In a new review article published in February in Environmental Entomology, Grace Gutierrez and Margarita Margarita López-Uribe, Ph.D., of Penn State University and colleagues at Cornell University and the University of Virginia investigated the ecology of the mason bee Osmia taurus (shown here) in the U.S. The species’ home range is in eastern Asia, and it was first discovered in the U.S. in 2002. Gutierrez and colleagues examined if O. taurus qualifies as an invasive species and compared its expansion in North America with that of another non-native mason bee, Osmia cornifrons, which was purposefully introduced in the U.S. in 1978 to facilitate pollination in orchards. (Photo by T’ai Roulston, Ph.D.)

By John P. Roche, Ph.D.

The mason bee Osmia taurus, a native of eastern Asia, was first discovered in the United States in 2002 in Maryland and West Virginia. Once here, its population increased rapidly, and it is now found from Florida to New Hampshire in the eastern U.S. The closely related non-native bee Osmia cornifrons was brought to the U.S. in 1978 to increase pollination in fruit orchards. But unlike Osmia taurus, populations of Osmia cornifrons remained stable and did not rapidly increase.

In a new review article published in February in Environmental Entomology, Grace Gutierrez, Margarita M. López-Uribe, Ph.D., and colleagues conducted a review of the current status of Osmia taurus in the U.S., compared its colonization to that of Osmia cornifrons, and examined if Osmia taurus can be considered invasive.

Mason bees pollinate a range of plants, including fruit trees in orchards. They lay their eggs in small cavities such as hollow stems or holes in wood left by other insects. Unlike the non-native European honey bee (Apis mellifera), which has colonies with a single fertile queen, mason bees are solitary bees and all females are fertile. Approximately 140 species of mason bees are present in the U.S.

Gutierrez, a master’s student in entomology in López-Uribe’s lab at Penn State University, and colleagues at Cornell University and the University of Virginia examined the breadth of previous research on O. taurus to investigate if the species could be considered invasive in the U.S.

“While we generally think about bees as species that have positive impacts on their environment because of their role as pollinators,” Gutierrez says, “in this case, the rapid spread and increase in relative abundance for Osmia taurus may have negative ecological effects, and it could potentially become invasive.”

What makes a non-native species “invasive”? And can a typically beneficial insect like a bee be deemed a threat to native species? Researchers explore these questions in a new review of the expansion of the non-native mason bee Osmia taurus since its U.S. arrival in 2002 and its effects on closely related native species. (Photo by Margarita López-Uribe, Ph.D.)

What makes a non-native species “invasive”? And can a typically beneficial insect like a bee be deemed a threat to native species? The non-native bee Osmia cornifrons (shown here) was brought to the U.S. in 1978 to increase pollination in fruit orchards. But, unlike Osmia taurus, populations of Osmia cornifrons remained stable and did not rapidly increase. (Photo by David Biddinger, Ph.D.)

The mites on this Osmia cornifrons mason bee are in the genus Chaetodactylus, and they attack pollen provisions of native and non-native Osmia species. (Photo by Margarita López-Uribe, Ph.D.)

An invasive species is one that both is non-native and causes harm to the environment, the economy, or human health. Osmia taurus is definitely non-native, one of 42 non-native bees identified in the U.S. so far. But does it cause harm?

Non-native bees could cause harm by competing for food or nesting resources with native bees, by helping the success of non-native species such as weeds, or by spreading pathogens or parasites that harm native bee species.

The native bee in the eastern U.S. that is the closest ecologically to O. taurus is Osmia lignaria, often known as the blue orchard bee (and its eastern subspecies Osmia lignaria lignaria). Osmia lignaria and Osmia taurus emerge at similar times of the year, use similar nesting cavities, and gather pollen from similar plants. But does O. taurus negatively affect O. lignaria populations? The blue orchard bee has undergone a steep decline in the U.S., but this decline began before O. taurus arrived. It is possible that O. taurus‘s population growth has been enabled through utilizing the ecological niche that opened up when the blue orchard bee declined.

Meanwhile, O. taurus can carry non-native pathogens. Studies have observed that O. taurus can be targeted by parasitic wasps, by Ascosphaera fungi, and by Wolbachia bacteria. If these pathogens spread to and harm native bees, that would classify O. taurus as an invasive species. Fungal pathogens have been found to be more abundant in areas where non-native species of Osmia such as O. taurus and O. cornifrons are found. But whether the fungal pathogens are spread by non-native Osmia species is still unknown. Additional research shows that some parasites, such as nosematid parasites and trypanosomatid parasitic protozoa, may not have negative effects on mason bee species.

Osmia taurus may also pollinate non-native plants, which could adversely affect native plants and native bees. But little information is available as to whether this occurs, and this topic is in need of more research.

Traits that can contribute to a species’ invasiveness include plasticity in life cycle, a broad tolerance to different temperatures, and being a generalist in terms of habitat requirements. In Japan, O. taurus is found in a wider range than O. cornifrons, which may be because O. taurus can live in a broader range of environmental conditions. Therefore, O. taurus has traits that could help it become invasive, but the traits don’t establish that it is invasive.

For those researching mason bees, López-Uribe says, “One of the main challenges is that Osmia taurus looks very similar to Osmia cornifrons. The two species are difficult to differentiate in the field. We recommend that Osmia cornifrons breeders should check at least a portion of their female cocoons (usually found at the back of the nest) to see if they are Osmia cornifrons or Osmia taurus before they ship them to different parts of the country.“

As for whether O. taurus should be considered invasive, after examining all the available data, López-Uribe says, “We conclude that Osmia taurus has invasiveness potential, but it is still unclear if it can be called an invasive species. We hope our review paper captures the attention of other entomologists to work on generating more data to investigate these questions.”

A great deal remains to be learned about Osmia taurus. What range of plants does it use for pollen? What are its habitat needs? What range of temperatures does it tolerate? Also, much more needs to be learned about parasites and pathogens of the species. And do different non-native bees form a mutualism in which they prefer non-native plants, benefiting the non-native bees and, by their negative effect on native plants, harming native bees? Future research will be able to fill in these gaps and provide a more complete picture of Osmia taurus and its ecological impacts on natural communities in North America.