Honeybee Decline: Tiny Parasites May be to Blame by Infecting Larvae

Honeybee decline worldwide continues to baffle scientists, and while invasive parasites have been blamed before, new research shows that a tiny single-celled parasite may have a greater-than expected impact on colonies by infecting larvae.

In the journal PLOS ONE, scientists report that a microsporidian called Nosema ceranae, which has been known to infect adult Asiatic and European honeybees, can also infect honeybee larvae. They also discovered that honeybee larvae infected with the microsporidian have reduced lifespans as adults.

Since 2006, beekeepers in North America and Europe have lost about one-third of their bee colonies each year due to "colony collapse disorder" (CCD). More recently, a study showed that honeybee numbers have dropped more than 40 percent during the year spanning April 2014 to April 2015.

While the exact cause is unknown, scientists have speculated that pesticides, pathogens, mites and certain beekeeping practices have all contributed to this decline. Nosema ceranae, a kind of fungal pathogen spread by spores, is also implicated in CCD because it reduces colony health and is widespread.

"Previous research suggested that Nosema ceranae could not infect honey bee larvae," James Nieh, a professor of biology at the University of California, San Diego, who headed the research effort, said in a statement. "But this was largely based upon indirect evidence: spore counts in newly emerged adult bees, which typically have low spore counts."

Nieh and his colleagues reached their conclusion after conducting experiments with larvae exposed to spores and reared in the lab. However, they point out that the extent of larval infection needs to be studied further using field bee colonies to determine the true impact of larval infection on colony health. Still, their findings are promising, especially considering that another recent study detected low levels of Nosema DNA in honeybee larvae, suggesting that larval infection can occur in field colonies.

"However, no study had directly investigated whether larvae could become infected with Nosema ceranae," said graduate student Daren Eiri, the study's first author. "Our study provides a direction to continue investigating this question outside the lab and in the field using entire colonies."

In addition, it may also help to solve a longstanding mystery.

"One puzzling aspect of Nosema ceranae infection is that infection in adult bees usually decreases after medication is given by beekeepers to a colony, but can later resurge," Nieh said. "Some of this resurgent infection could be due to transmission between bee colonies or to adult bees that have a low, but resistant level of infection."

"However," he added, "our study raises the possibility that brood are also infected. If so, this typically would not be detected for weeks until the emergence of adult bees. Generally, older adult bees are more heavily infected with Nosema. Thus, bees infected as brood may not develop high Nosema spore counts until they are much older adults, further delaying detection."

With this and future research, hopefully scientists may soon be able to figure out what is behind the mysterious declines in honeybee populations worldwide over the past decade, and how to save these precious pollinators.

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