For the tiny Varroa mite, a honeybee colony is like a buffet with all your favorite foods. There are tender pupae, newly-emerged adults, middle-aged nurse bees, and grizzled, three-week-old foragers. With all those options, which bee’s blood should a mite feast on?
When offered a choice by scientists, adult mites go for the nurse bees, and their young feed off the pupae. The mites’ appetites have devastating effects: they weaken the young pupae and transmit viruses to the adults, which have wiped out colonies worldwide. While scientists know about the effects of the mites’ preference, they’ve long puzzled over why adult mites love snacking on nurse bees. In a study published this June, American and Chinese scientists figured out that mites that feast on nurse bees have the most—and the healthiest—female offspring.
This study is the first step towards figuring out how beekeepers can better protect colonies from insatiable mites, which have been decimating American hives since 1987. “If you know your enemies better,” study author and Michigan entomologist Zachary Huang told MSU Today, “you can come up with new ways of controlling them.”
To start learning about these enemies, the entomologist focused on mite reproduction. When a female mite is ready to lay her eggs—her reproductive stage—she sneaks into a pupa’s honeycomb cell. There, she lays up to six eggs which hatch into one male and multiple females that feed off the pupa. After mating with their brother, the females leave to feed on adult bees—called the phoretic stage—or sneak straight into a new cell to lay their eggs. The scientists focused on figuring out why some mites went straight to the reproductive stage, while others gambled on the phoretic stage, which delayed reproduction and put them at risk for getting killed.
But first the scientists had to see if mites prefer nurse bees in the wild. The researchers set up seven bee colonies with newly-emerged adults, nurse bees, foragers, and one queen. After painting each bee group a different color, the scientists let the bees establish their colony and forage for food. Then, they introduced 200 hungry mites into each colony. Two days of devastation later, the scientists sorted the bees and rinsed each group with ethanol to knock loose and count the mites.
The results confirmed what the lab mites had already shown: mites love nurse bees. Approximately 1 out of 6 nurse bees was infested with mites, compared to 1 out of 10 foragers and 1 out of 17 newly-emerged adults. To get an idea why, the researchers captured female mites. They put some mites in a cage with only nurse bees, some with only foragers, and the last with newly-emerged adults. Several days later, the scientists found that the nurse-fed mites had had at least 50% more offspring than the other mites.
But how do the poor nurse bees make mites more fertile?
For one thing, the nurse bees were the least able to defend themselves by triggering infertility in attacking mites—a common honeybee trick that scientists are still trying to figure out. Only about 25% of nurse-fed mites were infertile, compared to 70% of the others. The scientists also speculate that, because the nurses stay in the hive, nurse-feeding mites are more likely to survive and to be close to pupae during reproduction.
Huang hopes to disrupt the mites by targeting their reproduction the way non-nurse bees do. “Instead of killing them [the mites] with a chemical,” Huang said, learning how to destroy the mites’ fertility “could lead to a more natural way of mite control and a better outlook for honeybees.”
Written by Megan Cartwright
Megan Cartwright is a freelance science/medical writer near Seattle. Before becoming a full-time writer, she worked as a scientist studying infectious diseases and vaccines, and earned her Ph.D. in Toxicology from the University of Washington. Megan has written for Slate and Bitesize Bio, and helps write blog posts and learning modules on chemistry for Visionlearning.