Bacterial imbalances can mean bad news for honeybees

A sister species of the Varroa destructor mite is threatening pollinators already hard pressed by pesticides, nutritional deficiencies and disease.

A team of U.S. Department of Agriculture scientists and their collaborators have established a strong link between honeybee health and the effects of diet on bacteria that live in the guts of these important insect pollinators.

In a study published in the November issue of Molecular Ecology, the team fed caged honeybees one of four diets: fresh pollen, aged pollen, fresh supplements and aged supplements. After seven days, the team euthanized and dissected the bees and used next-generation sequencing methods to identify the bacteria communities that had colonized the bees' digestive tract.

The team also compared the thorax (flight muscle) weight and size of each group's hypopharyngeal glands as measures of the diets' effects on bee growth and development. The glands enable nurse bees to produce "royal jelly," a substance that's fed to developing larvae, ensuring the hive's continued survival. The flight muscle weight represents the potential for work after the nurse bee transitions into the role of forager.

In general, bees given fresh pollen or fresh supplements fared better than bees given pollen or supplements that had first been aged for 21 days, said Kirk Anderson, senior author and a microbial ecologist with USDA's Agricultural Research Service (ARS) in Tucson, Ariz.

Bees fed fresh diets suffered fewer deaths, made better use of energy for growth and had lower levels of gut pathogens such as Nosema ceranae, according to Anderson and co-authors University of Arizona graduate student Patrick Maes, ARS lab technician Brendon Mott and Randy Oliver of Scientificbeekeeping.com.

In the study, the nutritional value of pollen lasted longer than that of supplement. Bees consumed significantly more aged supplement than aged pollen, but this didn't translate into long-term benefits. For example, bees consuming aged supplement had plump nurse glands but suffered significant losses in flight muscle, suggesting that nutrition diverted to feed developing larva came at a significant cost to the bees' own adult development. Poor development, in turn, can translate to early mortality or inefficient food collection when these nurse bees transition to the role of foragers.

Anderson said the effects of diet on gut bacteria populations (or "gut microbiome") are poorly understood but warrant study because of the implications for honeybee health and the insect's importance as a chief pollinator of 100-plus flowering crops.

Anderson said with continued research, new supplement formulations or usage practices could be created to improve not only the health of honeybees but also the bacteria that live within them.

Varroa mites

A sister species of the Varroa destructor mite is developing the ability to parasitize European honeybees, threatening pollinators already hard pressed by pesticides, nutritional deficiencies and disease, a Purdue University study has found.

Researchers found that some populations of Varroa jacobsoni mites are shifting from feeding and reproducing on Asian honeybees, their preferred host, to European honeybees, the primary species used for crop pollination and honey production worldwide. To bee researchers, it's a grimly familiar story: Varroa destructor made the same host leap at least 60 years ago, spreading rapidly to become the most important global health threat to European honeybees.

While host-switching V. jacobsoni mites have not been found outside of Papua New Guinea, Purdue researchers Gladys Andino and Greg Hunt said vigilance is needed to protect European honeybees worldwide from further risk.

"This could represent a real threat," said Andino, a bioinformatics specialist with Information Technology at Purdue. "If this mite gets out of control and spreads, we might have another situation like V. destructor."

Varroa mites are obligate parasites, meaning their lifecycle is inextricably entwined with that of their bee hosts. The mites can do serious damage to their hosts' health due to their relatively large size — "think of a tick as big as your fist," Hunt said. Mites latch on to bees and feed on their hemolymph, insects' rough equivalent to blood, leaving behind open wounds that are susceptible to infection. They can also transmit diseases such as deformed wing virus and have been linked to colony collapse disorder.

To gain insight into the biology behind V. jacobsoni's host switch, Andino and Hunt, professor of behavioral genetics and honeybee specialist, studied the differences in gene expression between V. jacobsoni mites that fed and reproduced on Asian honeybees and those that parasitized European honeybees. Knowing which host cues mites respond to and the genes involved could lead to potential control strategies, the researchers said.

"If we can understand the mechanism, we might be able to disrupt, block or manipulate that," Andino said. "But first we have to understand what is happening and which genes are involved in allowing the mites to shift to a new host."

Andino and Hunt sequenced and assembled the first V. jacobsoni transcriptome, a catalog of all of the proteins made by an organism that shows which genes are actively being expressed. They then used the transcriptome to compare gene expression in populations of V. jacobsoni.

Andino and Hunt said the mites' leap to European honeybees likely occurred within the last decade. Previously, V. jacobsoni mites were occasionally found on European honeybees but seemed unable to produce healthy offspring, limiting their destructive capacity.

Catching the host transition in its early stages will allow researchers to continue to investigate the complex genetic details behind the shift and monitor infected European honeybees, Hunt said.

"This happened once with one species of mite, and it looks like it's happening again. Maybe if we catch this as it's beginning, we'll be able to figure out why it's happening or, down the road, stop it."

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