By Morgan Roth

Imagine a pest that can invade honey bee hives, spread numerous diseases, deplete honey bee nutrition stores, ravage the honey bee immune system, and multiply rapidly: meet the Varroa destructor mite.

These destructive pests are responsible for heavy economic losses, caused by their infestation of beehives in almost every corner of the globe. But how did this problem start, what does this pest do, and what does the future look like for honey bees? These questions are answered in detail in a new article on the biology and management of Varroa mites, published in January in the open-access Journal of Integrated pest Management.

Varroa Mite History

Varroa mites were first documented by A. C. Oudemans in 1904 in mainland Asia. In their native habitat, Varroa mites do not seriously impact their Asian honey bee (Apis cerana) hosts. Varroa mites can only reproduce in Asian honey bee drone brood, and these bees are very efficient in removing mites and infected brood. However, in the 1970’s Varroa mites made their way to Europe and reached North America by 1987, with disastrous consequences for European honey bees (Apis mellifera), the species now domesticated and used to managed crop pollination services around the world. In European honey bee colonies, Varroa mites were able to reproduce in both drone and worker brood, and these bees were inefficient in mite removal.

Varroa Mite Biology and Damage

Figure 1: In its traveling stage, a Varroa mite female hitches a ride on the thorax of a European honey bee (Apis mellifera). (Photo by Morgan Roth, originally published in Roth et al 2020, Journal of Integrated Pest Management) Figure 2: A Varroa mite’s reproductive stage follows six steps: (1) a female mite enters a bee brood cell and (2) hides in brood food until (3) the prepupa stage, when feeding begins. (4) Laying of a male egg takes place after approximately 60 hours, followed by female eggs every 30 hours. (5) Mites undergo two developmental stages, reach maturity, and mate. (5) The mother and mature daughter mites then exit on adult bee. (Image by Morgan Roth, originally published in Roth et al 2020, Journal of Integrated Pest Management) Figure 3: Varroa mites can transmit deformed wing virus to European honey bees (Apis mellifera), with symptoms including deformed wings as seen in the worker bee shown at right here. (Photo by Morgan Roth, originally published in Roth et al 2020, Journal of Integrated Pest Management)

The Varroa mite has a unique lifecycle that takes place in two stages: a traveling stage (Figure 1 above) where adult female mites are transported on adult bees, and a reproductive stage (Figure 2). When a honey bee larva is about to enter the prepupal stage, a traveling female mite will enter the uncapped cell and hide. Once the cell is capped, she pierces a hole in the prepupa, begins feeding, and lays an unfertilized male egg followed by fertilized female eggs. Varroa mites go through two developmental phases prior to adulthood, after which the male mite will mate with any mature sister mites. When the bee pupa emerges as an adult, the mother mite and any mated daughter mites exit on the bee. Adult female Varroa mites are easily recognized by their reddish-brown color and are approximately 1.6 millimeters in length. They are also covered in bristling hairs called setae, which help them stay attached to their hosts as they travel and feed.

Until very recently, it was assumed that Varroa mites fed on bee hemolymph (blood), but it is now known that they feed on bee fat bodies. The honey bee fat body functions similarly to the mammalian liver, as it is responsible for detoxification. The fat body also plays a role in the bee immune system and nutrient storage. Varroa mites also spread various diseases, the most prevalent of which is deformed wing virus (Figure 3). Signs of deformed wing virus include shriveled wings, abdominal bloating, learning problems, and shortened lifespans, which damage the whole colony through loss of workers. Fat body depletion and disease prevalence are likely both responsible for the heavy overwintering losses that are now observed in most apiaries.

Varroa Mite Management

After the initial spread of Varroa mites to European honey bee colonies, synthetic acaricides were used to treat these infestations. However, these chemicals can detrimentally affect bees and buildup in hives. Additionally, overuse of acaricides has led to observations of acaricide-resistant Varroa mite populations. To combat acaricide resistance, it is vital that beekeepers follow Integrated Pest Management (IPM) methods. IPM methods take mite infestation levels into account, comparing them with an economic threshold prior to use of acaricide treatments. IPM methods start with the use of the least invasive methods and then move to more invasive treatments. These management methods fall under four general categories (Figure 4, above):

Cultural controls are the least invasive methods that can help limit Varroa mite success. Important cultural controls include sanitary practices, such as comb culling, not overcrowding hives, and choosing hive locations with good drainage and sufficient sunlight. Use of entrance reducers and hive decorations can help limit spread of mites between colonies by limiting drift, and breeding hygienic bees also falls within this category.

are the least invasive methods that can help limit Varroa mite success. Important cultural controls include sanitary practices, such as comb culling, not overcrowding hives, and choosing hive locations with good drainage and sufficient sunlight. Use of entrance reducers and hive decorations can help limit spread of mites between colonies by limiting drift, and breeding hygienic bees also falls within this category. Mechanical controls are slightly more invasive than cultural controls, especially drone brood removal, which is one way to trap and remove mites. Screened bottom boards and brood comb modification/rotation are also used as mechanical controls. Screened bottom boards are often used with other treatments, as they improve hive ventilation and ensure that any falling mites exit the hive.

are slightly more invasive than cultural controls, especially drone brood removal, which is one way to trap and remove mites. Screened bottom boards and brood comb modification/rotation are also used as mechanical controls. Screened bottom boards are often used with other treatments, as they improve hive ventilation and ensure that any falling mites exit the hive. Biorational acaricides include several organic acids, thymol, and several essential oils. Popular organic acids include formic, oxalic, and lactic acid treatments, which require specific seasonal conditions when used. These acaricides will rarely build up in wax and hive products over time, and formic acid is the only organic acid that will kill mites in capped brood cells.

include several organic acids, thymol, and several essential oils. Popular organic acids include formic, oxalic, and lactic acid treatments, which require specific seasonal conditions when used. These acaricides will rarely build up in wax and hive products over time, and formic acid is the only organic acid that will kill mites in capped brood cells. Synthetic acaricides can be used if mite populations exceed economic thresholds, and include Apistan (tau-fluvalinate), Checkmite (coumaphos), and Apivar (amitraz). It is important to use these compounds sparingly, as they can build up to harmful levels in the hive.

To avoid resistance development, rotation of treatment types is also encouraged when using an IPM model. It is hopeful that implementation of IPM methods will lead to a future where bees are encouraged to develop their own defenses and Varroa mites are kept in check through effective, informed, and data-driven management choices.

Morgan A. Roth, M.S., is a Ph.D. student in the Molecular Physiology and Toxicology Laboratory at Virginia Tech, working with advisor Aaron D. Gross, Ph.D., in Blacksburg, Virginia. Email: mroth11@vt.edu.