Around this time of year a number of inquiries are received centering around the cause of “half circle holes” in the foliage of bougainvilla, ash trees, rose bushes, and other plants with thin, smooth leaves … another mystery in the garden.
Undeniable evidence is the presence of similar semi-circular cuts all measuring about 3/4 inch diameter in the edge of many leaves. Close observation points to a stout, black, female leafcutter bee, an important pollinator in the western U.S. Further, the missing leaf parts are being used to construct individual cells within nest tunnels, usually holes she has either found or made in rotted or soft wood.
Leafcutter bees are native, solitary bees approximately the size of a common honeybee, yet darker with light bands on the abdomen. They do not produce hives or colonies, as social bees do. They are not aggressive and will sting only if being handled. Stings are very mild and much less painful than those of honeybees or wasps.
Adult female leafcutters live up to two months and may deposit 40 eggs during their lifetime. Males do not live long after mating so the rest is up to the female.
She may nest in large diameter rose canes, plant stems, or other found holes in wood. When the nest is established, she will begin collecting fragments of leaves with which to construct individual nest cells within.
To make a “bee loaf,” she will knead a collection of pollen, nectar, and her saliva. It may take her a number of trips to build a bee loaf large enough to feed one larvae from egg to maturity. When she has enough material, she will lay one egg on top, then seal the small chamber with chewed-up leaves.
This is all repeated until the entire nest hole, or tunnel, is filled. She then will build one final, thicker wall. Shortly after completing her work, she will die. The next generation will feed on the bee loaves she left behind. They will grow and transform into adults. The youngsters then chew their way out of the nest, find themselves a mate, and the cycle begins again. In our climate, there may be as many as three generations produced each year.
Leafcutters do not have pollen “baskets” on their hind legs like other bees, but collect pollen with hairs on their abdominal underside. They are valuable pollinators of wildflowers, fruit, vegetables, and other crops. Commercially they are often used for mass pollinating field crops such as alfalfa, blueberries, onions, and carrots. The U. S. Agricultural Research Service claims that one leafcutter bee in an alfalfa field can do the job of 20 honeybees.
Leaf cutting can damage the appearance, but usually does not harm the plant. Because these bees do not eat the leaf material, insecticides are ineffective. Fabric physical barriers such as cheesecloth may be the best solution to preventing damage.
Picture a small, docile bee chewing off same-size, perfect half-circles of leaf material … then flying away carrying the piece as large as herself or larger … with an awesome responsibility to continue her species. No longer do we have a mystery, but an inspiring story of strength and dedication in the garden.
Mary Kidnocker is a University of Arizona Master Gardener who lives in the Green Valley area. Her articles are featured weekly.