Bees Prefer Open Canopy Forests

While the decline in honey bees (genus Apis) in the United States is widely known, prompting the government to put forth strategies to protect the species, the status of other common bees, particularly those found in forests, is less well-known — until recently.

“Nobody knows the bees’ population trends,” said James Hanula, a research entomologist with the U.S. Forest Service, member of The Wildlife Society and lead author of a study published in the journal, Forest Ecology and Management. The study examines how changes in forest structure from open canopies to closed canopies could be contributing to the decline in bees and other pollinators in the Southeast.

“Most solitary bees haven’t been studied long enough to know whether they’ve been in decline or not, but in general, there’s a perception that they’re declining” Hanula said, adding that honey bees aren’t typically found in forests in large numbers so native pollinators are important.

According to Hanula, forests have changed dramatically since the early 1900s when there was very little forest cover. Since then, due to effort to reforest and exclude forest fires, forests have become much more closed with more intensive forestry and less thinning. “Bees would have done well in the forests of the early 1900s,” he said. “But now, forests are mostly denser with heavy shrub cover conditions that wouldn’t be good for bees.”

As part of their study, Hanula and the other U.S. Forest Service researchers captured 128 species of bees, including three common bumblebee species, in seven types of forest in the Oconee National Forest in Georgia. They studied bees in areas with dense young pines, thinned young pines, mature open pine with extensive shrub and sapling cover, mature open pine with extensive herbaceous plant cover, mature upland hardwood forest, mature riparian hardwood forest, and recently cleared forests.

The scientists then sampled bees using pan traps, which are used to catch insects attracted to the color, and measured the tree density of the overstory, the understory herbaceous plant and shrub diversity and cover, light penetration and leaf area index. They then set out to determine in which areas the number of bees and the number of bee species was the highest.

After reviewing their results, they found that there are more bees and species of bees in cleared forest and in mature pine stands with an herbaceous plant understory. The worst habitat for the bees seemed to be dense young pine that hadn’t been thinned yet, Hanula said. “It probably has to do with bees requiring more flowers and sunny forests for nesting habitat,” he said. “Ground nesting bees like to have bare ground to nest in and in sunny areas.”

But there are ways to provide bees and other pollinators with a more suitable habitat. Hanula said other studies have shown that reducing invasive shrubs such as Chinese privet (Ligustrum sinense) has a tremendous benefit for pollinators.

He also said thinning and burning pine stands, even at an early age, will benefit pollinators — and probably other wildlife as well. In another recent study, researchers found forest fire prevention also has a negative impact on the eastern diamondback rattlesnake (Crotalus adamanteus). Further, Hanula and his team found that the best long-term bee habitat was habitat that was being managed for red-cockaded woodpeckers (Leuconotopicus borealis).

Role of Parasites in Honey Bee Declines

Meanwhile, researchers continue to examine potential reasons for the ongoing decline in honeybee populations.

In a recent study published in PLOS ONE, biologists at UC San Diego uncovered another clue in the “colony collapse disorder” mystery that’s exact cause is unknown.

The researchers found that a parasite, or microsporidian, called Nosem ceranae, which infects adult Asiatic and European honeybees, can also infect honeybee larvae. Further, they found that once the larvae are infected with the microsporidian, they’re more likely to have a reduced lifespan as an adult.

“Previous research suggested that Nosema ceranae could not infect honey bee larvae,” said James Nieh, a professor of biology at UC San Diego who worked with graduate student Daren Eiri, the first author of the study, in a press release.

Researchers speculate that pesticides, pathogens, mites and some beekeeping pathogens have contributed to honeybees’ decline. The researchers suggest that Nosema cerenae, which is a fungal pathogen spread by spores, is a factor in colony collapse because of reduction of colony health that it causes.

And while a previous study found low levels of Nosema DNA in honeybee larvae, it was not yet determined whether or not the larvae could be infected.

“We hope that our study will spur further research into how Nosema ceranae is transmitted and into the potential infection of larvae in natural and managed honey bee colonies in the field,” said Nieh.

Header Image: Bumble bees such as Bombus auricomas, pictured above, appear to prefer open canopy forests over closed canopy forests, according to a recent study.
Image Credit: USGS