For many people, the most frightening thing about working with bees would be the constant threat of being stung. However, for beekeepers, the biggest concern is another type of tiny insect-like creature entirely.
The Varroa mite.
The Varroa mite is the world’s most devastating honey bee pest. Varroa mites (Varroa destructor and V. jacobsoni) are tiny red-brown external parasites of honey bees. Although Varroa mites can feed and live on adult honey bees, they mainly feed and reproduce on larvae and pupae in the developing brood, causing malformation and weakening of honey bees in addition to transmitting numerous viruses. To better understand the effect of Varroa on European honey bees, it is important to understand why Varroa has had and continues to have such as a devastating impact on European honey bees and how this mite has subsequently spread throughout the world.
Colonies with low infestation generally show very few symptoms, however as the mite population increases, symptoms become more apparent. Heavy Varroa mite infestations can build up in 3–4 years and cause scattered brood, crippled and crawling honey bees, impaired flight performance, a lower rate of return to the colony after foraging, a reduced lifespan, and a significantly reduced weight of worker bees. Colony symptoms, commonly called parasitic mite syndrome, include an abnormal brood pattern, sunken and chewed cappings and larvae slumped in the bottom or side of the cell. This ultimately causes a reduction in the honey bee population, supersedure of queen bees and eventual colony breakdown and death.
A little background
The varroa mite (Varroa destructor) is the most serious pest of honey bee colonies worldwide. This parasite was first detected in North Carolina in 1990, having been introduced to the United States just three years earlier. Virtually all feral (or “wild”) honey bee colonies have all but been wiped out by these mites, and beekeepers continue to struggle with varroa infestations in their hives. In North Carolina alone, the number of managed beehives has dropped by an estimated 44 percent since the invasion of the mites. It is vital, therefore, to understand the varroa mite and the options available for its control.
The mite is an external parasite which attacks both adult bees and the developing honey bee larvae. The adult mites have a flattened oval shape, are reddish-brown in color, and are about 0.06 inches wide (about the size of the head of a pin). The mated female mite enters the cell of a developing bee larva and lays up to six eggs. The developing mites feed on the pupae and, depending on the number of mites, may kill it, cause it to be deformed, or have no visible effect. While the males die in the cell, the adult daughter mites climb onto an adult worker bee and feed on its hemolymph (bee “blood”). The female mite can then repeat the cycle by entering cells of other developing larvae. Mites prefer drone larvae over worker larvae, but they will infest worker larvae and eventually kill the colony if preventative measures are not taken.
The mites can harm the bees indirectly as well. In addition to the obvious effects of mites feeding on developing and adult bees, the mites can also serve as vectors of several viruses that can kill bees. These secondary infections, facilitated by the mites compromising the bees’ immune systems, can cause a condition known as Parasitic Mite Syndrome (PMS), which can kill colonies within months of infestation.
Varroa mites are parasitic mites, which require a honey bee host to survive and reproduce. The Varroa mite is only able to reproduce on honey bee brood, while only adult female Varroa mites are able to feed on adult honey bees. Therefore, the entire life cycle of Varroa mite occurs within the honey bee colony.
Female Varroa mites are more likely to lay eggs on drone brood than on worker brood (10–12 times more frequently). This is due to the drone’s longer brood cycle. For this reason inspection of drone brood provides the best chance of detecting Varroa mite infections, however, worker brood also provides an effective means of detection.
The Varroa mite life cycle consists of the following stages:
Adult female Varroa mites enter honey bee brood cells (especially drone brood) at the pre-capping stage and lay two to five eggs after the brood cell is capped.
0.5 mm long eggs are laid on the bottom of the cells, on the walls, and sometimes directly on the larvae.
The first egg laid is a male, and subsequent eggs are female.
After hatching Varroa mites pass through two larval stages (called a ‘protonymph’ and a ‘deutonymph’) before developing into an adult. It takes about 5–6 days for male Varroa mites to develop and 7–8 days for female mites to develop.
