Extreme weather linked to climate change poses a greater threat than insecticides to wild bee populations that are essential for Michigan blueberry growers, according to a recent study.

Wild bees, which pollinate blueberries more effectively than bees and bumblebees, are subject to two main stressors that cause tension or strain: extreme weather events such as droughts, storms and early frosts, as well as chemicals from insecticides .

Blueberries are big business in the state, growing about 100 million pounds each year, according to Michigan State University Extension. SUBWAY

The results of the new research highlight the importance of providing suitable habitat for wild bees, according to Julianna Wilson, study co-author and interim specialist in tree fruit entomology at MSU Extension… about 100 million pounds of blueberries each year.

The study involved three rounds of wild bee trapping over a 15-year period on 15 highbush blueberry farms in Ottawa, Allegan and Van Buren counties. The researchers found 35% of the 162 known wild bee species in Michigan.

Locations of 15 blueberry farms in southwestern Michigan in the study. Image: "Agriculture, ecosystems and the environment"

Those counties, as well as Berrien and Muskegon, make up the state's top blueberry-growing region, according to the Michigan Ag Council.

They found that the abundance of bees, the number captured in traps, decreased by 61% and the species richness, the number of species present, decreased by 33% between the first sampling period (2004-06) and the second (2013-14).

Why the dramatic decline?

The study cited “extreme” weather in the spring of 2012, just prior to the second testing period, when the Great Lakes region “experienced record temperatures in March, leading to early spring flowering of trees and shrubs and the early appearance of bees. This was followed by severe frosts in April, which led to widespread flower loss and consequent reduced yields of fruit crops."

There was a “limited recovery in abundance and species richness” in the third sampling period (2017-18). During that time, "some species showed dramatic species declines," while others showed "stable or increasing prevalence," according to the study in the journal Agriculture, Ecosystems and Environment.

The authors are from the US Department of Agriculture, MSU, the University of Manitoba, and the Xerces Society for Invertebrate Conservation.

Some bee species are more vulnerable than others to such situations, according to the study, and "extremes of hot or cold climatic conditions are likely to affect species survival, especially those at the northern limits of their distribution." .

Rising spring temperatures push plants out of winter dormancy earlier, he said, increasing the risk that flowers will suffer frost damage and bees that depend on those flowers are at higher risk.

Regarding pesticides, the study said: "In agricultural systems, the higher prevalence of invasive pest species may make crop management more dependent on insecticides (which are) harmful to beneficial insects, including pollinators" such as the bees.

Shelly Hartmann, co-owner of True Blue Farms in Grand Junction and a member of the Michigan Blueberry Commission, said: “For many years, Michigan blueberry growers have supported a wide diversity of bees.

"Producers have installed pollinator plantations to support wild bees, avoid spraying during the day when bees are active, are implementing integrated pest management and select the least toxic option, when possible, to control pests", said Hartmann, who also chairs the California-based Highbush Blueberry Council.

The study found that contrary to what many people thought, pest management problems are not the main challenge facing those wild bees.

Wilson said: "The bees we were talking about are active bees in the spring, and the pest management program is later in the summer when those bees are no more."

Hartmann said: “Growers are balancing multiple goals and are focused on delivering a safe and healthy nutritious product to the market, while also supporting the land they grow on.

"Producers work hard to protect and improve their environment and well-being whenever we have the opportunity," he said.

However, it is not all doom and gloom.

Wild bees, Wilson said, have evolved “to take advantage of many opportunities and niches in the environment.

And if producers know that wild bees are more productive pollinators than honey bees and bumblebees, they have an incentive to preserve and provide habitat for the bees, he said.

The study said that a recovery in the richness of bee species suggests that populations "are relatively robust to these types of disturbances," referring to extreme weather events.

He called for constant monitoring of wild bees at the species level to determine the status of populations and identify the species most at risk.

Hartmann said: "In closing, bees are our friends."