Leafcutter Bee

With their incredibly precise and rapidly executed leaf-cutting ability, leafcutter bees might just be the Martha Stewarts of the insect world. However, these tiny pollinators are far more than DIY experts! Megachilidae is a family of over 4000 species of bees that includes leafcutter bees in the genus Megachile. The genus Megachile includes 50 subgenera and 1,500 species of solitary bees worldwide, making it one of the largest genera of bees. These bees are found on every continent except Antarctica. In North America, there are over 200 species of Megachile. The largest distribution is in the Western United States and Mexico. Leafcutter bees are important pollinators for many crops and wildflowers in North America. Keep reading to learn more about this genus of solitary bees!

Scientific Name

The name Megachile is derived from the Greek words mega meaning large or great, and cheilos, meaning lip or mandible. This name refers to the relatively large jaws of female leafcutter bees which they use to cut and shape leaves to build their nests. The name Megachile was first used by the German entomologist Johann Ludwig Christian Gravenhorst in 1829 to describe a group of bees with robust mandibles. Today, the genus Megachile includes over 1,500 species of bees scattered across the globe.

Leafcutter Bee: Appearance

Leafcutter bees, also leafcutting bees, are small to medium-sized bees, typically measuring between 0.25 -0.80 inches (6 to 20 mm) in length, with wingspans that range from 0.4-0.8 inches (10-20 mm). However, Wallace’s giant bees (Megachile pluto), have wingspans of up to 2.5 inches (63.5 mm)! Bees in the genus are typically black or dark-colored, though some species may have metallic or iridescent colors on their bodies. Most species have banded abdomens. In some species, females have dense yellow hairs, or setae, on their underbellies that are called scopae. Scopae are used to carry pollen.
Another distinguishing feature of female leafcutter bees is their large, powerful mandibles. Females employ their strong jaws to cut and shape pieces of leaves or petals to fashion their nests. The mandibles are often reddish-brown or yellowish in color and have a sharp cutting edge. Males in the genus tend to have smaller mandibles, like the pugnacious leafcutter bee (Megachile pugnata).
Leafcutter bees have hairy bodies, which helps them collect pollen and carry it back to their nests. Their wings are clear or lightly tinted, and they have six legs and two pairs of wings. Some species may have distinct markings or patterns on their bodies, which can be used to help identify them.

Leafcutter Bee: Behavior

Bees in the genus Megachile are solitary bees that do not live in colonies or have social structures.
Female leafcutter bees construct individual nests for their offspring by cutting circular pieces from leaves or petals( and using them to line individual cells. Each cell contains a ball of pollen and nectar, which serves as food for the developing larvae.

In many species of Megachile, the female will position her eggs in a particular order within their nests. This is known as egg loading or oviposition sequencing. Egg loading can serve a number of purposes, including maximizing resource allocation and minimizing competition among developing larvae. In some cases, the female will place male eggs closer to the entrance of the nest and female eggs farther back. This results in males emerging first. This phenomenon is known as protandry and increases the chances of successful fertilization and reproduction.

Leafcutter bees are generally not aggressive and will only sting if provoked or threatened. These bees have a short lifespan, living for only a few weeks to a few months, depending on the species. During their brief lifetime, they focus on nest building, foraging for pollen and nectar, and laying eggs.

Crop Pollinators

Leafcutter bees are important pollinators for many crops and wildflowers. Farmers often utilize managed populations of alfalfa leafcutter bees (Megachile rotundata) to ensure optimal pollination and higher crop yields. The bees are provided with nesting materials such as cardboard tubes or paper straws, which are placed in or near alfalfa fields during the bloom period. The bees visit the flowers to collect pollen and nectar, which they use to provision their brood cells. This process of collecting and transferring pollen from flower to flower results in the fertilization of alfalfa plants, producing higher yields. 

When it was introduced in North America (1940s), pollination by the alfalfa leafcutting bee increased U.S. alfalfa production by 200%! Because alfalfa leafcutter bees are active during a specific time window each year, their use as pollinators is carefully timed and managed by farmers to ensure optimal pollination and crop success. Megachile bees, particularly the alfalfa leafcutter bee (Megachile rotundata), are commonly sold to farmers for crop pollination.

Because Megachile bees are solitary and nest in individual cavities, this makes them ideal for alfalfa pollination, as they can be easily managed and transported to the fields. Farmers can purchase Megachile bees as cocoons or as pre-emerged adults, which are then released into the fields during the alfalfa bloom. Once the bees have pollinated the crops, they can be collected and their cocoons can be stored for the following season. The use of Megachile bees for crop pollination has become increasingly popular in recent years, as concerns over the decline of honeybee populations and their susceptibility to certain pesticides have led to a need for alternative pollinators.

