Somebody with a camera in Malgrat de Mar, a little beach town up the coast from Barcelona, caught a female giant resin bee in July 2024 with her head buried in the white flowers of a Japanese pagoda tree. That picture is now Figure 1 of a new study, and the study’s argument is pretty simple: the tree Spanish towns like to put on their sidewalks is feeding the bee’s spread across Catalonia.<\/p>\n\n\n\n
There are 150 records of the bee in the dataset, plus 21,383 records of the tree, and 82.67 percent of the bee records overlapped with the tree. Having one of those trees nearby turned out to matter at least as much as a good climate.<\/p>\n\n\n\n
Daniel Echeand\u00eda of the Universitat Aut\u00f2noma de Barcelona (UAB) and the Desertification Research Center (CIDE) in Moncada, Spain, led the paper. The idea came from Carlos Hern\u00e1ndez-Castellano, who’s now in the Department of Ecology at the Czech University of Life Sciences Prague and supervised it, and the rest of the authors are from CREAF, the University of Salzburg and BOKU University in Vienna.<\/p>\n\n\n\n
Hern\u00e1ndez-Castellano has been on this bee’s trail for a while. In October 2018, when he was at CREAF, he was quoted by ECOticias saying that “the giant resin bee probably arrived last year and has already established itself here.”<\/p>\n\n\n\n
Megachile sculpturalis is native to Taiwan, Japan, China and South Korea, it’s one of the biggest bees you’ll find in Europe, and it doesn’t live in a hive like a honeybee does (each female builds and stocks her own nest). The new paper calls it “the first non-native bee reported in Europe.”<\/p>\n\n\n\n
North Carolina got it first, in 1994, then France in 2008, then Catalonia in 2018, where it popped up at several sites. The authors think it “was likely introduced via the inadvertent transportation of nesting females in the global wood trade.”<\/p>\n\n\n\n
That’s the same hunch Constant\u00ed Stefanescu, Hern\u00e1ndez-Castellano’s colleague at CREAF, had in 2018, when he said that “this way of building nests suggests that it reached Europe through the wood trade from Asia.” Females don’t dig their own nests, they take over holes that are already there, old carpenter bee tunnels or gaps in dead wood, and pack them mostly with resin, hence the name.<\/p>\n\n\n\n
From far off it can be mistaken for the Asian hornet, and Stefanescu had a tip for that: “The giant resin bee, like all bees, has hairs that it uses to carry pollen, whereas wasps don’t have this hairiness. We can also tell them apart by their habits. This bee feeds on the pollen and nectar of flowers and makes its nests in holes, while the Asian hornet feeds on other insects, for example honeybees, and lives in colonies inside the nests they build.”<\/p>\n\n\n\n
The Japanese pagoda tree, Styphnolobium japonicum, is originally from China and gets planted all over the world as an ornamental. North of the equator it flowers in July and August, big showy bunches of white, sweet-smelling blossoms, which lands right in the middle of the bee’s season (June to September) when the females are hauling pollen home for their young.<\/p>\n\n\n\n
Barcelona has roughly 11,000 of them, according to the Spanish outlet Xataka. The authors write that “all studies published to date conducting palynological analyses across multiple regions of Europe point to S. japonicum as the main (or even exclusive) pollen source of M. sculpturalis,” with one earlier study as the odd one out.<\/p>\n\n\n\n
Back home in East Asia the bee isn’t that picky and mostly goes for kudzu, which won’t surprise anyone in the American South. Dan Suiter, a professor in the University of Georgia’s Department of Entomology on its Griffin campus, who now and then gets a resin bee mailed to him to identify, put it this way in a May 2021 university story: “We don’t see this bee very often. But we know that it’s invasive and uses kudzu as a food source.”<\/p>\n\n\n\n
So why the narrow menu in Europe? The Catalan team’s best guess is that it’s “possibly due to the limited availability of alternative resources with similar traits.”<\/p>\n\n\n\n
Catalonia covers about 12,399 square miles (32,113 square kilometers), and the authors call it one of the most invaded regions of the Iberian Peninsula<\/a>. They scraped bee sightings from GBIF, iNaturalist, a regional invasive species database, published papers, local entomologists and 74 city councils.<\/p>\n\n\n\n Finding the trees meant writing to botanists and town halls. Out of the 240 towns and cities they contacted, 173 wrote back (72 percent), and the tree was growing in 159 of them.<\/p>\n\n\n\n Then came the modeling, with location thrown in alongside climate suitability (a zero-to-one score for how much a spot’s weather resembles what the bee is used to back home) and whether a pagoda tree stood within about 1.6 miles (2.5 kilometers) of each point, a distance the team picked as a conservative guess at how far the bee goes to forage.<\/p>\n\n\n\n Climate helped the bee and so did the tree, but in the abstract the authors go further and write that “the availability of S. japonicum was at least as important as climatic suitability at shaping the distribution of M. sculpturalis.”