What could be eating round holes in the leaves of rose bushes?

Gardens & Landscapes March 26, 2010 Print Friendly and PDF
Leafcutter bees--important native insects of the western United States--use cut leaf fragments to construct their nest cells. They often are essential pollinators of wild plants. 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 creates concern. Leafcutter bees sometimes nest in the largest diameter rose canes but cause little damage because they restrict tunneling to the pith and rarely girdle the cambium. (Note: Other insects, including various hunting wasps and small carpenter bees more commonly tunnel and nest in rose canes.) After the nest is made, 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-inch in diameter from the edge of leaves. Although they cut many types of leaves, leafcutter bees prefer certain types, notably rose, green ash, lilac and Virginia creeper. 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, they carry them back to the nest and use them to fashion nest cells within the previously constructed tunnels. Most common leafcutter bees (Megachile spp.) 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 than that of honeybees or yellowjacket wasps. Leafcutter bees are solitary bees, meaning that they don't produce colonies as do social insects (honeybees, yellowjackets, 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 this time. Leafcutter bees nest in soft, rotted wood; thick-stemmed, pithy plants (e.g., 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. This sometimes causes 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. For more information see the fact sheet, Leafcutter Bees.

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This work is supported by the USDA National Institute of Food and Agriculture, New Technologies for Ag Extension project.