Habitat

Leafcutter bees live in a variety of habitats across North and South America, Europe, Asia, and Africa. As a widely distributed genus, leafcutter bees are found in grasslands, deserts, savannahs, and forests. They stick to areas with abundant flowering plants. Leafcutters nest in an array of locations, including hollow plant stems, dead wood, and soil crevices. Some species may also nest in the abandoned burrows of other insects or small animals. Leafcutter bees are considered important native pollinators and can be found in a variety of agricultural settings, including orchards, vineyards, and cultivated fields. In these settings, farmers may also provide artificial nesting sites, such as nesting blocks or tubes, to encourage the bees to nest in the area.

Leafcutter Bee: Diet

Leafcutter bees are considered generalist pollinators, which means they visit and collect nectar and pollen from a wide variety of flowering plants. The specific plants that they feed on vary depending on the specific species and their geographic location. Some common plants that leafcutter bees are known to visit for nectar and pollen include alfalfa, clover, cranberries, fruit trees, melons, peas, sunflowers, and wildflowers. However, as generalists, they will forage on whatever plants are flowering.

Predators

The genus Megachile has a number of natural predators that pose threats to both adult bees and their larvae. These predators include other insects such as wasps (Chrysididae, Mutillidae) and ants (Crematogaster ashmeadi); spiders, birds, and rodents. Despite the risks posed by predators, Megachile bees have evolved a number of behaviors and adaptations to help protect themselves and their offspring, such as nesting in protected locations and using their powerful mandibles to defend against attackers. Some species of Megachile construct decoy nests to fool potential predators.

Threats

Megachile face a number of threats that negatively affect their populations and overall health. One of the main threats is habitat loss and fragmentation from urbanization, agriculture, and logging. This can limit the availability of suitable nesting sites and food sources, as well as increase exposure to pesticides and other toxins. Climate change can also affect leafcutter bees by altering the timing and availability of blooming flowers and other plant resources. Other threats include disease and parasites, and competition from non-native species.

Leafcutter Bee: Conservation Status and Population

As a genus, Megachile does not have a specific conservation status assigned to it. However, many individual species within the genus may be considered threatened or endangered due to factors such as habitat loss, pesticide use, and climate change. The cranberry leafcutter bee (Megachile addenda) and the alfalfa leafcutter bee (Megachile rotundata), are considered important pollinators of agricultural crops and are actively managed by farmers and conservation organizations. Conservation efforts that aim to protect habitats, promote sustainable land-use practices, and reduce the use of harmful pesticides can help to support the health and populations of Megachile.

Population Data

There is limited population data available for the genus Megachile as a whole, as it is a diverse group of bees with many different species and populations. However, there are some studies that have looked at the population dynamics of specific Megachile species in certain areas. For example, a study of the alfalfa leafcutter bee (Megachile rotundata) in Canada determined that populations of the species are in decline, which is attributed to habitat loss and pesticide exposure. There is growing concern among researchers and conservationists about declines in bee populations, including those within the genus Megachile.

Leafcutter Bee: Lifecycle

The lifecycle of the genus Megachile begins with the emergence of adult bees in the spring when flowers and other plants begin to bloom. The male bees emerge first and spend their time finding appropriate foraging sites. Once she has emerged, the female begins constructing her nest in hollow plant stems, abandoned insect burrows, or other small cavities. The female bee collects pollen and nectar from flowers to provision her brood cells. Leafcutter bees deposit between 1-28 eggs in individual brood cells.

Once hatched, the larvae feed on the provisioned pollen and nectar. They will molt several times prior to spinning cocoons and pupating. Some species of leafcutter bees are multivoltine. These species will have multiple generations per year. The number of generations varies depending on the species and geographic location. The final generation of pupae overwinters in their cocoons, snug and safe. In the springtime when the flowers start to bloom, the adult male bees emerge from their cocoons, closely followed by the females, to repeat the cycle once more. The precise timing and duration of each stage in the lifecycle vary depending on location.

A Selection of North American Megachile:

• Alfalfa leafcutter bee (Megachile rotundata)
• Black and gray leafcutter bee (Megachile leucophaea)
• Eastern leafcutter bee (Megachile rubi) 
• Faithful leafcutting bee (Megachile fidelis)
• Flat-tailed leafcutter bee (Megachile mendica)
• Hoary leafcutter bee (Megachile pruina)
• Short leafcutter bee (Megachile brevis)
• Texas leafcutter bee (Megachile texana)
• Western leafcutter bee (Megachile perihirta)
• Wheeler’s leafcutter bee (Megachile wheeleri)