<\/p>\n\n\n\n When they added the tree, the models got more complicated but fit a lot better, which they say ends up “reinforcing that S. japonicum is more critical than suitability in determining M. sculpturalis presence.” Their best model got 85.56 percent of records right, and both species piled up in the big cities, Barcelona most of all.<\/p>\n\n\n\n The bee most likely came over from France through the lower eastern Pyrenees, which could be why there are more of them in the east, and the tree may be what’s drawing them west.<\/p>\n\n\n\n When one invader makes life easier for another, ecologists call it an “invasional meltdown,” and the authors conclude that “the non-native tree Styphnolobium japonicum facilitates the spread of the bee Megachile sculpturalis in a non-native range.” What they don’t know yet is whether the bee actually raises more young on pagoda tree pollen, or whether the tree gets better pollination out of the deal.<\/p>\n\n\n\n There’s also the housing problem, because University of Florida entomologists<\/a> Kristen C. Stevens, Cameron J. Jack and James D. Ellis write that giant resin bees “are known to displace native carpenter bees (Xylocopa spp.) from their cavities in order to nest in them,” though “the males are unable to sting, and the females, although able to sting, are not aggressive and usually fly away from humans.”<\/p>\n\n\n\n Hern\u00e1ndez-Castellano said much the same about stings in 2018: “Of course it can sting us if it feels threatened, but solitary bees are very cautious about attacking, since the reproduction and protection of the nest depend on a single bee, and the death of the mother would be catastrophic for the offspring.”<\/p>\n\n\n\n Back then he also said that “in other systems where the giant resin bee is exotic, very few cases of direct competition with other native bees have been detected, and here we have not detected any.” Eight years later the new paper points to growing evidence that the bee may cut native bee numbers<\/a> in the bee hotels<\/a> it moves into, and to people getting upset about it “nesting in private gardens, houses, and public wooden infrastructure, leading to removal efforts.”<\/p>\n\n\n\n Over in Georgia, Becky Griffin, community and school garden coordinator at UGA’s Georgia Mountain Research and Education Center in Blairsville, was rounding up volunteers to look for it in May 2021. “They asked me to gather the troops of bug watchers to help determine where in Georgia the sculptured resin bee is located,” she said. “We just want to document where they are.” Her trick for schoolkids was that the native carpenter bee is bald and smooth, or, in her words, “Carpenter bees have a shiny hiney.” She also had advice for adults: “If you look for the big bees, take note of the abdomen and thorax and that will give you a good, educated guess. We are starting to see small sightings all over the state already, and I think the peak time will be in the next couple of weeks as we see more summer-like temperatures.” Later in the same story she added, “Right now we are on a fact-finding mission.”<\/p>\n\n\n\n The authors are upfront that “co-occurrence patterns alone provide only a limited proxy for ecological interactions,” even if the separate pollen studies back them up.<\/p>\n\n\n\n One test, whether good weather only helps the bee where the tree grows, didn’t reach significance (p = 0.095). Cities also have more people out looking, which can skew the records, and the team tried to balance that by contacting nearly every municipality.<\/p>\n\n\n\n Citing earlier models<\/a>, they note the bee has filled only 25 percent of its potential range in Europe, so it’s still early in the invasion. They only had presence records, too, and they say abundance data would help clear things up.<\/p>\n\n\n\n “Initially, S. japonicum was not considered ecologically or socially problematic,” the researchers write, but its role in the bee’s spread “calls for a reassessment.”<\/p>\n\n\n\n They don’t hedge much: “we recommend against the use of S. japonicum in urban landscapes to prevent the facilitation and establishment of M. sculpturalis.” They also predict that pulling the trees out in an experiment would make the bee disappear locally, or at least keep it from showing up, although nobody has tried it.<\/p>\n\n\n\n Hern\u00e1ndez-Castellano had made a similar point back in 2018. Insects, he said, “are practically impossible to eradicate,” and “what we can do is avoid having exotic ornamental plants in gardens and parks, such as the Japanese pagoda tree.”<\/p>\n\n\n\n Ripping out or swapping street trees<\/a> comes with social, economic and planning headaches, the authors add, and “interdisciplinary teams are needed to develop ecological recommendations to prevent and eradicate invasive species in urban ecosystems.”<\/p>\n\n\n\nThe tree counted at least as much as the weather<\/h2>\n\n\n\n
An invasional meltdown between two newcomers<\/h2>\n\n\n\n
What the study cannot show yet<\/h2>\n\n\n\n
What cities should think about planting<\/h2>\n\n\n\n