Know Thy Enemy (Part 1): Who is Varroa Destructor?
Female Varroa destructor under a dissecting microscope.
Author: Heather Broccard-Bell, Ph.D., Honey Bee Health Researcher. Featured photo credit: H. Broccard-Bell.
Know Thy Enemy: Understanding Varroa
In the ongoing quest to improve the health, happiness, and productivity of our honey bee colonies, protection from varroa mites remains a top priority. Key to developing effective varroa control strategies is understanding how and why varroa do what they do. Admittedly, the current picture is incomplete—but we are making progress with scientists making new discoveries all the time. In this series of blog posts, I will go through what is currently known about varroa behaviour and biology, and how we can use that to improve varroa control strategies. My goal is to provide you with up-to-date information and share some lesser-known tidbits about varroa to help you best manage your colonies.
“If you know the enemy and know yourself, you need not fear the result of a hundred battles. If you know yourself but not the enemy, for every victory gained you will also suffer a defeat. If you know neither the enemy nor yourself, you will succumb in every battle.” ― Sun Tzu, The Art of War
PART 1 – Who is Varroa destructor?
By now, most of us are familiar with the varroa mite as a highly problematic pest of our beloved Western honey bee (Apis mellifera). Varroa hopped over from its historical host, the Asian honey bee (Apis cerana) only around 60 years ago. Genetic evidence suggests that the jump happened at least twice. The Asian honey bee had co-evolved with the mite for millions of years. Through a lengthy process of natural selection, the two species learned to live with one another well enough that varroa is no longer a major concern for Asian honey bee colonies. Unfortunately, the same is not true for Western honey bees.
Before we get into exactly why varroa is so bad and what we can do about it, let’s familiarize ourselves with exactly who Varroa destructor is.
Varroa belongs to a group of arachnids called parasitiform mites, that includes ticks. As an arachnid, the species falls into the same category as spiders. You might be forgiven for thinking that a bug is a bug is a bug—but in fact, varroa mites are more closely related to scorpions and horseshoe crabs than they are to bees!
Parasitiform mites get the energy they need to live and reproduce from their hosts. In the case of the varroa mite, this energy comes primarily from the fat bodies of honey bee brood and adults. Some of you have probably been taught that varroa feed on the hemolymph (blood) of bees, but we recently learned that this is incorrect! Although varroa do consume some hemolymph, this is not their primary food source, and they are not able to survive on hemolymph alone.
But what is this mysterious fat body? It is the collection of stored fat in the body of a honey bee. Much like you and me, honey bees store food energy in the form of fat to be used later. Perhaps not surprisingly, winter bees have larger fat bodies than summer bees. This fact likely contributes to varroa’s ability to overwinter with honey bees during broodless periods. Scientists have learned that the fat body also plays a vital role in the immune system of honey bees—an important finding that we’ll get back to when we talk about viruses in an upcoming segment.
The Varroa Life Cycle: A Complex Story Unfolds
Thanks to the similarities between Western honey bees and Asian honey bees, varroa have been able to successfully use the same approaches to survive and reproduce in both species. This approach largely relies on varroa’s ability to co-ordinate its life with that of the honey bee. And because varroa is blind, that mostly happens by sniffing out the honey bee’s pheromones.
Most of you are familiar with queen pheromone, but did you know that all members of honey bee colonies, from larvae to adult foragers and drones, use pheromones? A pheromone is a chemical or combination of chemicals that carries a message from one member of a species to another member of the same species*. “Chemical communication” is the oldest and most widespread way to send a message in the living world. Everything from bacteria to trees send signals to one another in the language of chemicals!
Honey bees produce pheromones using at least 15 different glands, sending chemical messages to other bees about almost every aspect of their lives. And varroa are listening in…
Shh… the Varroa Mites are Listening
During the early stage of life, a honey bee larva grows in an uncapped cell and produces certain types of pheromones. These chemicals are attractive to nurse bees and are thought to signal their help with capping the cell when the time is right. The pheromones also attract varroa mites. Pregnant female varroa—what we call “foundresses”—need to get into honey bee larval cells just before capping. Varroa foundresses, who usually arrive at a cell on the body of a nurse bee, simply eavesdrop on the messages sent by the honey bee larva, allowing them to find uncapped brood cells at just the right age. The foundress can then hop on in and make herself at home.
Drone larvae produce higher levels of some brood pheromones than worker larvae. We think this accounts for the fact that nurses visit drone cells more than worker cells, and goes some of the way to explain why varroa end up in drone cells more often.
The Journey of a Foundress Mite
Once inside a cell, the foundress preps for her next moves. Since varroa mites are blind, they rely heavily on the ability to sense which chemicals are present. Incidentally, we’ve learned that chemicals in brood food are also attractive to varroa. The first thing a varroa foundress does when she enters a cell is to settle into the jelly-like brood food at the bottom. She then extends a little tube up out of the goop so she can breathe—just like she’s snorkeling. To avoid being removed by an over-attentive nurse bee, the foundress hides in the brood food until the cell has been capped over and it’s safe for her to emerge.
