Metabarcoding reveals global variation in pollen collecting behavior of the invasive bee Megachile sculpturalis (Hymenoptera: Megachilidae)

In the quiet corners of forests and the bustling centers of cities, a silent exchange takes place every spring and summer. Bees visit flowers, gathering pollen to feed their young, while inadvertently carrying the genetic material of one plant to another. This relationship is the engine of much of the world's food production and wild biodiversity. However, when a species moves into a new land where it does not belong, this delicate balance can be disrupted. Invasive species are organisms that establish themselves in foreign environments, often outcompeting native life and altering how ecosystems function. For bees, their success in a new home depends heavily on what they can eat. Some invaders are picky, sticking only to the specific flowers they knew in their homeland. Others are opportunists, willing to eat almost anything they find. Understanding which strategy an invasive bee uses is crucial, because it tells us whether the insect will struggle to survive or thrive and spread, potentially reshaping the local landscape.

A team of researchers set out to uncover the dietary habits of a specific intruder: the giant resin bee, a solitary insect native to East Asia that has recently appeared in Europe and North America. To do this, they turned to a modern technique called DNA metabarcoding. Instead of trying to identify pollen grains under a microscope—a difficult task when grains are mixed together and look similar—the scientists extracted the genetic material from the pollen collected by the bees. By reading the unique DNA sequences, they could identify exactly which plant species the bees had visited. They gathered over 250 samples from bees and their nests across nine European countries, four American states, and a small number from the bee's native range in China. This global survey allowed them to see not just what the bees ate, but how their eating habits changed depending on where they were living.

The results revealed a story of two very different diets. In the bee's native range in East Asia, the insect behaved as a generalist, visiting a wide variety of plants from many different families. This flexibility is typical for successful invaders, allowing them to adapt to new environments. However, the picture changed dramatically in Europe. There, the giant resin bee displayed a striking preference for a single type of tree: the Japanese pagoda tree. In the European samples, this one tree species dominated the diet, appearing in the vast majority of the pollen collected. In some locations, nearly every sample contained nothing but pollen from this tree. The researchers found that this tree is widely planted in European cities, parks, and even along highways as an ornamental feature. It blooms late in the season, providing a reliable food source when other flowers have faded. The data suggests that the widespread planting of this non-native tree may have unintentionally helped the invasive bee establish a foothold in Europe, creating a situation where the bee and the tree support each other's presence in a new land.

In contrast, the bees in North America showed a different pattern. While they also visited a variety of plants, they did not rely on the Japanese pagoda tree, which was not detected in their pollen samples. Instead, the North American bees fed on a mix of native and non-native plants, including some that are themselves considered invasive weeds. The study identified specific interactions where the bee visited plants like purple loosestrife in the United States and kudzu (Pueraria montana) in both the United States and southern Switzerland. These are plants that spread aggressively and can harm local ecosystems. The researchers noted that when an invasive pollinator visits an invasive plant, it can create a feedback loop that helps both species spread faster, a phenomenon known as invasional meltdown. While the study did not prove that the bee is the sole cause of these plants' spread, the evidence strongly suggests that the bee is facilitating their success by moving their pollen efficiently.

The researchers also looked at how consistent the bees' diets were within their own nests. In Europe, the consistency was extreme. Bees living in the same city, or even the same neighborhood, brought back almost exclusively the same type of pollen from the Japanese pagoda tree. This was true even when the bees were nesting in different settings, such as private gardens or public parks, and even when other trees were available nearby. In the United States, the diet was more varied from nest to nest, reflecting the different plants available in those specific locations. The study also highlighted that the bees in Europe were feeding on a high proportion of plants that originated in East Asia, the same region where the bees come from. This supports the idea that the bee is sticking to plants it "knows," even when those plants are non-native to Europe.

Ultimately, the paper paints a complex picture of invasion. The giant resin bee is not a simple specialist that only eats one thing, nor is it a completely random eater. It is an opportunistic forager that adapts its behavior to the resources available. In Europe, it has found a massive, reliable food source in the form of a widely planted ornamental tree, which has likely helped it spread rapidly. In North America, it is utilizing a broader range of resources, including other invasive plants. The findings suggest that human decisions about which trees to plant in our cities have real consequences for wildlife. By planting large numbers of non-native ornamental species, we may be creating ideal conditions for invasive insects to thrive. The researchers conclude that managing urban green spaces with a focus on diverse native plants, rather than mass plantings of exotics, could help prevent the spread of invasive pollinators and protect the native bees and plants that are already there. This work provides the first global look at what this invasive bee actually eats, turning a vague concern about an intruder into a clear understanding of its relationship with the plants around it.