical honeybee with a distinctly segmented thorax covered in light brown hair. They have a bright orange pollen brush under their abdomen and from above this looks like a halo.
The males of the species are harder to identify, but at first glance, they look similar to the females. On closer inspection, male cutter bees have more hair both on and between the thorax segments. They tend to have a fluffy appearance and over time these hairs fade to a silver-white color.
The Art of Leaf Cutting
Leafcutter bees are active for up to six weeks during the summer months and after this time, they remain in their nesting chambers. This is when the bees eat, grow, spin cocoons, and remain active as their larvae mature.
When the summer temperatures reach 70ºF consistently, the new adult bees emerge. The nesting behavior of these bees takes place in artificial nests that they create in natural hollow reeds, wood stumps, cardboard straws, wood trays, and a wide variety of other locations. Adding these materials to your garden is a good way to encourage leaf-cutter bees to take up residence.
The female bee searches for suitable nesting sites and they will investigate numerous locations before they start to build. These bees require a tunnel depth of at least 6” with a sealed rear cavity to deter predators and a cavity diameter of around 6mm.
When the female has found a nest site, she will fly zigzag patterns near the entrance to memorize the location.
After this time the nest cannot be moved or there is a real risk that the bee will become disorientated and leave the area. The female will then gather the nectar and pollen for food, lay her eggs, and seal the egg chambers with the leaf and flower petal cuttings.
How the Leafcutter Bee Builds Their Nest
The building process starts at the rear of the nesting chamber with a protective leaf wall packed with pollen loaf and nectar. A single egg is laid on this food source and then the chamber is sealed with another leaf wall.
This process continues from the rear of the nesting chamber to the front and every extra layer adds more protection. These bees prefer softer and more flexible flower petals and leaves to build and seal the nests.
Some popular building materials include roses, lilacs, hostas, buckwheat, clover, alfalfa, and lamb quarters.
As the female forages for food and building materials, she will be covered with pollen dust that becomes trapped in her hair. This is spread to other plants she visits and this pollination is extremely effective. The female bee will work diligently until she has no remaining building space, and she will cap the end of the nest with a final thick layer of leaves.
This will keep predators and bad weather out of the nest to protect the larvae inside. During her lifetime, a female cutter bee will create 2-4 tunnels that each contain approximately 30 eggs. During the evenings, the female bee will stay in the nest entrance facing inwards and if the temperature drops under 64ºF or if the weather is bad, she will face outwards.
This facing is required to guard the entrance against insect intrusion and predation. If you have a cutter bee nest and you check out the entrance on a cooler night you may see the female watching, you.
The Role in Pollination
As we mentioned earlier, you may want to invite leaf-cutter bees into your garden. They are extremely good pollinators and the US Agricultural Research Service estimates that a single female can pollinate more than 20 honeybees.
This study looked at larger numbers too and 150 cutter bees can outperform 3,000 honeybees when it comes to pollination. So, if you want your garden to thrive it’s a great idea to have at least a few of these busy bees around.
Human Interaction and Conservation
Leafcutter bees have rapid and precise leaf-cutting skills and they are excellent builders. The Megachilidae bee family includes more than 4,000 species including the leaf cutters. The Megachile genus includes 1,500 solitary bee species worldwide and 50 subgenera.
In North America alone there are more than 200 Megachile bee species with the largest distribution in the Western United States and Mexico border areas. These are important species for crop pollination and a wide variety of wildflowers across North America.
Sadly, this bee species faces a number of threats, including pesticides, toxins, logging, and habitat loss. This limits access to food sources and suitable nesting sites. At this time, leaf cutter bees have no conservation status, but some species in the genus are considered to be endangered or under threat.
The two most important pollinators are the alfalfa leafcutter bee ((Megachile rotundata) and the cranberry leafcutter bee (Megachile addenda).
These two species are managed by both farmers and conservation organizations that focus on habitat protection using sustainable land use techniques. Reducing pesticide use is also important to protect leafcutter bees from harmful contaminants.
The leafcutter bee is a fascinating creature to observe, and they are extremely busy pollinators as they prepare their nesting sites. The little circles they cut in plant leaves and flower petals will not harm the host plants. But the additional pollination services they provide will certainly make your garden a healthier environment.
These bees are pretty docile too, they can be encouraged into the garden with hollow can bundles, or an old log drilled with 0.4” diameter and 6” deep holes. Resist the urge to apply preservatives or varnishes to the wood which can harm the bees.
Reducing the use of pesticides in the garden is a great way to keep them safe and productive. It is impossible to observe the construction of the nests, but when the female leafcutter bee seals the entrance, it is fascinating to watch them work.