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.

A role far beyond that of just honey production.

Who would ever have thought something so small could inflict such fear and caution not only into the lives of small children but also that of grown adults? No matter how old you are you never want feel that unexpected stab of pain that you experience when stung by a bee. When I was little, they sent me running and screaming into the house to mum on more than one occasion and back then I swore black and blue that these little striped devils with wings ruined my childhood playing outside in the garden (we unfortunately lived on an orchard and every year without fail, dad brought bee hives in to pollinate the trees). However, now a little older and a little wiser I realised that in many ways the world would be lost without them.

With the advent of agriculture, a huge variety of plant species have been deliberately spread throughout the world for cultivation. Many of these plants require insect pollination to produce fruit and seed and to achieve this; bees have been introduced to ensure adequate crop pollination and high yields and returns. The honey bee (Apis mellifera L.) in particular is heavily relied on to pollinate a wide variety of crops and today plays a role far beyond that of just honey production. In New Zealand in particular bees are crucial to our primary sector as our countries dependence on horticulture and agriculture is dependant largely on pollination from this little gem. This is true for many other countries worldwide also. They essentially provide what is known as an ecosystem service which can be defined as resources and processes that are supplied by natural ecosystems that benefit humankind.

Since the 1830’s eight species of bee have been deliberately introduced into New Zealand. The honey bee was introduced primarily for honey production but has become the most important insect pollinator of seed, vegetable, fruit crops and pastures. Remaining species such as the bumble bee (Bombus terrestris), alkali bee (Nomia melanderi) and leaf cutting bee (Megachile rotundata) were introduced specifically to pollinate crops such as red clover or lucerne. However, consideration must be given to the potential impact that expanding populations of introduced bees could have on native and exotic flora and fauna. Also worthy of consideration is with the introduction and spread of things like the varroa mite in New Zealand since 2000, it highlights the importance of also considering other species for pollination should pollination services become unavailable through the loss of honey bees.

Research has shown that the more manageable bees such as bumble bee species and leaf cutting bees offer the greatest scope as alternative or supporting pollinators to the honey bee in New Zealand and it has been demonstrated that bumble bees will nest in man-made boxes under New Zealand conditions. It was however stated that further research is needed to determine the exact requirements needed to increase their numbers within agroecosystems. Currently the major limiting factors associated with using the leaf cutting bee for pollination on a wide scale include low bee numbers and its inactivity at temperatures less than 16°C or in windy conditions. This is somewhat problematic for some of New Zealand’s more temperate regions.

Unfortunately some introduced species have been seen to potentially pose a threat to some of New Zealand’s native pollinators which may have implications for the pollination of some of New Zealand’s unique plant life. For example the male of the wool-carder bee is an aggressive defender of territory and has been seen to kill other bee species. However since it was only introduced in 2005 its potential impact on New Zealand’s biota, the possible extent of its range and its potential abundance have not been clearly defined.

Currently there are no plans to introduce any new bee species into New Zealand although a number of species have been identified as potentially useful crop pollinators. However, research on utilising both native and introduced species more effectively within New Zealand’s agroecosystems may lead to more robust and integrated pollination systems that provide more efficient and predictable pollination services. Thus providing security in New Zealand’s agricultural and horticulture industries on a large scale.

This blog was based on an article by:

Howlett, B.G. & Donvan, B.J. (2010). A review of New Zealand’s deliberately introduced bee fauna: current status and potential impacts. New Zealand Entomologist, 33, 92-101.