Mating occurs in the brood cell. The male Varroa mite dies inside the cell shortly afterwards.
Young female Varroa mites and the mother Varroa mites emerge from the brood cell with the emerging honey bee.
The daughter Varroa mites will lay eggs in other brood cells after 2 weeks. Adult female Varroa mites usually live for 2 months, but can overwinter between the sclerites (the hardened plates of the exoskeleton) of adult honey bees.
Varroa mite numbers increase slowly at first (population growth is exponential), and it may not be until the third year of infection that Varroa mite numbers are sufficiently high for the pest to be readily detected. Close inspection of brood, especially drone brood, will provide the greatest chance of detecting Varroa mite infections early.
Only adult female Varroa mites will parasitise adult honey bees. Adult males only feed on larvae and pupae and do not leave the brood cell once they have hatched. In contrast adult female mites are very mobile and move over the combs or between adult honey bees. This behaviour means that the Varroa mite can also act as an effective virus vector allowing the transfer of viruses between individual bees. The spread of viruses is a significant impact of Varroa mites.
Once on a honey bee the female mites crawl between the sclerites of the honey bees’ abdomen where it feeds on the bee’s haemolymph (the bee’s equivalent to blood). By riding on adult honey bees Varroa mites can be rapidly spread to new areas due to the swarming, robbing and drifting habits of honey bees.
The lifespan of Varroa mites depends on the presence of brood, and can vary from between 25 days, to around 5 months. During the summer, Varroa mites may live for 2–3 months, and if brood is present they can complete 3–4 breeding cycles. In winter, when brood is either not present or is limited, the Varroa mites over-winter on the bodies of adult bees through their phoretic life phase. An adult Varroa mite may live and feed on an adult bee for up to around 3 months. Adult female Varroa mites can live for up to 5 days without food.
Varroa population growth
Varroa mite population growth is determined by the number of female mites in the honey bee colony, the reproductive rate of female Varroa mites as well as the availability of brood and the type of brood that is available. This will likely vary throughout the year due to fluctuations in the amount and type of brood present in the colony. In areas where brood is present all year round and drone brood is often present, Varroa population growth will be faster than in areas where brood is not present year round (eg in cooler areas).
Drone brood is capped for longer than worker brood, so on average, more female Varroa mites are able to mature on drone brood. Research has shown that a female mite laying eggs in drone brood will produce on average 2.6 adult females compared to 1.6 adult females if eggs are laid on worker brood. This means that the reproductive rate of Varroa mites increases with the availability of drone brood. In the average temperate climate, it is estimated that mite populations can increase 12-fold in colonies having brood for half of the year and 800-fold in colonies having brood year-round. This makes Varroa mite very difficult to control, especially in warmer climates where colonies maintain brood year-round.
In the absence of any brood the mite population will slowly decline as older mites die and are not replaced. It has been estimated that the Varroa mite population declines by approximately 10% for every month that brood is absent. However, as soon as new brood is produced the Varroa mite population will begin to increase again.
Invasion and population growth
The rate of population growth will also be determined by the number of Varroa mites that first infect (invade) the colony. For example if a single Varroa mite was to enter a colony and infect drone brood a single generation would produce 2.6 new female mites, however if 10 mites entered the hive and reproduced in drone brood a single generation could result in 26 new female mites. In other words the larger the initial infection the quicker the Varroa mite population will reach damaging levels.
If there are multiple colonies in an area that are infected with Varroa mites (e.g. feral colonies or un-treated hives) they could act as a constant source of infection. Varroa mites cause colonies to weaken and honey bees naturally rob and drift between weakened hives, which result in the spread and invasion of Varroa mites between local honey bee populations.
Due to the exponential nature of population growth the introduction of a few extra mites can have dramatic effects on how quickly the Varroa mite population can reach damaging/detectable levels.
