Alfalfa Leafcutter Bee (Megachile rotundata)

Megachile rotundata (or the alfalfa leafcutter bee) is a species native to Eurasia that was introduced into the United States after the 1930’s because of a drop in seed production. This bee was brought into the US to increase pollination yields of Alfalfa for seed because honey bees are not the best pollinators of the crop. M. rotundata was also introduced to New Zealand (1971) and Australia (1987) for the same reasons. This solitary species is now widespread across the United States with many feral populations.

M. rotundata nest close up.

M. rotundata nest close up.

Alfalfa has a tripping mechanism that triggers the stamen (pollen reproductive organ) to strike the pollinator enabling pollen transfer to the stigma (where the pollen grain germinates). Honey bees learn to avoid the tripping mechanism and collect nectar without tripping the stamen to strike which decreases the possibility for pollen transfer. M. rotundata; however, is not as adaptable as the honey bee or it is willing to accept more abuse from the flower, taking many hits trying to collect pollen and get a taste of sweet alfalfa nectar to feed and make provisions for its young. Although the bee is preferentially oligolectic (narrow preference for pollen type) behavior, it is polylectic (opportunistic forager) collecting from various plant species if available. For commercial pollination of alfalfa, often both honey bees and M. rotundata are used to increase crop yields. M. rotundata uses its mandibles to cut oblong and circular leaf pieces to create a string of individual cells in which individuals emerge. I drew a sketch of what it would look like inside of the individual cells below as well as an image of what one side of the mandible looks like.

M. rotundata mandible and nesting tube diagram.

M. rotundata mandible and nesting tube diagram.

Normally a solitary bee, this adaptive species can be reared and kept in large next boxes containing hundreds of reproductive females (see images below). In nature this bee makes its nest in soft rotting wood, thick stemmed pithy plants, hollow reeds, radiators and even drinking straws. M. rotundata will nest in anything that is approximate to its body size.

Megachile rotundata female on nest box.

Megachile rotundata female on nest box.

But as you would suspect, putting a large quantity of solitary bees in overcrowded nesting sites opens the doors for opportunistic pests and pathogens. M. rotundata fall victim to three main problems: Chalkbrood, pesticide exposure and a parasitic wasp. Chalkbrood in M. rotundata is similar to that found in honey bees (Ascophera apis) but is actually a different species (A. aggregata). The parasitic wasp Ptesomalus venustus cannot reproduce without M. rotundata. The female waits until the M. rotundata larva spins its cocoon and then stings to paralyze the larva and oviposits onto the surface of the prepupa. Within 48 hours, the wasp larvae emerge and start feeding until the prepupae is almost completely gone, and then pupates itself.

Alfalfa blooming with M. rotundata nest box trailers for pollination.

Alfalfa blooming with M. rotundata nest box trailers for pollination.

Another nest box photo.

Another nest box photo.

M. rotundata is commercially supplied as prepupa and kept at 44-45 degrees Fahrenheit. They are then incubated at 80-81 degrees Fahrenheit for about 25 days and placed into the field (pictured below). The emerging male and female bees will soon start mating and reproducing in nearby next boxes (pictured below). I have also added images of a M. apicalis female and M. mendica male so you can differentiate the two sexes. Though they are not the same species, they are similar in size and show similar sexual dimorphism.

 Megachile apicalis female, notice the small white hairlike projections on the abdomen(scopa) for pollen collection and the pointed tip of the abdomen(T7).

Megachile apicalis female, notice the small white hairlike projections on the abdomen(scopa) for pollen collection and the pointed tip of the abdomen(T7).

Megachile mendica male, notice the abdomen tip(T7)  is round with a notch in the center.  Also note the absence of pollen collecting hairs(scopa).

Megachile mendica male, notice the abdomen tip(T7) is round with a notch in the center. Also note the absence of pollen collecting hairs(scopa).

After ~25 days in the incubator these M. rotundata adults are starting to emerge from their individual cells.

After ~25 days in the incubator these M. rotundata adults are starting to emerge from their individual cells.

Nest boxes set up for pollination.

Nest boxes set up for pollination.

Even with all odds stacked against M. rotundata and honey bees, they still get the job done and pollination of the crop occurs. I have heard numbers from 2-6 colonies of honey bees per acre are needed for pollination (3 seems to be the most common). For M. rotundata 1-2 gallons per acre are needed; this depends on the amount of honey bee colonies present in the production field. 1 gallon of M. rotundata holds around 10,000 bees and is sold for approximately 85 to 100 dollars.

Alfalfa feild in bloom 2014.

Alfalfa feild in bloom 2014.

Cotton starting to grow across the road from the Alfalfa.

Cotton starting to grow across the road from the Alfalfa.

Written By: Rob Snyder

Rob Snyder has written 62 post in this blog.

I currently work out of the Butte County Cooperative Extension in Oroville, CA as a Crop Protection Agent. I received my B.S. in biology from Delaware Valley College, PA. There I attained a majority of my entomological knowledge from Dr. Chris Tipping and Dr. Robert Berthold. After graduation, I was an apiary inspector for 2 years at the Department of Agriculture in Pennsylvania. In my third year there, I still inspected some colonies but I mainly focused on The Pennsylvania Native Bee Survey (PANBS) where I pinned, labeled, entered data and identified native bees to genus species. Leo Donavall assisted me in learning the basics on positive Identifications of the native bees. Around the same time I began working on coordinating kit construction and distribution for the APHIS National Honey Bee Survey. I was also fortunate to conduct many of these surveys with fellow co-worker Mike Andree and Nathan Rice of USDA/ARS throughout California. All of these experiences have led me to where I am today, working to assist beekeepers in maintaining genetic diverse colonies resistant to parasites while reducing the use of chemical treatments in colonies. The BIP Diagnostic Lab at the University of MD is in an integral part of this process by generating reports in which we can track change and report to beekeepers vital information in a timely manner which may influence their treatment decisions.

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