The parasitic mite Varroa destructor is the worst pest of western honeybees. And according to a new Scientific Reports study, these mites know to pick the best host: not the young bees, not old bees, but mid-age adults called nurses. The findings show just how highly adapted these mites are for parasitizing honeybees.
Varroa destructor and the virus they carry have been devastating colonies of western honeybees (Apis mellifera) around the world over the last decade. The mite’s life cycle has two phases. First, they feed on the blood-like vital fluid of adult bees during what’s called the phoretic stage. Then, during the reproductive stage, the mites lay eggs on immature, developing bees called pupae within the cells of honeybee combs (pictured above). The phoretic stage lasts for a few days, during which the mites frequently switch between different adult bees.
There’s still a lot about Varroa reproductive biology that we don’t understand. What, for example, is the significance of the phoretic stage? During those few days, the mites aren’t increasing their population, and they suffer higher mortality risks too: They might fall off their host bees or get groomed off by others.
To investigate, a team led by Zachary Huang of Michigan State University provided mites in field colonies with three choices: bees that emerged within the last 24 hours, nurses that were five to 11 days old, and foragers that were at least 21 days old. Nurses are in that part of their life cycle where they take care of larvae, and foragers go out for pollen.
They found that Varroa destructor prefers nurses over newly-emerged bees or foragers.
"Further, we showed that feeding on different hosts gave them different reproductive outputs,” Huang said in a statement. After the mites fed on their phoretic hosts for three days, they were transferred to honeycomb cells. Ten days later, the team counted their offspring. Turns out, the preference for nurses maximizes the fitness of the mites. Nurses likely provide better nutrition as hosts, and mites that fed on nurses had lower infertility rates. Although, the benefits of feeding on nurses aren’t immediate since each mite still needs a second host (that is, a pupa) in order to reproduce.
"If you know your enemies better, you can come up with new ways of controlling them," Huang explained. By understanding their reproductive biology, researchers can look for a way to regulate them without affecting the bees.
Image in the text: A Varroa mite on the back of a honeybee. Zachary Huang