Although most of our trees have long since bloomed, Tetradium danielli is currently exploding in color. Its multitude of beautiful white flowers in turn attract thousands of bees, explaining its common name, the “bee-bee tree.” Since Tetradium danielli is native to east Asia, the majority of the North American bees discovered its sweet nectar relatively recently (on an evolutionary scale at least). However, at some point during the last 30 years, one of its Asian pollinators, the giant resin bee (Megachile sculpturalis), reunited with the Tetradium in our landscape. For better or worse, this introduced bee, and the introduced tree which attracts it, may become increasingly familiar in our urban ecosystem.
First recorded in 1992 in South Carolina, giant resin bees have since spread up and down the eastern seaboard of the U.S. and Canada, west to the Dakotas, and may eventually reach all the way to California. These bees are easily picked out from the usual suspects of European honeybees and native bumblebees. To start, they are large. The females in particular dwarf more common pollinators, appearing closer in size to carpenter bees (genus Xylocopa). However, unlike carpenter bees, which have round, shiny, hairless abdomens, giant resin bees have long, cylindrical and ridged abdomens. Males, only 2/3 the size of the females, are distinguished by a bright yellow “mustache” just above their mandibles.
Unlike social bees, giant resin bees do not live in colonies. Instead, like other bees in the family Megachilidae, they are a solitary, cavity dwelling species. The males die soon after mating, leaving the females to find a nest site and establish a brood. Although females prefer to nest in holes in trees, they lack the appendages to excavate them. Instead, they rely on the bore holes left by beetle larvae and cavity excavating bees, such as carpenter bees. Once a female finds an appropriately sized cavity, she lays an egg and stocks it with provisions of pollen and nectar. She creates cell walls to separate each egg using sticky resin (thus the common name) which she collects from trees. When the cavity is full, she uses more resin to seal and waterproof the tunnel entrance.
While it is fascinating to observe thousands of giant resin bees foraging on our Tetradium, there is evidence that giant resin bees may out-compete native cavity-dwelling insects for nesting habitat. For example, researchers have observed giant resin bees killing carpenter bees after immobilizing them with gobs of sticky resin and then taking over their carefully excavated nest sites. While this may provide satisfaction to homeowners who dislike the industrious boring of carpenter bees, the long-term effect could very well be another factor in the decline of an important niche pollinator.
Giant resin bees also take advantage of the increasingly popular “bee hotels” built for solitary bees such as mason and leafcutter bees. While these types of habitat enhancements (such as the one in our Leventritt Shrub and Vine Garden) can be valuable, lack of maintenance results in a build up of parasitic mites as well as potential inhabitation by giant resin bees. Research also suggests that giant resin bees may prefer foraging on non-native species, such as Tetradium danielli, which are common in urban yards and parks where many “bee hotels” are built and installed. The fact that Tetradium itself has invasive tendencies means that this particular partnership may be expanding from yard to park to forest.
For those who read all of this as yet another doom and gloom environmental story, there are certainly positive steps to take. For example, when possible, purchase native plants for your home and garden that will support native organisms. Keep an eye out for non-native insects to help track, and potentially mitigate, any damage they may cause. For those of you with bee hotels, don’t forget to clean them!
But since none of those actions will reverse time or put the bee back in the bottle, perhaps also find time to wonder at the amazing combinations of international biodiversity that exist all around us. The giant resin bee and the bee bee tree are, against all odds, enjoying their reunion during this warm New England summer. For better or worse, reunions such as this may only become more common.