By Whitney Cranshaw, Specialist of Entomology, Colorado State University Cooperative Extension
Damage and Diagnosis
Although leafcutter bees are rarely caught in the act, their presence is easily detected because of the unique, semicircular cuts they make along the edges of leaves. A variety of plants are used, but several are preferred, including rose, lilac, ash and Virginia creeper.
Leafcutter bees are important and native pollinators of many flowers. Most are fairly similar in size and sometimes even color to honeybees. However, as they collect pollen, it is packed under their abdomen, rather than in the "pollen baskets" found on the legs of honeybees.
Leafcutter bees are a type of solitary bee and do not make colonies like those of the social honeybees or bumblebees. Each female creates her own nest, which is usually excavated out of partially rotted wood or made in the pith of canes and hollow-stemmed weeds.
The nest cells are lined with the leaf fragments the bees cut. The cuts are made rapidly, typically in 10 seconds or less, and are carried back to the nest underneath the bee. The constructed cell is then packed with a plug of pollen and nectar and an egg is laid on it.
When finally sealed with a bit more leaf material, the cells look like a miniature cigar butt. In larger nests, a dozen or more such cells may be stacked one after the other.
The immature bees feed on the stored food and turn into a whitish grub by late summer. They remain in the nest over the winter, developing into adult bees the following spring and emerging. One generation is produced per year, with peak activity of leaf-cutting typically in late June and July.
Female leafcutter bees can sting, but it is not very painful. More important, they are non-aggressive and sting only when handled or accidentally confined.
Except in some isolated rural plantings, damage by leafcutter bees is usually just a curiosity and produces little injury. It is also difficult to prevent leaf-cutting, and there are no effective insecticides or repellents. Cheesecloth and other types of netting are sometime used to exclude leafcutter bees during peak periods of activity.
On the other hand, the pollination service that leafcutter bees provide is valued by some people who may even wish to help them out a bit. Access to suitable nesting areas limits leafcutter bees, and artificial nests can be provided to assist them. A simple "bee board" that leafcutter bees will often use involves drilling holes into blocks of wood. Holes should be from three-sixteenths of an inch to a quarter-inch in diameter and drilled about 6 inches deep, at a slightly oblique angle. The use of pithy plants around the yard such as sumac, elderberry and raspberry, also can provide nesting sites for the insects.
Photographs courtesy of Judy Sedbrook.
© CSU/Denver County Extension Master Gardener 2010
888 E. Iliff Avenue, Denver, CO 80210
Date last revised: 01/05/2010