Varroa mite research may improve bee health

Varroa mites can be seen feeding between this honey bee’s abdominal plates. Researchers are starting to learn more about the feeding habits of this pest.  |  University of Maryland photo

Scientists discover that mites target bees’ fat body located in the abdomen, which may help find new methods of control

Honey bee colonies worldwide face many threats, including disease, poor nutrition, and habitat loss. But the greatest threat is the varroa mite Varroa destructor.

Getting control of this pest means learning how it operates.

New research from the University of Maryland has shown that the mite has a voracious appetite for one particular organ in the honey bee called the fat body located in the abdomen.

This organ is vitally important to bees. It not only breaks down toxins and stores nutrients, it produces antioxidants and the wax covering parts of a bee’s exoskeletons. It helps manage the bee’s immune system and plays an essential role in metamorphosis, regulating the timing and activity of key hormones.

For decades, many people believed that varroa mites, like many mites and ticks, feed on the bee’s blood (more accurately called hemolymph in insects). That belief has persisted since the 1960s when several studies that were originally written in Russian were mistranslated. And yet there were hints that a blood-thirsty mite actually didn’t make sense.

“It was really difficult to understand how mites could get as much nutrients from blood because blood doesn’t have a lot of nutrients,” said Dennis vanEngelsdorp, professor of entomology and co-author of the study.

“(In addition), their poop is the poop of an insect that is water-deprived and not having excess to water.”

Given the rate at which mites grow and reproduce and the fact that a bee’s blood is low in nutrients, mites would need access to a lot more hemolymph than they could get from a single bee. Not only is a mite’s excrement dry but its mouth parts are adapted for digesting soft tissue with enzymes. In comparison, blood-feeding mites have mouthparts specially adapted for piercing membranes and sucking fluid.

“It’s quite an amazing system,” said vanEngelsdorp. “They are consuming the fat inside the bee and sucking up a sort of milk shake fatty slurry. It’s more nutritious (than blood) and explains why their populations can grow as fast as they can. It’s amazing that the eggs they produce are such big eggs compared to the size of the mites. Each egg is nutrient packed and, as soon as it hatches, it’s a viable organism. The fat body is where the bee stores its protein and it does a lot of their hormone regulation. If they don’t have that fat, they can’t rear the next generation of brood in the spring and that will explain a lot of these high rates of mortalities.”

A series of experiments uncovered the varroa mites’ feeding location, the first being to observe where on the bees’ bodies the mites attached. The researchers found that, when feeding on immature bees, they will eat anywhere but when feeding on adult bees they concentrate on the abdomen.

“More than 90 percent of mites we found on adults, fed there,” said Samuel Ramsey, UMD alumnus. “Fat body tissue is spread through the bodies of immature bees. As a bee matures, the tissue migrates to the underside of the abdomen. The connection was hard to ignore but we needed more evidence.”

Applying a technique called freeze fracturing, liquid nitrogen was used to freeze the mites and their bee hosts.

“In the (Advanced Imaging and) Microscopy Lab, the scientists are experts at taking images of insects and plant tissue,” said vanEngelsdorp. “They basically froze the bee with the mite really quickly. You have to coat it with a thin layer of gold so you can bounce electrons off of it. Then you can take the picture.”

Using powerful scanning electron microscopes to visualize the wound sites, they saw clear evidence of the mites feeding on fat body tissue. The images gave them excellent views of exactly what the mites’ mouthparts were doing. They could clearly see the digested pieces of fat body cells.

“The mites were turning the bees into cream of honey bee soup,” said Ramsey. “An organism the size of a bee’s face is climbing on and eating an organ. It’s scary stuff.”

The next experiment was to feed bees with one of two fluorescent dyes. One was uranine, a water-soluble dye that glows yellow. The other was Nile red, a fat-soluble dye that glows red. If the mites were feeding on blood (the water-soluble dye), their bellies would glow yellow and if they were eating fat, they’d glow red. According to Ramsey, they glowed bright red like a setting sun.

“We’ve been talking about these mites like they’re vampires, but they’re not. They’re more like werewolves.”

The final experiment was to raise varroa mites on an artificial diet regimen for seven days. Some were fed hemolymph and some fed fat tissue. Those on the bees’ blood died quickly; those on the fat tissue thrived and produced six times more eggs.

Now, the long-term goal is to take this knowledge, apply it to new methods of mite control and find a way to interfere with their reproduction.

“The long-term hope is that, with this fat-based feeding, there may be products that can be fed to the bees that get into the fat that can deter the varroa mites,” said vanEngelsdorp. “We can better understand and predict when colonies could die because maybe we could try to measure their fat. The goal is to find a product that is more targeted.”

Studies continue on the finer details of the research to develop those applications. The study, called Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph, was published in the Proceedings of the National Academy of Sciences.

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