Across the planet, honeybee health has been declining, and mysteriously so. But now, researchers studying a global, tiny, single-celled parasite have discovered that, contrary to conclusions drawn from previous studies, honeybee larvae are being infected. When they mature into honeybee adults, they end up living shorter lives. The findings, published in PLoS ONE this week, could help us to better understand the sad decline of this important pollinator.
In the last decade, beekeepers in the US and parts of Europe have suffered a colony loss of about 30 percent every winter. The cause for these collapses isn’t entirely understood, though multiple factors have been identified; mite infestations, pesticides, pathogens, or some interaction between them are thought to have a role. One of these pathogens is the globally distributed microsporidian called Nosema ceranae. This fungal parasite is transmitted by spores produced in midgut tissue, and it’s known to infect adult Asian and European honeybee species. There’s some evidence that the pathogen lowers honeybee health by degenerating digestive tissue and impairing flight behavior, which leads to malnutrition and reduced food intake for the colony, respectively. Based on indirect evidence, Nosema ceranae was believed to infect only adult honeybees. Not so.
A team led by UC San Diego’s Daren Eiri raised Apis mellifera honeybee larvae in the lab and infected them with Nosema ceranae spores obtained from infected bees in Chonburi, Thailand. The team gave the larvae a single dose of between zero (control) and 40,000 spores just once in their brood food three days after they hatched from their eggs.
When the bees entered their pre-pupa stage of life at eight days old, the team dissected some of their midguts. Spores had developed intracellularly in the midgut cells of 41 percent of the pre-pupae who were exposed as larvae. And compared to controls, exposed bees had significantly elevated spore counts. Pictured below, you can see the developing spores (labeled sp) at relative magnifications from 400x to 1400x (the label lu indicates the lumen, or the interior of the midgut):
A separate group of treated bees grew into adulthood, which takes about 19 days. Not only did they show an elevated spore count; their longevity was also decreased. A 40,000-spore treatment decreased the age at which adult bees died by 28 percent. Surprisingly, a lower dose of 10,000 spores resulted in even greater infection in adults. The team thinks that the higher dose may have triggered a stronger larval immune response. This reduced the number of spores they had as adults, but it also further reduced their lifespans.
But to determine the real impact of larval infection on colony health, larval infection will need to be examined in the field. “No study had directly investigated whether larvae could become infected with Nosema ceranae,” Eiri says in a news release. “Our study provides a direction to continue investigating this question outside the lab and in the field using entire colonies.”
Images: shutterstock.com (top), 2015 Eiri et al. / PLoS ONE (middle)
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