Biodiversity lowers pathogen levels in bees

A study in Michigan finds that bee communities with higher diversity had the lowest levels of three viral pathogens

A study by researchers at the University of Michigan has shown that the most diverse bee communities have the lowest levels of three common viral pathogens.

The researchers netted and trapped more than 4,000 bees representing 60 species and they were collected at winter squash farms across Michigan. All but one species in the research, the European honeybee, were native bees.

The most abundant bees were the European honeybee (Apis mellifera), the eastern bumblebee (Bombus impatiens), the squash bee (Eucera pruinose) and several species of sweat bee of the genus Lasioglossum.

When they were tested for deformed wing virus, black queen cell virus, and sacbrood virus, the researchers found that lower viral presence was linked to greater biodiversity in the local bee community.

“This result is exciting because it suggests that promoting diverse bee communities may be a win-win strategy to simultaneously reduce viral infections in managed honeybee colonies while helping to maintain native bee biodiversity,” said lead author Michelle Fearon, postdoctoral fellow in the department of ecology and evolutionary biology.

“We thought that the diversity of native bee communities would be correlated with fewer viral infections for honeybees, but we did not expect to see the same pattern for other native bees as well.”

They predicted that pollinator biodiversity would either increase or have no effect on infections in native bees.

“It was a happy surprise to find a consistent pattern that pollinator biodiversity helps to keep multiple types of viral infections low in honeybees and several native bees,” she said.

Fearon and her colleagues collected 4,349 bees at 14 Michigan winter squash farms over two summers. The winter squashes included acorn, butternut, and spaghetti squashes and pumpkins.

Honeybees were present at all the sites and a diversity of native bees were not only in the squash fields but along field edges. Native pollinators were actually more common visitors to the squash flowers than honeybees at many of the locations. Squash flowers are very large and provide good pollen and nectar resources, which are attractive to the native bees.

“Squash bees were one of the most common visitors that I saw on the squash flowers,” she said. “These are a solitary native bee species that specializes on squash plants. In fact, its entire life cycle is tied to the plants because they nest in the soil below the plants and males often sleep inside the closed flowers. These bees are very effective pollinators for squash plants because they only visit other squash flowers, ensuring that all the pollen deposited comes from the same species.”

Honeybee visitation to the squash flowers was more sporadic.

“At one site, the farmer had honeybee colonies placed next to the field to aid with pollination, but I found that almost none of the honeybees were visiting the squash flowers. Instead, squash bees were doing the majority of the pollination in those fields. Native bees that specialize on a certain type of crop can be more effective pollinators compared to generalist honeybees that visit a wide variety of plants.”

The study is the first to show that high levels of diversity in bee communities can help to dilute the harmful effects of pathogens. This ability to diminish the pathogens’ impact is known as the dilution effect and it is the first time it has been demonstrated with pollinator viruses.

But the concept of the dilution effect is met with some opposition because some ecologists maintain that biodiversity does not always lead to reduced impact of pathogens, Fearon said.

“There are many examples in the literature where host biodiversity has no effect on the spread of pathogens and others that show the opposite pattern called the amplification effect where greater biodiversity tends to increase pathogen prevalence.”

She said that they did not find any evidence for the amplification effect among any of the four host species or three viruses included in the study. But how or to what extent viral infections spread among bee species may be as much about exposure as evolution.

“I think that species that are good or bad hosts for spreading viruses may depend on both evolutionary relatedness to honeybees and whether bees are solitary or that live in large groups,” she said. “Viruses infecting honeybees may be more likely to spill over into bumblebees because they are closely related but less likely to be transmitted to more distantly related native bee species.”

Bee species with large colonies and close contacts have lots of potential for transmission compared to solitary bees nesting alone. But she stressed that more research is needed to better understand what characteristics decide the “good versus bad” bee hosts for viruses.

Fearon has two key recommendations for beekeepers and landowners.

“First, I recommend planting large patches of diverse, native wildflowers,” she said. “Hedgerows and cover plantings with diverse flowers have been shown to attract more native bee species, which increases pollination of both native plants and crops and benefits bee health. Wildflower patches also help to keep pest populations down by promoting beneficial predatory insects.

“Second, I recommend reducing the use of insecticides and fungicides that can have toxic effects or kill bees. When pesticide use is required, spray in the late afternoon or evening when pollinators are less active.”

Many of the squash farmers were pleased with the level of pollination and crop yields but most underestimated the diversity of pollinators that visited their fields. The results of the study have encouraged them to not only learn more about the bee species on their farms but how they can further increase pollinator diversity.

Fearon has a follow-up study underway to explore how natural areas and the quality of bee habitat keep pollinator communities healthy.

The study was published in the journal Ecology.

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