Reproduced by permission of Whitney Cranshaw
Colorado State University Extension Entomologist
Life History And Habits
Most common leafcutter bees ( Megachile species) are approximately the size of the common honeybee, although they are somewhat darker with light bands on the abdomen. They also have very different habits. Leafcutter bees also 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 2 months and lay some 35-40 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, 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 are also concerns about leafcutter bees nest that 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 as they restricting tunneling to the pith and rarely girdle cambium. Furthermore, other insects, including various hunting wasps (Pemphredon species) and small carpenter bees far more commonly tunnel and nest in rose canes.
After the nest has been produced, leafcutter bees collect fragments of leaves to construct individual nest cells. The bees cut leaves in a very distinctive manner, making a smooth semicircular cut about 3/4-in 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. This injury is often only a minor curiosity. However, where leafcutter bees are abundant and concentrate on cultivated plantings, the removal of leaf tissues can be very damaging. Serious damage most often occurs in isolated, rural plantings.
However, 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.
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 hollowed 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.
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