Bee parasite is more werewolf than vampire | Science News for Students

Bee parasite is more werewolf than vampire

That may finally explain why Varroa destructor mites are a nightmare for a beehive
Mar 4, 2019 — 6:45 am EST
a photo of a honeybee with a pink varroa on it' abdomen

Varroa destructor mites (one shown in pink on a honeybee) have long been a plague for honeybees. But they may have been misunderstood for decades. It may not so much bee “blood” but fat they’re after.

UMD, USDA, PNAS

Scientists have given a tiny mite the ominous name Varroa destructor. This parasite lives on honeybees. Scientists had thought it was a “vampire,” living off of bee blood. But tests with fake bee larvae reveal that the mite may not be so much a bloodsucker as a fat slurper. That makes the mite more werewolf than vampire, the researchers say.

The parasite invaded North America in the 1980s. Since then, it has become one of the biggest threats to honeybees. Based on research from the 1970s, scientists thought that the mites fed on the bee version of blood. It’s called hemolymph (HE-moh-limf). But the mites are actually after the fat, proposes Samuel Ramsey. He is an entomologist or insect specialist. He researched this idea while at the University of Maryland in College Park.

That insight might help people create mite-killing medicines to feed to bees, says Aaron Gross. He’s a toxicologist at Virginia Tech in Blacksburg. While there are mite-killing pesticides, beekeepers could really use some new choices, he says. One big reason: Some groups of mites don’t die when treated.

Ramsey’s rethink started with Varroa biology. Some things didn’t seem to fit with the blood hypothesis. Many bloodsucking species have a flexible body that can swell when the animal drinks a lot. Some have guts with fancy filters that pull nutrients from water. Varroa mites, though, don’t have these traits. And hemolymph is so weak and watery that Ramsey found it odd mites live on it.

So he spent about a year inventing artificial bee larvae. He made the fakes out of gelatin-based pill capsules. These let him test how well mites survived when fed different things. He filled the fake larvae with different foods. Some held only hemolymph. Mites feeding on them lived an average of just 1.8 days. Other fakes had mixed food inside. Only a few mites survived the whole seven-day test. However, all survivors dined on half or solely fat.

Those tests plus other signs suggest mites need bee fat, Ramsey and his colleagues argue. They described their tests January 29 in the Proceedings of the National Academy of Sciences. Rather than sucking blood, the mite “is feeding on flesh more like a werewolf,” Ramsey concludes.

Focusing on fat

In another part of the study, the researchers fed adult bees two stains. Nile red stained their fat. Yellow uranine stained their “blood.” Skilled microscope specialists then took images of the mites’ guts after feeding on fat. Guts glowed red. When the parasites fed on bees with stained hemolymph, the mite guts looked ghostly dim.

Says Ramsay: Those tests show that mites target adult bee fats.

a microscopic image of a cross section of a bee showing a mite wedged under the bee's protective plate
This microscope view shows a cross section of the outer layer of an adult honeybee’s abdomen. A mite (shown in pink) is wedged under a bee’s protective plate. A new study found that almost all mites appear attached in this general area — within reach of fat.
UMD, USDA, PNAS

“The experiments seem to be reliable, and the results are convincing,” says Peter Rosenkranz. He studies bee health and directs Germany’s Agricultural State Institute. It’s based at the University of Hohenheim in Stuttgart.

Thinking about fat could explain more about how mites harm bees, says Ramsey. An organ in bees known as the fat body breaks down pesticides. It also helps the year’s late larvae grow so that they can live extra long and survive the winter. The mites might make pesticides and winters worse, he says.

Damaging bee fat also may reduce bees’ immune response, says Lena Wilfert. She’s an evolutionary ecologist at the University of Ulm in Germany. Mites spread viruses among bees. Those viruses might do more to bees without normal fat bodies.

When a larva is ready to metamorphose into an adult, nurse bees seal the larva in a cell. A female mite can slip into the cell right before sealing. There, she lays eggs. When that bee later emerges as an adult, the mother mite and her daughters hitch along. They often then switch to nurse bees. Those nurses have especially large fat bodies. The mites clamp onto an area near the bee fat body. But the mites aren’t just riding, Ramsey says. Wound openings on bees are shaped like mite mouthparts. Also, internal damage shows up in images taken through a microscope.

Creating those decoy larvae from gelatin capsules to study mite behavior in the lab wasn’t easy. The smallest pills still have walls too thick for mites’ teeny mouthparts to pierce. Ramsey had to work out a way to carefully replace the bottom of a pill with a film stretched to a thickness of only 15 micrometers — about the width of a very thin human hair. Rubbing the film over real bees to transfer scent eventually coaxed mites to taste the fakes. Rearing mites without bees has been a very hard problem. However, Ramsey says, “I’m very, very, very bad at giving up.”

