Honey bee research set to save species
Cross-section of a honey bee's abdomen with a parasitic varroa mite (pink) feeding on fat body tissue. [UMD/USDA/PNAS]
Electron microscopy reveals that honey bee parasites feed on fatty organs, not blood, contradicting decades-old research and opening the door to new treatments to control the mites that kill these bees.
While honey bee colonies around the world are at risk from myriad threats, recent research suggests that one threat stands well above the others: a parasitic mite, varroa destructor, which specialises in attacking honey bees.
For decades, researchers have assumed that varroa mites feed on blood, like many of their mite and tick cousins.
But researchers from the University of Maryland and colleagues now believe that these mites actually prey on a honey bee organ - the 'fat body' - that carries out similar functions as the the human liver while also storing food and boosting bees' immune systems.
The varroa mite is wedged between the abdominal plates of a honey bee's exoskeleton. [UMD/USDA/PNAS]
"Losing fat body tissue impairs a bee's ability to detoxify pesticides and robs them of vital food stores," highlights Dr Samuel Ramsey from Entomology at Maryland. "The fat body is absolutely essential to honey bee survival."
As part of their research, Ramsey and colleagues first studied where on the bees' bodies the varroa mites attached for feeding.
"In adult bees, we found a very strong preference for the underside of the bees' abdomen," says Ramsey. "More than 90 percent of mites we found on adults fed there."
"As the bees mature, the fat body tissue migrates to the underside of the abdomen. The connection was hard to ignore, but we needed more evidence," he explains.
Ramsey and his team then directly imaged the wound sites where varroa mites gnawed on the bees' abdomens.
The researchers used liquid nitrogen to freeze the mites and their bee hosts, essentially taking a physical "snapshot" of the mites' feeding habits in action.
Then, using scanning electron microscopy at the Electron and Confocal Microscopy Unit at the US Department of Agriculture in Beltsville, Ramsey and his colleagues saw clear evidence that the mites were feeding on fat body tissue.
Feeding site of varroa on adult honey bee imaged via low-temperature SEM. Location of the mite (A). The mite is wedged beneath the third tergite of the metasoma (B). When removed, a detailed impression of the mite can be observed in the intersegmental membrane in addition to a wound where the mouthparts of the mite would be (black arrow in C). Note, the ambulacra, or foot pads, of the mite (white arrows) remained attached to the membrane when the mite was extracted (C and D). Higher magnification of the wound reveals distinct grooves in the wound matching the modified chelicera of the mouthparts of the mite, colorized for clarity [moveable digit (yellow), corniculus (green)] (F). (A and C) [Ramsey et al PNAS January 15, 2019]
"The images gave us an excellent view into the wound sites and what the mites' mouthparts were doing," says Ramsey. "We could see digested pieces of fat body cells. An organism the size of a bee's face is climbing on and eating an organ. It's scary stuff. But we couldn't yet verify that blood wasn't also being consumed."
To clarify this, Ramsey and colleagues fed bees with one of two fluorescent dyes; uranine, a water-soluble dye that glows yellow, and Nile red, a fat-soluble dye that glows red.
If the mites were consuming hemolymph, Ramsey expected to see a bright yellow glow in the mites' bellies after feeding. If they were feeding on fat bodies, on the other hand, Ramsey predicted a telltale red glow.
"When we saw the first mite's gut, it was glowing bright red like the sun. This was proof positive that the fat body was being consumed," he says.
Varroa were fed on nurse bees given only one of the two fluorescent biostains and imaged via confocal laser scanning microscopy. Mites that fed exclusively on bees with biostained hemolymph (A) showed fluorescence only marginally above the control (B). Mites fed on bees with fluorescently stained fat body show such high levels of Nile red in the digestive system that the shape of the gut can be clearly observed via fluorescence imaging (C) [Ramsey et al PNAS January 15, 2019]
"These results have the potential to revolutionize our understanding of the damage done to bees by mites," highlights lead researcher, Professor Dennis vanEngelsdorp, from Entomology at Maryland.
"Fat bodies serve so many crucial functions for bees. It makes so much more sense now to see how the harm to individual bees plays out in the ways that we already know varroa does damage to honey bee colonies," he adds. "Importantly, it also opens up so many new opportunities for more effective treatments and targeted approaches to control mites."
The varroa mite that has consumed honey bee fat body tissue tagged with Nile red fluorescent dye [UMD/USDA/PNAS]
Research is published in the Proceedings of the National Academy of Sciences.