Bees, the earth's most important pollinators, took another devastating hit last year, despite increased efforts to reverse the decline.

U.S beekeepers lost 44 percent of their total colonies from April 2015 to March 2016, an increase of 3.5 percentage points over the previous year, according to the findings of an annual survey released Tuesday.

Colony loss during winter jumped to 28.1 percent, from 22.3 percent a year earlier. In addition, beekeepers experienced a second straight year in which summer loss rates rivaled those of the colder months, which typically are more extreme.

The summer losses, in particular, are "cause for serious concern," according to Dennis vanEngelsdorp , an assistant professor of entomology at the University of Maryland and project director for the Bee Informed Partnership , which conducted the study with funding from the U.S. Department of Agriculture.

"Some winter losses are normal and expected ," vanEngelsdorp said in a statement. But 59 percent of responding beekeepers reported winter colony losses, far exceeding the 17 percent rate considered acceptable.

"The fact that beekeepers are losing bees in the summer, when bees should be at their healthiest, is quite alarming," vanEngelsdorp said.