Power Words

(more about Power Words)

agriculture     The growth of plants, animals or fungi for human needs, including food, fuel, chemicals and medicine.

behavior     The way something, often a person or other organism, acts towards others, or conducts itself.

biology     The study of living things. The scientists who study them are known as biologists.

cell     The smallest structural and functional unit of an organism. Typically too small to see with the unaided eye, it consists of a watery fluid surrounded by a membrane or wall. Depending on their size, animals are made of anywhere from thousands to trillions of cells. Most organisms, such as yeasts, molds, bacteria and some algae, are composed of only one cell.

colleague     Someone who works with another; a co-worker or team member.

compound     (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.

develop     To emerge or come into being, either naturally or through human intervention, such as by manufacturing. (in biology) To grow as an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape.

development     (in biology) The growth of an organism from conception through adulthood, often undergoing changes in chemistry, size and sometimes even shape.

ecology      A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

entomology     The scientific study of insects. One who does this is an entomologist.

evolutionary ecologist     Someone who studies the adaptive processes that have led to the diversity of ecosystems on Earth. These scientists can study many different subjects, including the microbiology and genetics of living organisms, how species that share the same community adapt to changing conditions over time, and the fossil record (to assess how various ancient communities of species might be related to each other and to modern-day relatives).

fat     A natural oily or greasy substance occurring in plants and in animal bodies, especially when deposited as a layer under the skin or around certain organs. Fat’s primary role is as an energy reserve. Fat also is a vital nutrient, though it can be harmful if consumed in excessive amounts.

gelatin     A substance made from animal collagen, usually bones and cow or pig hides. It starts out as a pale colored, tasteless powder. It contains proteins and amino acids. It can make jiggly desserts (like those known as Jell-O). But this substance also is used in yogurt, soups, candies and more. It can even be used as the basis of the clear capsules used to hold single-serving amounts of dry medicines.

generation     A group of individuals (in any species) born at about the same time or that are regarded as a single group. Your parents belong to one generation of your family, for example, and your grandparents to another. Similarly, you and everyone within a few years of your age across the planet are referred to as belonging to a particular generation of humans. The term also is sometimes extended to year classes of other animals or to types of inanimate objects (such as electronics or automobiles).

gut     An informal term for the gastrointestinal tract, especially the intestines.

hemolymph     A fluid in invertebrate animals that’s similar to blood in vertebrates.

immune     (adj.) Having to do with the immunity. (v.) Able to ward off a particular infection. Alternatively, this term can be used to mean an organism shows no impacts from exposure to a particular poison or process. More generally, the term may signal that something cannot be hurt by a particular drug, disease or chemical.

insect     A type of arthropod that as an adult will have six segmented legs and three body parts: a head, thorax and abdomen. There are hundreds of thousands of insects, which include bees, beetles, flies and moths.

insight     The ability to gain an accurate and deep understanding of a situation just by thinking about it, instead of working out a solution through experimentation.

larva     (plural: larvae) An immature life stage of an insect, which often has a distinctly different form as an adult. (Sometimes used to describe such a stage in the development of fish, frogs and other animals.)

micrometer     (sometimes called a micron) One thousandth of a millimeter, or one millionth of a meter. It’s also equivalent to a few one-hundred-thousandths of an inch.

microscopic     An adjective for things too small to be seen by the unaided eye. It takes a microscope to view objects this small, such as bacteria or other one-celled organisms.

mite     An invertebrate belonging to the broad group of animals known as arachnids, which also include spiders and ticks. None of these are insects, although like insects they belong to the larger umbrella group, called arthropods (named for their members’ segmented legs).

nutrition     (adj. nutritious) The healthful components (nutrients) in the diet — such as proteins, fats, vitamins and minerals — that the body uses to grow and to fuel its processes. A scientist who works in this field is known as a nutritionist.

organ     (in biology) Various parts of an organism that perform one or more particular functions. For instance, an ovary is an organ that makes eggs, the brain is an organ that makes sense of nerve signals and a plant’s roots are organs that take in nutrients and moisture.

parasite     An organism that gets benefits from another species, called a host, but doesn’t provide that host any benefits. Classic examples of parasites include ticks, fleas and tapeworms.

pesticide     A chemical or mix of compounds used to kill insects, rodents or other organisms harmful to cultivated plants, pets or livestock; or unwanted organisms that infest homes, offices, farm buildings and other protected structures.

Proceedings of the National Academy of Sciences     A prestigious journal publishing original scientific research, begun in 1914. The journal's content spans the biological, physical, and social sciences. Each of the more than 3,000 papers it publishes each year, now, are not only peer reviewed but also approved by a member of the U.S. National Academy of Sciences.

proportion     The amount of a certain component of a mixture relative to other components. For example, if a bag contains 2 apples and 3 oranges, the proportion of apples to oranges in the bag is 2 to 3.

toxicologist     A scientist who investigates the potential harm posed by physical agents in the environment. These may include materials to which we may be intentionally exposed, such as chemicals, cigarette smoke and foods, or materials to which we are exposed without choice, such as air and water pollutants. Toxicologists may study the risks such exposures cause, how they produce harm or how they move throughout the environment.

Citation

Journal:​ ​​ S.D. Ramsey et al. Varroa destructor feeds primarily on honey bee fat body tissue and not hemolymph. Proceedings of the National Academy of Sciences. Vol. 116, January 29, 2019, p. 1792. doi:10.1073/pnas.1818371116.