Effect on honey bees
Firstly, the process of Varroa feeding causes the loss of haemolymph during brood development, which significantly decreases the weight of the hatching bee. The weight loss depends on the number of Varroa mites in the cell and the level of reproduction taking place, but even a single female Varroa mite in a brood cell can result in an average loss of body weight of 7 per cent for hatching worker bees, and between 11–19 per cent for hatching drone bees. This subsequently leads to an impaired flight performance. This feeding behaviour also causes reduced hypopharyngeal glands (the glands that secrete royal jelly) which affect the honey bees’ ability to feed developing brood in the hive.
Secondly, worker bees which were parasitised during their development begin their foraging life stage earlier, but also have a significantly reduced lifespan. Infected worker bees and drones also display a decreased capability of non-associated learning, prolonged absences from the colony and lower rate of return to the colony, which may be due to a reduced ability to navigate.
Lastly, a significant impact of adult Varroa mites feeding on developing brood, as well as adult honey bees is through the transmission of viruses. Varroa mites feed on the honey bee’s haemolymph during brood development, as well as on adult bees. This results in Varroa mites acting as an effective vector for numerous viruses. There is also some evidence that Varroa feeding can reduce the effectiveness of the bee’s immune system, so they are more affected by viruses in the presence of Varroa, as well as other pests such as Tracheal mite or Nosema sp. This can cause common symptoms in the honey bee population, such as deformed and shrivelled wings, legs or abdomens, as well as symptoms specific to these other pests.
Together these effects mean that honey bee viruses and other pests become more damaging when Varroa is present in a colony. Further information on the honey bee viruses and their interaction with Varroa mite infestation is provided below.
Effect on honey bee colonies
Small numbers of Varroa mites infesting a colony will usually cause no obvious harm. As the Varroa mite population increases more individual bees are affected, which eventually weakens and affects the colony as a whole. However, in poorly managed colonies where infestation is allowed to increase, signs of damage to the entire colony start to become evident. The time for obvious colony symptoms to appear depends on many circumstances, but can take as long as 2–3 years.
Severe infestation slows the replacement of old adult bees with healthy young bees and may lead to the rapid spread of harmful bee viruses in the colony. At this stage, the normal processes of foraging, brood rearing and colony defence diminish and the colony’s entire social organization begins to deteriorate – a process known as colony collapse.
As infestation increases, the reproductive capacity of the honey bee colony also decreases. Drones which have been parasitised during their development have a significantly lower chance to mate and infested colonies produce less swarms.
When Varroa mites are present at high levels the larvae and pupae start displaying abnormal brood symptoms that appear similar to American foulbrood (AFB), European foulbrood (EFB) and Sacbrood virus but are not caused by these diseases. These symptoms are usually apparent in the final stages of colony breakdown by Varroa mite, and are termed parasitic mite syndrome (PMS).
Parasitic mite syndrome showing distorted larvae and pupae, no eggs in the cells and Varroa mites present. Rob Snyder, www.beeinformed.org
PMS is the name used to describe a group of symptoms, which include:
Scattered brood pattern, as well as bald brood and patches of neglected and ‘dead’ emerging brood.
Crawling or even crippled adult bees, with some partly chewed through dead bees unable to break free from the cell after the brood cycle.
Supersedure of queens.
Sunken and chewed cappings – similar to AFB, EFB and Sacbrood virus.
Larvae may appear off-colored (yellowish or brown).
Larvae appear slumped in the bottom or side of the cell (similar to Sacbrood virus) often with Varroa feeding on them. It has been suggested that this symptom is caused by starvation.
Some larvae die and become a dried, soft and easily removed scale.
Unlike foulbrood diseases, the affected larvae do not smell and do not rope out when tested using a ‘ropiness test’.
Unexplainable reduction in the bee population
From the beekeeping point of view there exist certain thresholds for economic damage and for irreversible damage. In the early stages of Varroa mite infection, honey production and pollination are not significantly affected. Clinical symptoms may also not be evident, which results in the infection going undetected. However, as the Varroa mite population increases to damaging levels, the colony begins to weaken, which results in a reduction in honey production and pollination activity. Infected colonies will become weakened to the point that they collapse unless the colony is treated to reduce the mite population. For options on how beekeepers overseas manage Varroa mite, see the Management tab of this page.
