The honey bee and the varroa mite

For many years now honey bees have been under attack by a little mite known as Varroa destructor. Infestation of the varroa mite has now become an epidemic of global proportions where it plagues bee colonies in many parts of the world- threatening to destroy entire honey bee populations in the western world if left untreated. Originally this mite was confined to only the Asian honey bee Apis cerana however, its shift to a new host the western honey bee Apis mellifera is what has left devastation in many colonies worldwide. The damage caused by this little mite is considered to be one of the crucial drivers for the periodic colony losses that were observed in the USA and Europe during 2006 and it is currently considered to be a major threat to the important role honey bees play in agriculture worldwide.

Unfortunately despite the strict regulations of New Zealand biosecurity that have not permitted live bee imports into New Zealand for at least the last 40 years the varroa mite has still made its way onto our shores. It was first discovered in the north island in 2000 and sadly it has more recently been found to be established in the south island also. While it is unknown exactly how the mite arrived in New Zealand, speculation suggests it most likely arrived with either an illegal introduction of queen bees from a varroa infested country, or in a bee colony swarm that established on or in a shipping container which survived the journey to New Zealand without detection.

In short the varroa mite is an external parasite of honey bees that attacks adult bees and their developing larvae, or young. It can both weaken and kill honey bees of managed hives and it has also been seen to devastate feral and wild colonies as shown in Europe where feral colonies are now extremely rare. Female mites are larger than their male counter parts at all developmental stages in their lifecycle and are fairly large in size- roughly 1.1 x 1.6mm. They can be identified by their reddish to dark brown body’s that are hard, flattened and oval in shape.


The varroa mite is very closely linked to its honey bee host and as a result it does not have a free living stage in its lifecycle. Except for distribution by swarms or foragers the mites spend the majority of their lifecycle within the dark of the honey bee nest, preferably within sealed brood cells. The lifecycle of the varroa mite females consists of two distinct phases- the phoretic phase on adult bees and the reproductive phase within the sealed drone and worked brood cells. On the adult bees the female mites are usually hidden under the sternites of the bee where they are transported to the brood cells for their reproduction or they are spread to new hosts by foraging and swarming bees. In contrast the males and nymphal stages of the mite are short lived and they can only be found within the sealed brood cells.

The individual honey bees are damaged in a variety of ways however it is the developing larvae and pupae that are the most sensitive stage to varroa mite attack. The mites impact on the bees by sucking substantial amounts of hemolymph from both the adult bees and from the host stages within the sealed brood cells. This can cause weight losses in the bees which can later be associated with a significantly shorter lifespan and evidence has shown that prolonged absences from the colony and lower rates of return to the colony may be associated with a reduced ability to navigate. Furthermore, it is a vector for various honey bee viruses including the kashmir bee virus, sacbrood virus, acute bee paralysis virus, Israeli acute paralysis virus and the deformed wing virus. These are spread through the direct injection of virus particles into the hemocoel of honey bee pupae and activation of latent virus infections through the additional injection of foreign salivary proteins.

The final breakdown of a honey bee colony is associated with the typical ‘‘parasitic mite syndrome” such as scattered brood, crawling or even crippled bees and unexplainable reduction of the bee population. The damage threshold however is not correlated with a fixed number of mites per colony. It is instead highly variable and depends on the bee and brood population, the season and the presence of bee viruses. The question has also been raised on the possibility of multiple factors acting together as a cause for bee damage or colony losses. These can include the integration of factors including pathogens, environmental factors such as pesticide use and climate all of which have been associated with the infamous so called colony collapse disorder where the exact cause of the sudden high colony mortality in the US in the year 2006 has still not been ascertained.

Unfortunately, once the varroa mite has become established in a colony it cannot be eradicated. Instead beekeepers in infected areas need to monitor the mite levels within their hives and take action before mite numbers rise to damaging levels. One possible way of doing so is by using various organic and inorganic miticides and research is also been carried out on biological control options and selecting for bees with tolerance to mite infestations.  

Visual examination of infected hives is not an effective way to monitor for varroa however if the following signs can be observed it suggests that the varroa mite has most likely become established

• Unexpectedly low bee numbers
• A patchy pattern on brood frames as would be seen with a heavy sacbrood infestation
• Small reddish-brown mites on the bodies of bees, and on uncapped drone pupae
• Weak crawling bees, possible with deformed wings
• Sudden hive population crashes, or hives being found in autumn with honey stores but no bees 

A recently updated varroa control manual contains detailed information on varroa management and is available from the National Beekeepers Association website.

Interesting until the year 2000 the varroa destructor was thought to be varroa jacobsoni and therefore many of the varroa article from the last century refer to varroa jacobsoni even though varroa destructor was the research topic. 

A few definitions

Phoretic- A relationship between two organisms

Sternite- A sclerotized plate forming the sternum of a segment

Brood cell- A cell in bee comb used for the rearing of a larva

Hemolymph- A liquid equivalent to blood in insects

Ministry of Agriculture and Fisheries. (2009). Varroa mite. Retrieved on May 10^th 2011 from http://www.biosecurity.govt.nz/pests/varroa

Rosenkranz, P., Aumeier, P. & Ziegelmann, B. (2010). Biology and control of varroa destructor. Journal of Invertebrate Pathology, 103, S96-S119.