Oct. 3, 2017
Varroa mite on an adult worker honey bee (photo by Stephen Ausmus, USDA-ARS), and proportionally sized mite, if scaled up to a human-sized host (photo by Landi Simone, Goose Rock Farm).
The varroa mite is the #1 threat to honey bees and the beekeeping industry around the world. These parasites infest just about every honey bee colony in the world, except for those in Australia. Originally a pest of the Asian honey bee, Apis cerana, these mites made the jump to a novel host, Apis mellifera, after people began moving the gentle European honey bee around the world. The trade and movement of bee stocks has exposed honey bee populations to many new and exotic diseases and pests, many of which have made their way to the U.S. in recent years.
The varroa mite was introduced into North American in the mid-1980's and within a few years had spread coast to coast. Varroa is largely blamed for the demise of nearly all feral honey bee colonies in the U.S. within a few years. About the same time, U.S. beekeepers were faced with record low honey prices, due to competition from cheaper honey imported from Asia. Many beekeepers went out of business, while others began to move bees around for crop pollination in order to make ends meet. With fewer wild bees and fewer beekeepers, pollination services were increasingly needed, especially as the scale of agricultural production continued to grow. However, this increased movement of bees to meet these needs, combined with the widespread shipment of bees for hobbyist beekeepers, has resulted in the near universal distribution of honey bee pests and diseases, further compounding the bees' problems.
While these parasites are tiny to us (about 1/16 inch across), they are actually one of the largest parasites known, in proportion to the size of their host's body. Varroa mites themselves are able to chemically mimic the odor of honey bees, to invade and escape detection in the colony. Once in the bee hive, mites damage the bees in multiple ways in their unusual life cycle. They can feed directly on the hemolymph (blood) of adult bees, which weaken the bees and shortens their already brief lives, Doing so also spread viruses and other disease pathogens. Even more dangerously, the mites reproduce only in sealed brood cells of honey bees. Safe beneath the wax capping, a female mite lays eggs, and then she and her offspring feast on developing bee pupae. This feeding caused significant damage, reduces the bee's life expectancy, and transmits viruses while the mite reproduce exponentially. Some viruses have even adapted to reproduce more quickly inside the mites. Mites that pick up virus particles from one bee will vector that virus to other bees when feeding again later.
Now, nearly 30 years after their introduction, varroa mites are still thriving. They appear able to quickly evolve resistance to the many chemical treatments and medications that beekeepers have tried to get rid of these pests -- a problem that has been made worse by over-use of the few effective products that have been available. There are many varroa treatement products available, but unfortunately none are the silver bullet that bees and beekeepers urgently need. Chemical pesticides leave residues in the wax honey combs and cannot be applied while the bees are storing honey for human consumption. Many also have sub-lethal effects on bee health. Organic acids and various essential plant oils can be useful to kill mites, but care must be used when applying any of these, as they may be very sensitive to temperature, or may work only when the hive is broodless (a very short period of time in the south). Some products can also affect honey quality, or even be very dangerous to bees and beekeeprs if used improperly.
New research led by Zachary Huang, at Michigan State University, may lead to a whole new direction of mite control. Using a process called RNA interference (or RNAi), scientists were able to effectively "silence" specific genes in the varroa mites in order to determine the specific role of the genes in the mite's biology. Using this technique, the team was able to identify two genes that caused high mortality in the mites when knocked out, and another four genes that appeared to control mite reproduction. Other research has shown that a mixture of specific double standed RNA molecules (dsRNA) can be fed to bees, and varroa mites will take up these molecules when feeding on the bees' blood.
Using RNAi in bees and in other medical applications is not new, but it's a science that has not yet been perfected. Other researchers have worked to stop honey bee viruses from reproducing using similar techniques. While the process can work well in a laboratory, getting it to perform well in the real world is much more difficult. Beekeepers need a product that can be fed directly to bees, that has a long shelf life, and does not have any adverse effects on the honey bees themselves. Working at a molecular level poses many challenges. Not least among these is the ability to identify one or more specific gene that are critical to mite survival and/or reproduction, but cannot be found in honey bees or any other organism that may be affected, and then knock it out without interfering with the host's health or biology. That's a tall order of very specific needs, and is therefore likely to be a ways off in the future.
But, as the frontiers of this research continue to expand, and more precise tools are available for scientists to use, then this technique may open up other areas of research. The same technique could be applied to mosquito control, household pest control, or field crop pests. Someday, therapeutic RNAi medications may even be able to help humans to suppress conditions caused by genetic disorders.