I have been assigned the task of reporting to you about the evolution of the species known as the Varroa mite. Below I have assembled an informative essay giving you all the important information you will need to know about this very devilish organism. Please be sure to use it however need be. Thank you.
The Varroa Mite is known to be one of the most destructive taxons known to bees, hence its alternative name, the Varroa destructor. Before getting into detail about this specific taxon, it’s important that you understand the nature of the phylum it belongs to. The Varroa Mite belongs to the phylum Arthropoda – the largest phylum in the animal kingdom which is the phylum of about 84% of all known species (Barnes, 2017). The Arthropoda phylum is comprised mostly of insects but also of some crustaceans such as lobsters and crabs (Barnes, 2017). The most distinguishing feature of arthropods is their exoskeletons which are primarily composed of chitin – a complex sugar (polysaccharide) secreted by the underlying epidermis (Barnes, 2017). Their bodies are strongly segmented meaning it’s fairly easy to spot where each main external body part begins and ends (Barnes, 2017). Arthropods also bear jointed appendages on their segment(s) which range in number and size (Myers, 2001). Most arthropods tend to carry a pair of compound eyes accompanied by one to several ocelli which altogether work create an enhanced field of view (Myers, 2001). See figure 1 for a common arthropod diagram.
Though Arthropods vary vastly in many of their characteristics, they possess three main characteristics in common, as shown above (“Phylum – Annelida and Arthropoda,” n.d.).
The rest of the Varroa mite’s taxonomic information is as follows:
Kingdom – Metazoa Class – Arachnida Order – Parasitiformes Family – Varroidae Genus – Varroa
Now that the Varroa Mite’s phylum is better understood, there are many distinct physical features which the Varroa Mite possesses that must be noted as it can be useful in distinguishing this mite from other arthropods. Varroa mites are bilaterally symmetrical, microscopic creatures, and when it comes to similarities between male and female Varroa Mites, that’s about it. Adult females mites differ in size compared to the males as they are typically 1.00 – 1.77mm long and 1.50 – 1.99mm wide whereas the adult male mites are smaller and range from 0.75 – 0.91mm long and 0.71 – 0.88 mm wide (“Varroa Destructor,” 2017). Another difference between the two sexes is body shape. Females are curved and oval-shaped making it easy for them to fit in the abdominal folds of the bees which allows them to suck blood in an area protected from the bees’ cleaning habits (“Varroa Destructor,” 2017). Females also have claws that allow them to grasp onto the bee, and ventral setae that helps them remain attached (Ellis & Nalen, 2016). Males on the other hand are more spherically-shaped and lack the more developed traits prevalent in the females as sucking adult bee blood is not very significant in their existence (“Varroa Destructor,” 2017). As if it couldn’t be any easier to tell them apart, they are also distinguishable by colour with the females displaying a reddish-brown colour and the males a yellowish-white (“Varroa Destructor,” 2017). For a detailed image of the Varroa Mite (male and female), see Figure 2.
A side-by-side comparison of the male and female Varroa mites showcasing their different size, shape, and colour (“Bee Mite ID,” 2016).
These nasty mites also have a nasty upbringing. In fact, if you thought you had a less than enjoyable upbringing, just wait until you hear the upbringing and life cycle of the Varroa mite. Firstly, these mites can be found anywhere that their bee hosts are found, and this basically means all over the world. However, Australia and places of arctic climate are the only large areas in the world which Varroa Mites have not yet invaded (“Varroa Destructor,” 2017). See Figure 3.
Figure 3 Distribution map of the Varroa mite. Specific regions where Varroa Mites live are not specified; only the countries they are present in (Ellis & Nalen, 2016).
They are believed to be native to East Asia but have been introduced almost worldwide (“Varroa Destructor,” 2017). Being wherever their bee hosts live means they typically spend most of their lives within bee hives. The Varroa mite has two life stages: reproductive and phoretic (Moore, Wilson, Skinner., 2016). Let’s begin with how they enter the world – the reproductive stage. When a mature female mite is ready to give birth, she jumps off of the host bee which she was feeding on into a vulnerable, uncapped brood cell in the hive (Moore et al., 2016) – particularly those containing drones as they have longer brood cycles (“Exotic Pests,” n.d.). See Figure 4 to observe Varroa mites in uncapped brood cells.
Female Varroa mites (the little brown specs) are spotted on bee larva inside brood cells that have yet to be capped (“Exotic Pests,” n.d.).
After the brood cell is capped by the other bees, the female mite begins to feed on the pupating larval bee (Moore et al., 2016). About three days later, the mother mite lays her first egg which is always a male (unfertilized) (Moore et al., 2016). By laying only one male egg, the mother mite increases the number of female mites she can reproduce in the next generation. This is important because male mites, unlike female mites, cannot survive outside the cell as their only job is to mate, then die (“Varroa Destroyer,” 2017). The 4-5 eggs that come afterwards are all females with each one being laid approximately every thirty hours over the span of about a week (“Varroa Destructor,” 2017). Almost immediately, the developing female mites begin to feed on the pupating bee as well as mate with their brother (Moore et al., 2016). This asexual reproduction occurs by the male mite transferring sperm through hollow tube-like structures stemming from its mouthparts into the female’s openings at the base of her legs (Ellis & Nalen, 2016). This seems to be the only type of interaction that takes place between Varroa mites during their lifetime; after mating in their first few days, they are all on their own for the rest of their lives. Since male mites cannot survive outside the cell, the females must mate with him before the bee emerges or else the females will remain sterile (“Varroa Destroyer,” 2017). Once all sisters have mated with their brother they continue their journey to becoming adults. The male will take 5-6 days to fully mature and 7-8 days for the females – a very r-strategist-like characteristic (“Varroa Destroyer,” 2017). The development of the Varroa mite is dependent on the time it takes for its bee host to develop; consequently not all mites reach maturity before the bee surfaces from the brood cell, and immature females cannot survive outside the brood cell (“Varroa Destroyer,” 2017). It is believed by scientists that the bees’ pheromones or hormones are necessary for the mite to reach full maturity (“Varroa Destroyer,” 2017). The male mite will die before the bee is ready to leave its cell, and the surviving females will emerge from the cell attached to the bee or by climbing out onto another bee (Moore et al., 2016). Here begins the phoretic stage where the pregnant females spend their time feeding on bees, switching hosts (which allows them to transport between different bee hives), and spreading disease (Moore et al., 2016). See Figure 5 to observe a bee in the phoretic stage.
