Researchers have found that a combination of factors is responsible for bumblebee deaths.

CORVALLIS — Pesticides may kill bees, but not all bee deaths are caused by pesticides.

That is the conclusion of a new study published July 10 by researchers at Oregon State University investigating bumblebee mortality beneath flowering linden trees.

The issue was thrust into the national spotlight in 2013, when 50,000 bees died at a Target parking lot in Wilsonville, after landscapers sprayed 55 trees with the pesticide dinotefuran to control aphids. Dinotefuran is one of a class of pesticides known as neonicotinoids that are highly toxic to bees.

“It was really dramatic,” said Sujaya Rao, a professor of entomology and one of the study’s lead researchers. “They were literally falling dead in front of people’s eyes as they came to shop.”

While that incident — the largest single loss of native bees ever recorded — was undoubtedly due to the pesticide, Rao said it prompted OSU researchers to take a closer look at bumblebee health.

Rao has spent 17 years at OSU working on native bees in agricultural crops. She left in 2017 to become the department head for entomology at the University of Minnesota.

According to the study, the phenomenon of bee deaths around linden trees dates back to the 1970s in Europe and North America. Neonicotinoids were not introduced until the 1990s and are considered to be safer for farmworkers.

Rao said people are quick to blame pesticides for every bee death, but the study determined a rare combination of factors is also likely to blame. These include low temperatures, nectar volume and “tree loyalty” among certain bees that ultimately leads to starvation.

Linden trees are a popular choice for planting in cities and urban areas. Rao said they flower profusely, and provide lots of pollen and nectar for bees.

The problem is that some bees are drawn to the same trees over and over again, ignoring other flowering plants nearby and failing to get enough nutrition.

“When you have something that attractive to bees, lots of bees come to it and everybody is foraging,” Rao said. “It’s like a whole bunch of us trying to drink punch from a bowl. There won’t be enough to go around.”

That is especially troublesome during cool Northwest mornings, when temperatures drop below 86 degrees Fahrenheit. When ambient temperature is low, bees need more energy in their thorax to fly. If they are already experiencing an energy deficit, they might simply fall to the ground.

“They keep trying to fly, and they’re just not able to do that,” Rao said. “When they cannot fly, they cannot get to food. They cannot get to the nest, so they just die. It’s very sad.”

Not every linden tree causes bee mortality, and not every bee that forages in lindens dies, Rao said. Lindens are still great trees, she added, and cities should not stop planting them.

But under the right conditions, Rao said it is possible for some bees to die around lindens, even in the absence of pesticides.

“It does happen in western Oregon year after year after year,” she said. “You cannot control the temperature.”

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