According to The Xerces Society for Invertebrate Conservation, in 2013 the largest number of native bee casualties ever recorded occurred at a Target parking lot at Argyle Square in Wilsonville.
The event, where over 50,000 bumblebees died, and other similar episodes in the Portland area, prompted policy changes at the local and state level: the Oregon State Legislature and Department of Agriculture banned the use of certain pesticides from being used on linden trees (as in the Target lot) and created labeling requirements for such pesticides while the City of Wilsonville implemented programs to introduce pollinator habitats in parts of the city and change its pest management practices.
Recently, the event helped inspire a new study that suggests that pesticides are not the only cause of mass bee deaths.
The study, which was conducted by former Oregon State University professor Sujaya Rao on linden trees in western Oregon and published in PLOS ONE — a peer-reviewed science journal — found that the convergence of factors such as temperatures lower than 86 degrees, it being late in the blooming season, a low amount of available nectar and bumblebees' attachment to resource-depleted linden trees can lead to mass deaths. The researchers determined this hypothesis by finding higher numbers of bees crawling on the ground because they lost the energy to fly in the morning when temperatures were cooler and watching bees continue to forage for nectar on Linden trees with a low amount of nectar.
"Late blooming linden trees with low nectar volume may not provide the caloric requirements for both flight and thermoregulation (the body's ability to regulate temperature)," the study reads. "Bees that continue to forage despite cool temperatures and low nectar volume in flowers are thus at risk of starvation."
After a different study in 2013, the ODA determined that the Wilsonville event was caused by the application of the neonicotinoid dinotefuran. And Andony Melathopoulos, an assistant professor of Pollinator Health Extension in the Department of Horticulture at Oregon State University, said that the ODA's verdict is still accurate.
"The levels of the pesticides of the trees was very high," he said. "This other effect (found in the study) may have been taking place but the magnitude of bees dying (at the Wilsonville event) is not what we're seeing in terms of bees dying naturally."
However, he said the new study shows that other mass bee casualties thought to be caused by pesticides may have actually been caused by environmental conditions and that the study is valuable for researchers, investigators hoping to determine the cause of future bee deaths and, potentially, policymakers.
"It's clear that we have a lot of cases where there are dead bumblebees that are under trees that there is no connection to pesticide use," Melathopoulos said.
One of the key findings of the study, according to Melathopoulos, is the verification that bumblebees sometimes continue to attempt to collect nectar from linden trees that have already exhausted all of their nectar. And this quirk leads them to expend energy fruitlessly, drop to the ground and die. The study posits that the alkaloid trigonelline, which the study found in Linden nectar, and other alkaloids cause the bees to do this because they hinder memory and learning in insects.
"If trigonelline affects bee learning and memory as has been demonstrated for caffeine and nicotine, skewed resource loyalty could be a factor in the linden-bee mortality phenomenon," the study reads. "Bumble bees that experience a high nectar reward during early bloom may develop a strong loyalty to it and continue to return throughout bloom, even when nectar volume is low."
Melathopoulus hopes this phenomenon and its cause are explored further.
"People had seen similar things where bumblebees were on plants that were super attractive and then were falling on the ground. No one had understood the mechanism until now," Melathopoulus said. "This is a great study. It really puts the pieces together for us."
Melathopoulus wasn't exactly sure what cities and other entities can do to prevent bee deaths in light of the study but said more planted areas that have plenty of pollen and nectar can help.
"The big message for people is to focus on creating really rich garden habitats for pollinator habitats and that will help a lot," he said.
Wilsonville Natural Resources Manager Kerry Rappold, for his part, wasn't sure what specific policies could be associated with the findings but said more research into the impact of alkaloids could be helpful.
Melathapoulus said the study shouldn't dissuade cities and other entities from planting linden trees but that they should not do so in places like a parking lot or streetscapes, where the honeydew emanating from aphids in the trees can fall onto cars, which is what led to the application of dinotefuran in Wilsonville. And Rappold said the City has a similar philosophy about linden trees.
"The benefits of those trees (nectar and pollen contents) outweighs the small amount of (potential) mortality," Melathapoulus said.
Rappold is proud of the programs he helped institute in Wilsonville after the 2013 catastrophe. Through the integrated pest management plan, they adopted Oregon State University's list of which pesticides to use and avoid. And along with the pollinator habitat program, they recently started a backyard habitat program where experts help Wilsonville residents form a more productive and healthy habitat. He is also glad that groundbreaking research precipitated from the event.
"I think it's been a great catalyst for raising awareness, for stimulating this type of research hopefully for policy changes that can benefit pollinators," he said. "Locally out of that horrible situation there was a lot of good that came out of it. Hopefully that momentum carries forward."
Melathopoulos echoed that point.
"Oregon has some of the best programming around pollinators and managing trees than anywhere in the US," he said. "That incident catalyzed us to take pollinator protection seriously and this research project fits into that."
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