The Giant Resin Bee

Feb 23, 2020

The Giant resin bee (Megachile sculpturalis) is an invasive species from Eastern Asia accidentally introduced to the United States in the mid 1990’s. It is now present in most states East of the Mississippi river and is expanding Westward. It has been observed pollinating 43 different native species but it especially likes to pollinate the introduced plants from Asia (apparently including Kudzu). Much of the info in this article is from the taxonomic paper on the Giant Resin Bee in the biodiversity data journal and from the department of entomology of the University of Florida (UF/IFAS).

The first specimen of Giant resin bee was collected in 1994 in North Carolina. It is suspected that the bee was introduced through international trade. The range rapidly expanded across North America from the original location, reaching southwest to Alabama by 1999, north to Canada by 2002, northwest to Wisconsin by 2004, northeast to Maine by 2008, and westward to Kansas by 2008.  By that time almost all states East of the river had been colonized. I started to observe The Resin bee in 2016 in Mississippi and its population has been growing since then in Hudsonville. The western edge of its range seems to stretch now to western Texas, Oklahoma, Kansas, Nebraska, and South Dakota, areas of the western coast, along with sections of Mexico. This species has also been introduced into Europe in the mid 2000’s, through the port of Marseille, and is currently established in France, Switzerland, and Italy.

The Giant Resin Bee is a Megachile (Callomegachile), easily differentiated from native bees by a large narrow elongated body. US native Megachilids, such as leafcutter bees, are considerably smaller than the giant resin bee, ranging in size from 5 mm to 24 mm. Female Resin bees range from 22 – 27 mm, while males are considerably smaller at 14 – 19 mm with a distinctly wide yellowish “moustache” on their lower face. The large cylindrical body is typically black and yellow-brown in color. The head is dark with yellow-brown setae (hairs). These golden hairs can be found on the thorax and the first segment of the abdomen. The female has a more pointed abdomen while the male’s abdomen has a blunt edge. See picture above of a male with its square abdomen.  The moustache of golden hair is visible on the picture below. Giant resin bees have smoky-colored wings that contain dark marks near the edges. Their wings, while resting, are often held in a V shape.

Megachile sculpturalis are solitary bees that do not form colonies. They are nevertheless gregarious and females build their nests in the same area as other females. They are known to be tunnel nesters, however I have found one nest in a large cavity between two boards and the nest was not organized as a tunnel (see picture below). The females create their nests in narrow grooves in tree cavities, crevices, downed logs and other debris sometimes used by other bee species. They are known to displace native carpenter bees (Xylocopa spp.) from their cavities in order to nest in them. During the summer months, female Megachile sculpturalis begin constructing brood cells of wood particles and mud, very similar to those of the native carpenter bee (Xylocopa virginica). They will use existing carpenter bee cavities in order to lay their eggs because their mandibles cannot chew through wood to create cavities of their own. They will use other material with pre-existing cavities as nesting habitat such as rotting wood. After several pollen collecting trips, the females form a pollen ball containing pollen and resin in each brood cell and lay a single egg on the pollen ball. She will seal the cell using a mixture of wood particles, mud, or resin. After the first cell is sealed, the bee will repeat the process and lay another egg within a separate brood cell. The female can make as many as 10 cells per nest. Larvae remain in their cells and feed on the pollen balls. They will consume this food throughout the winter. The larvae will pupate during the spring and the adults emerge in early summer. This female fills in a cavity with mud and resin. The bee hotel was mainly used by a Giant Resin Bee. The pointy abdomen, different to the squarish tip of the male abdomen is visible on the picture.

This female giant resin bee was found dead next to her nest built between two boards. The nest is not organized in the form of a tunnel but in the form of a fan starting from the angle of the board. It is made of solid resin.

Although their large appearance may make them intimidating, they are largely harmless. The males are unable to sting, and the females, although able to sting, are not aggressive and usually fly away from humans. The overall ecological impact of this bee has been considered benign, since no drastically negative or positive outcomes have been detected by their presence thus far. They are known to pollinate up to 43 different species of plants in the US and do exhibit preferential pollination of plant species from their native habitat. The giant resin bee has been observed to leave puncture marks on the petals of two types of flowers, the everlasting pea and the Japanese pagoda flower. This is a helpful visual marker for researchers studying what plants the giant resin bee visits. This behavior does leave a permanent mark on the flowers, and in other species, a mark like this could have a negative effect on the pollinating ability of the flower.

These bees may negatively affect a native carpenter bee, Xylocopa virginica. The carpenter bees are known to chew long narrow cavities in the eaves of homes, which the giant resin beeswill use as nest sites. Because of this, Megachile sculpturalis may actually compete with Xylocopa virginica for nesting habitat.