Varroa destructor is an ectoparasitic mite that can cause European honey bee colonies to collapse by spreading the Malformed Wing Virus as they feed. A study published in PLOS Pathogens by Zachary Lamas and colleagues at the USDA-ARS and the University of Maryland suggests that a relatively small number of mites may contribute to a large number of infected bees.
Arthropod disease vectors transmit pathogens as they feed on susceptible hosts. However, little is known about how Varroa feeding dynamics spread viruses in adult honeybees. To better understand varroa mite parasitism on honeybees, researchers conducted a series of experiments.
First, they used fluorescent microbeads to test whether Varroa fed on adult bees each time they entered a known feeding position. They then determined whether microspheres could be transferred from a Varroa to an adult bee via Varroa feeding by having Varroa feed on bee pupae injected with fluorescent microspheres.
In the third experiment, researchers saw mites switch from adult bee host to host. The researchers then observed how a single mite could spread pathogens by feeding on multiple bees and calculated the relative risk of Varroa parasitism on adult workers.
Mites with a high virus content and which changed the most often contributed to the highest mortality in adult honeybees. Varroa are promiscuous eaters and change hosts quickly.
Mites that switched hosts with the highest frequency accounted for nearly three times as many parasitized hosts as their counterparts that switched less frequently. Future studies are needed to better understand the mechanisms that drive mites to switch hosts.
According to the authors “Our work shows that virus spread is driven by Varroa actively switching from one adult bee to another as they feed. Relatively few of the most active Varroa parasitize most bees. The ability to parasitize and infect multiple adult bees provides the best explanation to date for the maintenance and subsequent host-to-host spread of viruses among the long-lived worker bees common in these dense and vulnerable colony populations.”
- Lamas ZS, Solmaz S, Ryabov EV, Mowery J, Heermann M, Sonenshine D, et al. (2023) Promiscuous feeding with multiple adult honeybee hosts enhances the vectorial capacity of Varroa destructor. PLoS Pathhog 19(1): e1011061. DOI: 10.1371/journal.ppat.1011061