Great Plains Gardening

By Edith Koeller

July 25, 2003

How many of you are familiar with the leafcutter bees that I mentioned in my last article? Some years they can be a problem aesthetically speaking and other years one is hard pressed to find the damage. I have not seen any signs so far this summer.

According to CSU fact sheet 5.576 "insecticides are ineffective for prevention of leaf cutting. The only known control of leaf injuries is to cover susceptible plants with cheesecloth or other loose netting during periods when leafcutter bees are most active. Numbers of leafcutter bees in an area can be reduced if breeding sites are eliminated, although these might be difficult to detect. Look for rotting boards, with sawdust pushed out of excavated tunnels or thick-stemmed plants with hallowed openings. Leafcutter bees can be prevented from tunneling into rose canes by sealing exposed pith as canes are pruned. This can be easily achieved by placing a thumb tack, bit of sealing wax, or white glue on the opening."

"Leafcutter bees are important native insects of the western United States that use cut leaf fragments to construct their next cells. They often are essential pollinators of wild plants, and some leafcutter bees are even semi-domesticated to help produce alfalfa seed. However, their habit of leaf cutting, as well as their nesting in soft wood or plant stems, often attracts attention and concern."

"Most common leafcutter bees are approximately the size of the common honeybee, although they are somewhat darker with light bands on the abdomen. They also have different habits. Leafcutter bees are not aggressive and sting only when handled. Their sting is very mild-much less painful that that of honeybees or yellow jacket wasps."

"Leafcutter bees are solitary bees, meaning they don’t produce colonies as do social insects (honeybees, yellow jackets, ants, etc.). Instead, individual female leafcutter bees do all the work of rearing. This includes digging out nesting areas, creating nest cells, and providing their young with food. Adult females may live up to two months and lay some 35 to 40 eggs during that time."

"Leafcutter bees nest in soft, rotted wood, thick stemmed pithy plants such as rose, and in similar materials that the bees can easily cut through and excavate. Nest tunnels may extend several inches deep and coarse sawdust may be deposited at the entrance, sometimes causing confusion with other wood-nesting insects such as carpenter ants. However, leafcutter bees restrict their tunneling to soft, rotted wood and do not cause damage to homes or other wooden structures."

"There also are concerns about leafcutter bee nests in canes of roses, excavating the pith of canes that have been pruned. Leafcutter bees will sometimes nest in the largest-diameter rose canes, but cause little damage because they restrict tunneling to the pith and rarely girdle cambium. Furthermore, other insects, including various hunting wasps and small carpenter bees more commonly tunnel and nest in rose canes."

After the next is produced, leafcutter bees collect fragments of leaves to construct individual nest cells. The bees cut leaves in a distinctive manner, making a smooth, semicircular cut about ¾ inches in diameter from the edge of leaves. Although many types of leaves will be cut, leafcutter bees will preferentially select certain types - notably rose, green ash, lilac, and Virginia creeper." I have had them cut my raspberries leaves in years past also. "This injury often is only a minor curiosity. However, where leafcutter bees are abundant and concentrate on cultivated plantings, the removal of leaf tissues can be damaging. Serious damage most often occurs in isolated rural plantings."

"Leafcutter bees do not eat the cut pieces of leaves that they remove. Instead, these are carried back to the nest and used to fashion nest cells within the previously constructed tunnels. Each leaf-lined cell is then provisioned with a mixture of nectar and pollen. An egg is then laid and the cell sealed, producing a finished nest cell that somewhat resembles a cigar butt. A series of closely – packed cells are produced in sequence so that a finished nest tunnel may contain a dozen or more cells forming a tube 4 to 8 inches long. The young bees develop and remain within the cells, emerging the next season."

"There are a great many parasites that act as important natural enemies of leafcutter bees. As a result, leaf cutting activity may vary widely from year to year. Parasitic bees and wasps, velvet ants, and certain blister beetles are among the most important enemies of leafcutter bees and other solitary bees."

"At least one species of leafcutter bee is cultivated for agricultural use in Colorado. Megachile rotundata is used for the pollination of alfalfa grown for seed, a function that it does far more efficiently than honeybees. These leafcutter bees are semi-domesticated by providing them predrilled "bee boards" that the leafcutter bees use for nest construction. At the end of the season, the nest cells with developing bees are collected and carefully stored, to be released the subsequent season in synchrony with alfalfa bloom."

Leafcutter bees. Friend or foe? What do you think?

Until next time…