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Mysterious Crop Circles

Leafcutter bees are carving circles in the leaves of bushes.

Leafcutter bee
Figs. 1–3. A female leafcutter bee makes a circular cut in 10 seconds and uses the tissue to line its nesting cavity. Damage to the plant is minimal. Leafcutter bees are outstanding pollinators and may be attracted into nesting tubes.
Several years ago I was called by the police to investigate a pasture where crop circles appeared. It was like a circus at the scene. A UFO organization was videotaping. Scientists were taking samples. The farmer recalled his cat was dancing during the night the circles appeared.

The circles of dead grass in the valley were real. Lab tests showed no traces of herbicide. Were these spots caused by aliens? Maybe. The mystery was never solved.

Personally, I think the dead grass was caused by temporary flooding in the valley, but that wasn’t the answer the UFO crew was looking for.

Today, there are new cases of crop circles in North Dakota. These circles are on bushes (Fig. 1). Is this another invasion of aliens?

No, these circles are caused by leafcutter bees. The bees carve semicircles out of leaves to line their nesting cavities and protect their eggs from predators (Fig. 2).

A leafcutter bee is an impressive pollinator and our friend in the garden. The bee carries its golden pollen loosely on the underside of its abdomen (Fig. 2). Since she keeps the pollen dry, it falls off easily when she flies from flower to flower. She is a much more efficient pollinator than a honey bee, who tucks pollen in “baskets” in its hind legs. A honeybee does a great job of gathering pollen, but not in spreading it.

Leafcutter bees are solitary and busy. They nest in small cavities found in rotted wood or hollow stems. They are non-aggressive. These bees only sting when handled or trapped, and its effect is no more painful than a mosquito bite.

You can build a nesting site for leafcutter bees. A group of hollow tubes is placed where it gets the morning sun (Fig. 3). Another option is to drill holes (3/16 to 1/4 inch in diameter and 6 inches deep, at a slight angle) in a block of wood. The bees will nest in pithy plants such as sumac and raspberry. 

Written by Tom Kalb, Extension Horticulturist, North Dakota State University. Published in the NDSU Yard & Garden Report, July 13, 2016.

Sources: Cranshaw, W. 2010. Leafcutter bees. Colorado State University Cooperative Extension: Fort Collins.
Crown Bees. 2016. http://crownbees.com/learn-leafcutterbees.

Photos were made available under Creative Commons licenses specified by the photographers: Jack Skipworth, Rolf Brecher and Picture Esk.

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