A fully-developed baby bee exits its brood cell with a Varroa mite on its thorax. The Varroa mite is now in the phoretic stage (“Exotic Pests,” n.d.).
Eventually these Varroa mites reproduce will just like their mother did with them. Although they are iteroperous (can produce offspring several times), female Varroa mites only mate once, at birth, because their sperm transport system degenerates after their first mate (Ellis & Nalen, 2016). Depending on the availability brood in a bee colony, the phoretic stage lasts accordingly – therefore if there is lots of brood in the bee hive, Varroa mites will likely return to the reproductive stage, but if there isn’t much brood (such occurs in cold climate) then the reproduction will have to wait until there is brood again (Huang, 2013). The cycle from reproductive to phoretic stage repeats until death and Varroa mites can live anywhere from 1 – 5 months (“Varroa Destructor,” 2017).
The ecological role of the Varroa mite is very simple, and to put it as kindly as possible, their role is to ‘regulate’ the quantity of bees in their ecosystem. However, to put it much more realistically, they do what’s in their nickname which is ‘destroy’ bees. Varroa mites can become a serious problem for bee colonies. If left untreated, Varroa mites can destroy entire colonies of bees; such an event can take place in as little as one year (Moore et al., 2016). When these mites feed on the hemolymph of bees and brood, they often transmit crippling and even fatal diseases that frequently leave their victims with devastating repercussions; such that often weaken and shorten their lifespans (“Varroa Destructor,” 2017). Examples of viruses/diseases Varroa Destructors can transfer to bees are: sacbrood, deformed wing virus, black queen cell virus, chronic bee paralysis virus, acute bee paralysis virus, Kashmir bee virus, Israeli acute paralysis virus, slow bee paralysis virus, etc. (Moore et al., 2016). In 2013, a study was conducted by the USDA-APHIS to find the presence of several viruses in sample bee colonies from 27 U.S. states. The results are shown in Figure 6 below:
A graph showcasing the prevalence of viruses in sample bee colonies examined from 27 U.S. states (Moore et al., 2016).
A decline in bee population as a result of Varroa mite infestations also lead to a direct effect on humans. About one third of human food comes from insect pollinated crops, and about 80% of these crops are pollinated by the European honeybee (“Varroa Destructor,” 2017). Due to lack of adequate pollination from declining bee numbers, the agricultural sector is vulnerable to significantly lower crop yields than usual (“Varroa Destructor,” 2017). In fact, the CSIRO estimated that preventing Varroa mites from entering Australia over the next 30 years would lead to Australian economic savings of $21.3 – 50.5 million per year (“Varroa Destructor,” 2017). Additionally, the activities of Varroa mites have severely affected apiculture. Since the introduction of Varroa mites to New Zealand in 2000, there has been a 50% reduction in the number of beekeepers as a direct result (“Varroa Destructor,” 2017). Thus, those who rely on beekeeping to make a living are forced to give up their livelihoods and look elsewhere for income. Sometimes, beekeepers determined to keep their jobs, will forfeit their organic status by using varroicides (a pesticide that is not approved by organic food certification officials) to combat the infestation problem in the most effective way (“Varroa Destructor,” 2017). As you may be able to tell, this species is no ‘species of concern (vulnerable/endangered)’ but it is definitely a ‘concerning species’. However, there are ways for humans to deal with the problem of Varroa mites, even though these ways may not be as effective as using varroicides. One non-chemical way beekeepers may deal with Varroa mites is by removing all the drones from the colony (because Varroa mites prefer drone blood) (“The Honeybee,” 2010). Another way to combat Varroa mite infestation is to use something called Apistan strips which are to be hung in the brood nest for four weeks (Bessin, n.d.). However the most effective way to deal with them without losing organic food status is by using a synthetic chemical compound that is based on Chrysanthemum flowers which acts as a neurotoxin on these mites without affecting the bees, in the right dosage (“The Honeybee,” 2010). Unfortunately, over the years Varroa Mites have become increasingly resistant to this neurotoxin, so pesticides, like varroicides, seem to remain the most effective option for killing Varroa mites.
In conclusion, Varroa mites are the most threatening and torturous species to bee populations (besides possibly humans) and every beekeeper should assume their bee colonies contain Varroa mites. Once detected, treatment for killing Varroa mites should not be delayed as they reproduce quickly and effectively and can lead to colony collapse. Thank you BRIAN for including my research in your “Encyclopedia of Biodiversity and Evolution” and I hope I have provided the sufficient information you need on the Varroa mite.