Megachile bees

Mar 16, 2020

The Megachilidae family is made of bees which characteristics are the front wing with two submarginal cells, large  mandibles and most strikingly the abdominal scopae (collecting hairs) for the female. Hence, unlike other bees, the pollen is not transported along the hind legs but under the abdomen as shows the picture. The name Megachile means ‘large Lipped’ and refers to the characteristic mandibles.

The Megachile bees, one genus of the family Megachilidae, are also called Leafcutter bees due to their habit to cut circular pieces of fresh leaves to coat their nests and separate the cells. The genus counts 1503 referenced species in the world with 144 in US and Canada. 44 species in the East of the US. It is a solitary bee, which cavities can be above the ground (dead trees, plant stems, rock cavities, mud dauber wasps nests), or underground like the Osmia for example. However for their propensity to nest in wood, they are easily transported and can exist in areas where bees are otherwise few and far between. These bees have benefitted from globalization, but also from the trade routes of the last 300 years, among which the slaves trade. Megachile concina for example is thought to have been brought to the United States from the West Indies sometimes after world war II, and may have been brought to the West Indies from Africa sometime in the 1800’s. Also see the article on Megachile sculpturalis (Giant Resin Bee) and how it invaded the USA in the 1990’s.

The sharp mandibles allow the females to cut leaves or petals starting from the edge progressing inward. The females will overlap oval shaped cut leaves to construct cylinders within the nest cavity and will obliterate the cigar with a circular shaped piece of leaf once the nest has been provisioned with pollen and nectar. An analysis run in 2016 on 3 megachile species identified 54 different plant species used, out of which 48 had some antimicrobial properties.

Leaf cutters usually have one generation per year, at least this is verified in the upper North of the US, it is not clear in Mississippi. Females emerge from the nest, mate with males and will look for a suitable nesting site. Most leaf cutters over winter as prepupae (development stage prior the pupa stage, equivalent to chrysalis stage) and will finish their development the following spring emerging mainly late June and July. This being said I can see some species in early May and some late September. I am not capable to identify most of the species I find. Below picture is a female Megachile from Dordogne in France, found by my parents as I was on vacation. All the other pictures have been taken in Holly Springs, Mississippi.

Females present a broad flattened abdomen with rows of scopae (collecting hairs) on the underside. The mandibles are large to cut the leaves and the wings are usually held outward at 45 degrees to the body when foraging. Males will bear a rounded abdomen with the last segment pointing downward. Many males will show long hairs on their forelegs, which they use to cover the eyes of the female when they mate. Special glands oil the males forelegs that will help the female to determine the male identity and suitability. The picture below shows a female on a Butterly milkweed. Unlike most flowers, milkweeds do not produce loose pollen. Waxy masses of milkweed pollen are grouped into sacs called pollinia. Bees don’t collect pollinia to use as food for their larvae the way they do with the loose pollen of other flowers. A visiting insect trying to reach the nectar offered at the top of the star-shaped corona will slip one of its legs or another appendage inside the anther slits between the hoods. The pollinia inside the stigmatic chamber sticks to the insect’s setae or tarsal claws. By pulling its leg out of the slit, the insect extracts the pollinia and carries them off to another milkweed flower. The flower will be pollinated successfully if the donor pollinia remain in the recipient anther slit. the pollinia is visible on the picture attached at the tip of the hind leg of the bee. It also is visible on the picture that the bee has no pad between the tarsal claws (called ariola), which is a distinctive characteristic of the Megachile.

The picture below shows the strong mandibles of the female.

Here below a male Megachile captured by a Crab spider ambushed in a Heliantus. The downward pointing last segment of the abdomen is noticeable. Males lack collecting hairs under the abdomen. Also the pale light colored hair in between segments is typical. Bees in general are subject to predation by spiders, and noticeably crab spiders, at the end of August and September.

The hair adaptation of the megachille makes them capable pollinators and many species are managed by growers to aid in the pollination of several high value crops. The alfalfa leafcutter bee for example (Megachile rotundata) is essential to seed production for alfalfa in the United States.