Varroa destructor

There is some evidence that harm from both Varroa mite and associated viruses they transmit may be a contributing factor that leads to colony collapse disorder (CCD).[2] While the exact causes of CCD are not known, infection of colonies from multiple pathogens and interaction of those pathogens with environmental stresses is considered by entomologists to be one of the likely causes of CCD.[20][21] Most scientists agree there is not a single cause of CCD.[22]

Beekeepers use several methods for monitoring levels of Varroa mites in a colony.[23] They involve either estimating the total number of mites in a hive by using a sticky board under a screen bottom board to capture mites falling from the hive or estimating the number of mites per bee with powdered sugar or an ethanol wash.[24]

Monitoring for mites with a sticky board can be used to estimate the total number of mites in a colony over 72 hours using the equation:

${\displaystyle p(c)={\frac {3.76-b}{-0.01}}}$

where b is the number of mites found on the sticky board and c is the number of estimated mites in the colony. However, the bee population in a colony also needs to be known to determine what population of mites is tolerable with this method.[24]

Mite counts from a known quantity of bees (i.e., 300 bees) collected from brood comb are instead often used to determine mite severity. Mites are dislodged from a sample of bees using non-lethal or lethal means. The bees are shaken in a container of either powdered sugar, alcohol, or soapy water to dislodge and count mites. Powdered sugar is generally considered non-lethal to honey bees, but lethal methods such as alcohol can be more effective at dislodging mites.[25][24] 3% of the colony being infested is considered an economic threshold damaging enough to warrant further management such as miticides, though beekeepers may use other management tactics in the 0–2% infestation range to keep mite populations low.[24]

Varroa mites can be treated with commercially available acaricides that must be timed carefully to minimize the contamination of honey that might be consumed by humans. The four most common synthetic pesticides used for mite treatments with formulations specific for honey bee colony use are amitraz, coumaphos, and two pyrethroids, flumethrin and (tau-fluvalinate), while naturally occurring compounds include formic acid, oxalic acid, essential oils such as thymol and beta acids from hops resin (e.g. lupulone). Many of these products whether synthetic or naturally produced can negatively affect honey bee brood or queens. These products often are applied through impregnated plastic strips or as powders spread between brood frames.[24]

Synthetic compounds often have high efficacy against Varroa mites, but resistance has occurred for all of these products in different areas of the world. Amitraz (e.g., Apivar strips) will typically kill 75–90% of mites, but is often not very affordable. Use of coumaphos has decreased over time due to low efficacy and resistance. Pyrethroids are used because a concentrations that will kill mites have relatively low toxicity to honey bees. Naturally-occurring compounds such as formic acid can range 35–75% mite mortality with efficacy varying based on temperature, brood population, and proximity to the chemical within the hive. Formic acid can also cause brood or queen mortality during high temperatures. Oxalic acid is used during broodless periods in the hive; it increases grooming behavior in bees and increases mite dislodging, and there are no known cases of Varroa mite resistance to this chemical.[24]

Compounds derived from plants have also been assessed for mite management. Thymol is one essential oil with efficacy against mites that varies, similar to formic acid, with temperature and brood population as well as being harmful to bees during high temperatures. Other essential oils such as garlic, oregano, and neem oil have had some efficacy in field trials, though most essential oils that have been tested have little to no effect. Essential oil use is widespread in hives with many of those uses being off-label or in violation of pesticide regulations in various countries. Hop beta acids are lupulones obtained from hop plants and have been used in products marketed for mite control. They have low toxicity to humans and bees, but efficacy is mixed ranging 43–88% mite mortality.[24]

Varroa mites can also be controlled through nonchemical means. Most of these controls are intended to reduce the mite population to a manageable level, not to eliminate the mites completely.[24]

Screened bottom boards are used both for monitoring and can modestly reduce mite populations by 11–14%. Mites which fall from the comb or bees can land outside the hive instead landing on a solid bottom board that would allow them to easily return to the nest.[24]

Varroa infest drone cells at a higher rate than worker brood cells, so drone cells can be used as a trap for mite removal. Beekeepers can also introduce a frame with drone foundation cells that encourage bees to construct more drone cells. When the drone cells are capped, the frame can be removed to freeze out mites. This labor intensive process can reduce mite levels by about 50–93%, but if trap cells are not removed early enough before mites emerge, mite populations can spike. This method is only viable in spring and early summer when drones are produced.[24]

Heat is also sometimes used as a control method. The mites cannot survive temperatures near 40 °C (104 °F), but brief exposure to these temperatures do not harm honey bees. Devices are marketed intended to heat brood to these temperatures, though the efficacy of many of these products has not been reviewed.[24][26]

Powdered sugar used for estimating mite counts in hives has also been considered for mite management as it or other inert dusts were believed to initiate grooming responses. Long-term studies do not show any efficacy for reducing mite populations.[24]

Researchers have been able to use RNA interference by feeding honey bees mixtures of double-stranded RNA that reduced Varroa mite infestation up to 50%.[27][28]

Efforts also have been made to breed hygienic honey bees, such as Varroa sensitive hygiene with resistance to Varroa mites.[29][30][31] Hygienic behaviors include:[29]

• Workers removing pupae heavily infested with mites, which kills both the developing bee and immature mites.
• An extended phoretic period in adult female mites.
• Grooming or removal from the brood cell increases adult mite mortality.
• Mites removed from host pupae are at an incorrect life stage to re-infest another pupa.

Hygienic behavior is effective against diseases such as American foulbrood or chalkbrood, but the efficacy of this behavior against mites is not quantified well; colonies with this behavior alone do not necessarily result in Varroa mite resistant colonies that can survive without miticide treatments. There are minimal trade-off costs to hives that have this behavior, so it is being actively pursued in bee breeding programs.[29]