The honey bee prepupa (the stage after capping, just before the larva begins to transition into an adult bee) takes about 5 hours to eat the remainder of its brood food after capping, exposing the varroa foundress. At this point, the foundress moves up in the cell, and opens a hole in the body of the honey bee prepupa. The hole will be used as a feeding site for herself and her offspring, who will not be strong enough to tear through the developing honey bee’s cuticle on their own. The foundress makes use of the feeding site herself at this time, since she needs proteins from the larva to produce eggs. Like so many other aspects of varroa’s life, the process of oogenesis (egg production), which begins now, is triggered by the presence of honey bee brood pheromones.
After feeding, the foundress defecates on the wall of the cell, creating a white patch of feces that is readily visible to any curious beekeeper. The fecal patch will serve as a gathering point for the foundress’ offspring, and mating will happen at this location (ew!).
About 70 hours after capping, the foundress lays her first egg. Pheromones given off by the honey bee larva shortly after capping trigger the foundress to lay an unfertilized egg. Just like for honey bees, unfertilized varroa eggs develop into males. It takes approximately 6.5 days from the time an egg is laid until the resulting male is mature. He needs to shed his skin (“moult”) several times, undergoing several developmental phases after the egg hatches. In the meantime, the foundress busies herself laying fertilized eggs, once every 30 hours, that will develop into females.
Female mites undergo a similar development process, reaching maturity around half a day faster than males. In total, the foundress will lay 3-4 eggs. Not all of her offspring will have time to reach maturity before the cell is uncapped and the mature honey bee emerges. On average, only 2-3 mature females will be produced per foundress during each brood cycle.
Scientists and beekeepers alike consistently find that varroa prefer drone cells to worker cells. Most agree that the preference is because drone cells are capped for 3 days longer than worker cells. The extra time allows the foundress to produce more mature daughters.
A Peculiar Kind of Family
Once the male and the first female offspring (his sister) have reached maturity, they begin to mate. For male varroa, the most attractive females are ones that 1) hang out on the foundress’ fecal patch and 2) have moulted most recently. This seemingly bizarre kink makes sense from the varroa perspective. A male will continue to mate with a female until a new, more recently-moulted female arrives on the scene. Thus, females receive the most attention when they are the most fertile and no fertile female gets missed.
Foundress varroa only mate once, and in another close parallel of honey bee biology, store the sperm from that mating for the rest of their lives. Foundresses can go through the cycle of entering a honey bee cell to lay eggs several times—typically up to 3 under field conditions.
One question that always comes up when people discuss the varroa mating system: what is the effect of all that inbreeding? The short answer: we don’t yet fully understand how varroa avoid the pitfalls of inbreeding. Although we lack a complete explanation, we have recently identified at least two ways mites can gain genetic diversity. I’ll discuss these and inbreeding in general in an upcoming post.
It’s Go Time
About 14 days after capping, mature drone honey bees emerge from their cells—and along with them, several newly-mated female varroa mites. The male remains in the cell, where he dies. At this point, the newly-mated females and the original foundress varroa usually hitch a ride on nurse bees, whose cocktail of pheromones is particularly attractive to the mites. In an interesting twist, scientists recently learned that unmated females can also emerge from the cell to hitch rides on nurse bees. These virgins go on to infest a new cell, where they lay an unfertilized egg, and then mate with their resulting son.
The Phase Formerly Known as Phoretic
Varroa can live and feed on adult bees for anywhere from hours to months. In the past, this was referred to as the “phoretic phase”—but phoresy is an association between organisms where one hitches a ride on another without causing harm. We know that varroa do parasitically feed on adult honey bees. In fact, some have suggested that this feeding is required for varroa reproduction. In any case, it’s clear that the relationship between varroa and adult honey bees is not a harmless one, and researchers now prefer to use the term “dispersal phase.”
So, we know varroa read honey bee pheromones. But did you know that mites can also influence the pheromones produced by honey bees? For the most part, varroa remain on nurse bees during the dispersal phase, which keeps them within the same colony. However, when a colony starts to get overcrowded with mites, the pheromones given off by workers and nurse bees change. Soon, varroa will begin to hop onto foragers as well. Aboard foragers, varroa leave the colony, where they can be transferred onto bees from other colonies at foraging sites, through drift, or even robbing.
Up Next: A Look into Varroa Inbreeding
Although scientists have learned a lot about varroa during the past few years, many details remain to be filled in. I hope you collected at least one new tidbit from this post. I invite you to join me in part two of this series, where we take a closer look at the problems of inbreeding and what we have learned about some ways varroa overcome them.
About Heather Broccard-Bell, Ph.D.
Dr. Heather Broccard-Bell is the Honey Bee Health Researcher at NOD Apiary Products. She is a scientist and educator with over 15 years research and teaching experience. Heather has been focused on investigating issues surrounding honey bee health and communication since 2014. When Heather’s not in the lab, you can usually find her in the bee yard or on a trail hiking with her many pawed pals. If you’d like to ask Dr. Heather Broccard-Bell more about her work or this article, you can email her at firstname.lastname@example.org
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*Although I have used the term “pheromone” throughout this article, technically when one species eavesdrops on another species’ pheromones, we call them kairomones, not pheromones.