mason bee

mason bee, (genus Osmia), genus of more than 350 species of solitary bees mostly native to the Northern Hemisphere and known for their habit of using mud in the construction of their nests. Like most other bees, mason bees do not produce honey, and they do not sting unless provoked. They are considered to be native pollinators, which means that they coevolved with the flowering plants in the areas in which they live and are well adapted to local climates, flower structures, and other ecological relationships in their habitats. The blue orchard bee (Osmia lignaria) is one of the few non-honeybee species that is managed in agriculture; it is an especially efficient pollinator of fruit trees.

Mason bees are found in a variety of habitats, ranging from deserts to forests to prairies, and many are generalist pollinators that can use a variety of flowering plant species. Despite their apparent adaptability and their ecological and economic importance, mason bee populations are in decline in many places. Habitat loss and degradation, invasive species and competition from non-native bees, and increased usage of pesticides are thought to be driving factors. There is also concern that anthropogenic climate change may lead to a phenological mismatch, in which the emergence of adult bees does not coincide with the flowering of their preferred nectar and pollen sources, as well as to high temperatures that become too extreme for some species. According to the U.S. Department of Agriculture’s Bee Biology and Systematics Laboratory, 17 of the 66 native species of mason bees in the United States are in decline.

Physical description

Mason bees can be metallic green to blue or black. Many have hairs on the head and thorax whose color is similar or in contrast to the colors of the body. The bees range from 9.5 to 16 mm (0.4 to 0.6 inch) in length, the males being slightly smaller than the females. Most mason bees have a robust body that is typically smaller than that of the European honeybee (Apis mellifera), though a few species are large enough to be confused with a bumblebee.

Members of the family Megachilidae, which also includes leaf-cutter bees, characteristically transfer pollen by carrying it on their abdomen in specialized hairlike structures called scopae. Although the scopal hairs are not typically very visible unless laden with pollen, their color can be used in the identification of some mason bee species.

The so-called small mason bees are members of the related genus Hoplitis.

Life cycle

Like most other bees, mason bees are solitary and do not form hives. Some species nest in aggregations, but they do not live in organized colonies and are not considered to be social insects. Instead, females create their own nests and provide for their own larval offspring. Mason bees nest in hollow reeds or other plant stems or in small crevices or existing aboveground tunnels built by other insects; some species are ground-nesting and excavate their own tunnels. Many species also make use of human-made “bee hotels,” which provide holes and canes of different sizes. The ideal nest is typically about 8 mm (0.3 inch) in diameter and about 15 cm (6 inches) deep.

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A nest is developed one or two eggs at a time over several days. Each egg is deposited into its own brood cell in the reed or tunnel, provisioned with food, and then separated from the others in the nest by mud partitions or leaf mastic (chewed leaf sections). The deepest part of the nest contains the fertilized eggs, which will develop into females; they are laid at the back of the nest for additional protection from predators. The unfertilized, male eggs, which compose the majority of the brood, are laid closer to the entrance. After each egg is laid, the female collects pollen and nectar and takes it back to the nest, where she mixes it with saliva and rolls it into a ball. Each egg is thus given a food allotment, though the female eggs are given slightly more provisions than the male eggs. Once an egg is provisioned, the mason bee seals the opening of the brood cell with leaf mastic or mud and begins to work on the next cell. A female can make as many as seven such nests in her brief life.

The eggs hatch a few days after they are laid, and the larvae feed on the food provisioned by the mother bee. After about 10 days, usually in August or September, each larva spins a cocoon and pupates within its brood cell. Although pupation results in an adult bee, young mason bees do not leave their nests yet. Instead, they go into diapause, essentially to hibernate during the winter months. Once the temperature is consistently at least 10–13 °C (50–55 °F) in the spring, the bees chew their way out of their cocoons and brood cells. Males emerge first since their eggs are closer to the nest entrance. They wait for the females to emerge a few days later, at which point the males mate with the females and then die. A female can store the semen until she is ready to lay fertilized eggs for female offspring. Adult females begin building and provisioning their nests approximately three to four days after mating and lay one or two eggs per day until they die about one month later.

There are several parasitic organisms that can injure or kill mason bees in the nest. Parasitic chalcid wasps can pierce a mason bee’s cocoon and lay their eggs inside; the wasp larvae eventually consume the pupating bees. Pollen mites can overwhelm a nest and kill the eggs inside. These are usually introduced as hitchhikers on the female bee’s body as she provisions the nest with food.