Varroa and viruses
Honey bee viruses are among a number of pathogens that can contribute to the ill health of a colony. For many years, scientists have known that honey bees host a number of viruses. Although honey bee viruses are capable of killing honey bees, the presence of a virus generally does not cause the death of adult honey bees or larvae, and symptoms are only expressed when the colony is under some form of stress, which may be caused by hive movement, climatic conditions such as cool and wet weather conditions, or poor nutrition.
The majority of honey bee viruses are considered ‘silent’, that is, they are present in many honey bee colonies but they are at such low levels that there is an absence of any clear disease symptoms. There is now evidence that many honey bee viruses are associated with Varroa mite presence and levels in a colony and it is the viruses, not the Varroa mites themselves, which cause the majority of the damage that bees experience while hosting the mites.
Viruses are transmitted throughout a hive in one of two pathways. Viruses can be vertically transmitted; where by the parent (queen bee or drone) becomes infected and passes the virus on to its offspring, which are infected from birth. Alternatively, viruses can be horizontally transmitted, which is when infected individuals transmit the virus to uninfected individuals. Commonly this would involve the transfer of viral particles with food or by the removal of waste material. Vectors, such as Varroa mites, can horizontally transmit honey bee viruses within a colony, as well as transmit viruses which can be vertically transmitted by queen bees or drones.
These viruses being vectored by Varroa can induce major disease outbreaks, which allow the viruses to exhibit all of their potential virulence. As Varroa mites feed on the developing larvae and pupae, they not only take away essential nourishment from the developing bees, they also create an opportunity for the viral particles to enter the developing bee. In these circumstances the virus particles are able to multiply within the Varroa mite’s mouthparts, which are then directly injected into the developing bees. Under these circumstances, viral infections can be lethal and lead to immuno-suppression within the developing honey bees, which subsequently activates these ‘silent’ viruses.
It is believed that the eventual colony breakdown and collapse of a Varroa infested hive, with the typical crippled and deformed bees, scattered brood nest, loss of coordinated social behaviour and rapid bee de-population is an effect of multiple viral infections, rather than the direct parasitisation of individual bees by Varroa mite.
More information about these exotic viruses:
Acute bee paralysis virus (ABPV)
ABPV causes paralysis of infected adults and white eyed pupae. ABPV is spread by Varroa mites, or through the ingestion of viral particles contaminating the salivary glands of adult bees, which then contaminates food sources. The virus is also reported to be vertically transmitted from the queen or drone to their offspring. The virus is present in Europe, North America, South America, Africa and New Zealand. It is closely related to the Kashmir bee virus.
Deformed wing virus (DWV)
Not all honey bees will show symptoms of DWV. It is thought that when the colony is under stress (which may be the case following a Varroa mite infection) the honey bees’ immune system becomes suppressed and the bees become more susceptible to the disease. Honey bees that show symptoms typically have deformed wings (often crumpled or greatly reduced), and shortened abdomens, paralysis may also be observed in some cases. The virus is also associated with reduced life span, patchy brood and reduced colony populations. DWV is known to be spread by the Varroa mites and by Tropilaelaps mites. The virus can also be spread between bees in food and faeces, or vertically transmitted from the queen or drone to their offspring. DWV is present in Asia, Europe, Africa, North America and South America
Slow paralysis virus (SPV)
SPV causes paralysis of the front two pairs of legs with symptoms becoming apparent approximately 10 days after the bee is infected. This usually occurs just a few days before the honey bee dies. Transmission is thought to occur via the ingestion of viral particles between adult honey bees. SPV is also associated with, and transmitted by Varroa mites. Vertical transmission of the virus from the queen or the drone to their offspring has not been recorded. Slow Paralysis Virus has been reported from Britain, Fiji and Western Samoa.