Garden Tips: Leafcutter bees good thing to see in garden

When we think about important pollinators of fruit and vegetable crops, we tend to think of honey and bumble bees. However, there are other bees in the garden that are also valuable pollinators.

Unlike colony- or hive-dwelling honey bees, many wild bees are solitary. Solitary bees have separate nests built by the females. My favorite solitary bee, the alfalfa leafcutter bee, is a common resident of local yards and gardens.

The alfalfa leafcutter bee is not native to Washington. Experts think it was accidentally introduced into the U.S. from Asia in the 1930s, then was deliberately imported in the1940s because it had been found to be an excellent pollinator for commercial alfalfa, legumes, blueberries and vegetable crops. Adult alfalfa leafcutter bees range in size from 0.25 to 0.8 inches in length, and are black in color with five bands of white to silverish hair on top of their abdomens. They are effective pollinators of alfalfa because they are more adept than honey bees at prying open alfalfa flowers to gain the nectar and pollen held within.

The alfalfa leafcutter bee is solitary and builds its nests wherever it finds suitable small cavities, such as in tree trunk cracks, holes in the soil or tunnels left behind by boring insects. They are often blamed for the hollowed out canes found in garden roses, but these tunnels were created by cane borers. While the leafcutter bee does not create the tunnels, it does use them for building its nests.

When it comes to cutting almost perfect circles of leaf tissue, female leafcutter bees are definitely guilty. The females use their jaws to cut out the leafcutter circles that they use to create cells in cigarlike nests in whatever small cavity they have found. A female may use up to 15 circles to create one cell in which she lays a single egg, then packs it with pollen and nectar that she has collected.

The pollen and nectar in the cell will feed the baby bee after it hatches into a larva. Later, each larva transforms into a pupa, then spins a cocoon, and finally emerges as an adult bee. A leafcutter bee nest may have one or more cells, depending on the size of the hole used by the mama bee.

While leafcutter bees are responsible for the one-quarter to one-half inch holes found in the leaves of roses and other ornamental plants, the damage poses little threat to a plant’s health. When it comes to pollinating alfalfa, experts say that alfalfa leafcutter bees are 15 times more effective at pollination than honey bees. They are also valuable because they are generalists that pollinate many types of garden flowers and vegetables.

Gardeners should not be dismayed when the see the characteristic round cut-outs created by the adult females. They are an encouraging sign that there are active pollinators working in the garden, so rejoice and bee happy when you see them.

This story was originally published January 8, 2017 at 8:32 AM with the headline "Garden Tips: Leafcutter bees good thing to see in garden."