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Alfalfa Leafcutting Bees Like Nests That Face North, Study Shows

alfalfa leafcutting bee (Megachile rotundata) at nest box

Alfalfa leafcutting bees (Megachile rotundata) are solitary bees that lay eggs in narrow cavities, either natural or artificial, such as in a next box. The species is a critical pollinator of alfalfa crops. A new study led by researchers at North Dakota State University examined how the microclimates on different sides and levels of nesting boxes influence the bees’ egg-laying preferences and ensuing number of offspring. (Photo by Whitney Cranshaw, Colorado State University, Bugwood.org)

By Edward Ricciuti

As they say in the real estate business: “Location, location, location.” The same goes when the female of the agriculturally important alfalfa leafcutting bee (Megachile rotundata) shops around for a nest site. According to new research, these little bees are much more picky about where they deposit their eggs than previously thought, a finding that may benefit producers who supply them to farmers for pollinating crops.

Ed Ricciuti

Ed Ricciuti

Alfalfa bee breeders can take advantage of the tendency of females to nest precisely where temperatures are most suited to survival of their young, according to a new study published last week in Environmental Entomology. The bees are exceedingly demanding about picking a site in nest boxes used for commercial production that is not too hot and not too cold but—with apologies to Goldilocks—where the temperature is just right for their eggs and larvae. The study showed that the right location in a nest box may be only a couple of inches away from the wrong one.

Alfalfa bees nest in cavities the diameter of a soda straw; in fact, paper straws are used by commercial producers to line cylindrical nest holes in solid foam blocks that are stacked up inside nest boxes. The holes in the blocks form rows horizontally and vertically, like the blocks in a crossword puzzle. The boxes used in the study each had 36 blocks with four holes each. In boxes or in nature, females lay a half dozen or so eggs in a hole, walling off each egg with a piece of leaf—thus its name of “leafcutting”—to create individual cells.

It turns out that the typical nest box, about the size of an average bird house, creates varying microclimates, within which the temperature of an individual nest is determined by its position in the box and the direction it faces. A nest box, in effect, has mini climate zones. The study was carried out with 3D-printed nest boxes, with 144 nest cavities each. The boxes were posted in an alfalfa field in Fargo, North Dakota, and observed daily.

nest box experiment design

A study led by researchers at North Dakota State University examined how the microclimates on different sides and levels of nesting boxes influence egg-laying preferences and the number of offspring of alfalfa leafcutting bees (Megachile rotundata). The box design, shown here, allowed the researchers to measure temperature of each nest cavity with button-size sensors and observe differences based on directional position and other environmental conditions. Results showed significant variation in temperature among the cavities and that M. rotundata females preferred to nest in cooler cavities on the northern sides of the nest box. (Image originally published in Wilson et al 2020, Environmental Entomology)

“Our research shows that, even with the same atmospheric temperature at these nesting boxes, each bee, depending on block, experienced a different temperature,” says lead author Elisabeth S. Wilson, who conducted the research while a graduate student at North Dakota State University.

Temperature variations made a big difference in nesting success, the research suggests. Nests on the cooler, northeast side of boxes, preferred by females, had a 15 percent increase in the number of offspring compared to the southeast side. The common practice in production is to orient nest box shelters toward the southeast, the report notes. The researchers also found the females preferred to nest in the top row of the nest box, which had the most shade and limited temperature range. It also seemed to afford protection from the wind.

In general, the entire north-facing side of the box was more desirable than the side facing south. Southwest was the hottest, with a temperature 1.5 degrees higher than the 45 degrees Celsius that marks the threshold above which developing young start dying. Northeast recorded the lowest maximum temperatures.

These results, according to Wilson and colleagues, demonstrate that growers can manipulate the microclimate of bee nest boxes to increase nesting and consequent numbers of offspring. That means a lot, considering it takes 40,000 to 60,000 bees per acre to pollinate an alfalfa seed crop. Introduced to the United States from Eurasia in the late 1930s, the bee also is used on canola, carrots, and several other crops.

“I designed this study thinking about the eventual use and how this research could impact the farming community,” says Wilson. Many efforts to improve bee populations consume large amounts of time and money, she notes. Simply turning around a nest box to face in the optimum direction is much easier and cheaper. “Turning around a nest box is a low-time investment, free, and relatively risk free,” says Wilson.

Ed Ricciuti is a journalist, author, and naturalist who has been writing for more than a half century. His latest book is called Bears in the Backyard: Big Animals, Sprawling Suburbs, and the New Urban Jungle (Countryman Press, June 2014). His assignments have taken him around the world. He specializes in nature, science, conservation issues, and law enforcement. A former curator at the New York Zoological Society, and now at the Wildlife Conservation Society, he may be the only man ever bitten by a coatimundi on Manhattan’s 57th Street.

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