Little is known about Bermuda’s leafcutter bee, but this tiny insect is driving a new wave of genetic research.

If you’ve never heard of the leafcutter bee, you are not alone. Dr Mark Outerbridge, senior biodiversity officer at the Department of Environment and Natural Resources (DENR), hadn’t heard of Bermuda’s only native bee species until he joined the department in 2013, and read the Protected Species Act: “I actually met a few creatures that way,” he laughs.

Leafcutter bees are solitary, and therefore unlike the more familiar honey bee which is a colonial bee species, they do not create hives or have a queen. Globally, they are found almost everywhere there are flowering plants; however, in Bermuda, they are only known to nest in the limestone cracks and crevices around Castle Harbour. This makes them particularly vulnerable to rising sea levels and coastal erosion.

They earned their name because female leafcutter bees use the cutting edges of their mandibles, or ‘insect jaws,’ to harvest soft, flexible leaves to line their nests. In Bermuda, their leaves of choice are rose, peach, Virginia creeper, nicker bean, sea oxeye and prickly pear.

Despite being listed as a Level 2 vulnerable species, little was known about Bermuda’s leafcutter bee (Megachile pruina pruina, to give it its scientific name) until now, thanks to Jenn Rose. A Californian, with a personal connection to Bermuda, Rose was studying at the University of Copenhagen and heard about this “neglected group that no one knows about.” After reading the “Recovery Plan for the Leafcutter Bee” by Dr Outerbridge, she gave him a call.

Her research involved extracting the DNA from the leg of a Bermuda Natural History Museum specimen so she could explore the species’ evolutionary history and uncover the potential origins of our source population. She also evaluated their genetic diversity. Her resulting master’s thesis was completed in December and has the working title “Evolutionary Genomic Analysis Reveals a Unique Lineage of Megachile Pruina Found in an Isolated Population in Bermuda.” By comparing the DNA sequence from the Bermuda specimen to specimens from the US, Rose discovered that our leafcutter bee population “may represent a unique lineage no longer found among populations in its native range in the United States.”

Rose’s research revealed that while there was a “strong separation between Bermuda’s population and the United States,” there were “no significant differences” observed among the US populations. She also discovered that Bermuda’s species samples “were more closely related to an unidentified species collected in Mexico as well as two species collected in three locations in the Caribbean.” It is possible, she explains, that the source population for Bermuda’s leafcutter bees was not sampled or has become extinct. She also said that this species might have historically existed or may still exist in the Caribbean and Latin America. However, “if the source population no longer exists, the bee population in Bermuda could potentially represent a unique and isolated evolutionary lineage, echoing the familiar narrative of the Bermuda skink.”

How can we determine if these bees are endemic instead of just native? “We need to do a second step of comparing it morphologically [morphology is the scientific study of the structure and form of animals and plants]and seeing where those differences lie to see if it’s truly its own lineage or not,” says Rose. “At the moment, I would say native. Further analysis could support that it’s unique to Bermuda.” Unfortunately for Bermuda, Rose now has her hands full doing a PhD on community and pollinator ecology at the University of the Basque Country, but Dr Outerbridge is hopeful that her research could inspire another student to take up the cause. “Nothing galvanises researchers like somebody waving a paper saying they’re unique to Bermuda,” he says. “We have a few critters like that, where we have this possible endemic sub species. If you believe in evolution, that’s how it happens. It will start off as a native species, then give it enough time and reproductive isolation it changes genetically to become its own separate species.”

Leafcutter bees don’t live in colonies. Instead they nest in hollow-stemmed plants and rock crevices. The female will harvest leaves to line the nest with, after which she will deposit her eggs and pollen.

Dr Outerbridge would now like to follow Rose’s suggestions for further research. Firstly, to gather morphological data to compare with “their cousins on the mainland,” which would complement her genetic research and hopefully confirm if it is truly a different species. Next, he would like to study their abundance and distribution to confirm whether or not they exist outside of Castle Harbour and, if they don’t, determine what is limiting their spread. This could more accurately establish how vulnerable they are and what needs to be done to better protect them. Thirdly, their nesting. “Once you start looking into their life history, you begin to appreciate what challenges they are facing,” he explains. This is something that has been studied more extensively in the US, where their leafcutter bees will, he adds, “find a hole, say it’s the length of a pencil, they will create cell, egg, cell, egg, all the way until they run out of space.” The cell is where they put the pollen, which is food for the pupae.

Interestingly, the final egg is always a male: “If there are ten in a line, the first nine are females and the tenth is male, because his job, he emerges first, and he’s out waiting for the females of the population to emerge, and when the females emerge, they mate very quickly.”
From the nests Dr Outerbridge has observed in Bermuda, he suspects that they are only making one nest, one cell, one egg, but he hopes, one day, to take the trial work done in the US and apply it to Bermuda to find out for certain.

Given how small the population of Bermuda’s leafcutter bees is believed to be, what impact do they actually have on our environment and, if nothing is done to protect them, what could happen if they die out? Firstly, they are pollinators, a process essential for our survival. Furthermore, solitary bees such as the leafcutter are, points out Dr Outbridge, “excellent pollinators.” This is because they are “quite clumsy.” In the flower, he explains, “they bumble around a little bit, and in that process, they’re knocking pollen around and shaking things up.”

In addition to this, he adds, they must perform some role in Bermuda’s coastal environment: “Our coastal environment is still very much how it would have appeared hundreds of years ago, because it’s such a tough environment. So, the plants and animals that live in that environment are adapted to that. If we lose the leafcutter bee, there will be a subtle change.”

How to Spot a Leafcutter Bee
Leafcutter bees are difficult to spot, but the DENR wants to hear from anyone who may have seen one. The best time to find them is around April, May and June when they are at their most active. While their known nesting sites are on the east end coastline, they could be anywhere on Bermuda. Unfortunately, they are difficult to identify, not just because of their size but also because they look like hover flies; however, they are particularly distinctive when carrying pollen as it will be on their body hairs, not their hind legs. They are also slightly bigger than the honey bee and have much larger eyes that resemble “big, dark goggles.”

If you think you have seen a leafcutter bee or may have one on your property, contact the DENR at 236-4201.