Varroa and the health of a honey bee colony

What varroa is

Varroa destructor is an external parasitic mite about 1.5 mm across — large enough to see, and enormous relative to a bee. Its original host is the Asian honey bee, Apis cerana, with which it has coexisted for a very long time and which has behavioural defences against it. Sometime in the twentieth century it jumped to the western honey bee, Apis mellifera, which had no evolved defences at all, and it has since spread to almost every part of the world where honey bees are kept. Australia held out until 2022.

The mite reproduces inside sealed brood cells. A female enters a cell containing a larva just before the workers cap it, lays eggs, and her offspring feed on the developing bee. When the bee emerges, the mites emerge with it. This is why varroa populations track brood rearing, and why a colony's mite load grows exponentially through a season.

The life cycle, step by step

The WOAH diagnostic standard describes two phases. In the dispersal phase the female rides on adult bees, mostly nurse bees, feeding on them. In the reproductive phase she enters a brood cell just before it is capped. Her eggs begin developing 26 to 30 hours after capping; the first she lays is a male, followed by up to five or six females about 30 hours apart. The young mate inside the cell, and the mated daughters leave with the emerging bee — one to two from a worker cell, two to three from a drone cell, which is why the mite prefers drone brood and why drone brood is where a beekeeper looks for it.

Two consequences follow. A mite in sealed brood is out of reach of anything applied to the adult bees, so a colony with a lot of brood hides a lot of its mites. And a colony's mite load can rise from unnoticeable to damaging within a season, because each cycle multiplies it.

Why it is worse than a parasite should be

A mite feeding on a bee is damaging. A mite acting as a hypodermic needle between bees is catastrophic, and that is the real mechanism. Varroa transmits and amplifies viruses — deformed wing virus above all — and it does so in a way that bypasses the bee's ordinary defences. Colonies collapse from viral disease vectored by varroa far more often than from the direct feeding damage.

Two corrections worth making explicitly. First, varroa does not only drink the bee's blood. For decades it was described as feeding on haemolymph; a 2019 study concluded that it feeds primarily on the fat body, an organ analogous to a liver, and the WOAH standard now names both haemolymph and fat body. That helps explain why the damage to immunity and overwintering is so much greater than the volume taken would suggest. Second, a colony that looks strong in autumn can be dead by late winter, because the mites damaged the long-lived winter bees the colony's survival depends on. WOAH puts the collapse of an untreated colony at one to four years.

How it is managed

• Monitoring first. An alcohol wash or sugar roll of a measured sample of adult bees gives a mite load per hundred bees; a sticky board under the colony shows whether the load is rising or falling but not how high it is. Treating without measuring is how resistance develops and how colonies are lost anyway.
• Organic acids — oxalic acid and formic acid — which are effective, cheap and have not generated widespread resistance. Oxalic acid works best when there is no sealed brood for mites to hide in.
• Thymol and other essential-oil-based treatments, temperature-sensitive but useful in the right window.
• Synthetic acaricides such as pyrethroids and amitraz, which work and to which varroa has repeatedly developed resistance where they have been used as a sole strategy.
• Biotechnical methods: drone brood removal, which exploits the mites' preference for drone cells, and induced brood breaks that interrupt the reproductive cycle.
• Breeding for resistance. Varroa-sensitive hygiene and similar traits are real, heritable and being selected for, and this is the only approach that offers a permanent answer rather than an annual one.

Timing matters as much as the treatment. Mite numbers usually peak in late summer, just as the colony shrinks its brood and starts rearing the long-lived bees that must carry it through winter. Winter bees parasitised during development live shorter lives, and a treatment given after they are reared kills mites without undoing that damage — which is why Britain's National Bee Unit times control to protect the winter bees, not merely to finish before winter. Mites also arrive from outside: collapsing, mite-heavy colonies pass them to their neighbours mainly by being robbed, and drifting bees carry them over short distances, so a colony treated on time can still be re-infested from a neglected one nearby.

The current position, and its date

Varroa biology does not change. What does change is how well the available controls work and how much of a beekeeper's year the mite consumes, and both moved during 2024, 2025 and 2026 in ways worth stating with a date on them. The list below was verified on 20 August 2026; the resistance table under it on 30 October 2026.

• Amitraz resistance is documented rather than anecdotal. A 2025 systematic review of seventy-four studies found resistance recorded in the United States, Mexico, Argentina, Czechia, France and Spain, with resistance ratios spanning a wide range.
• It is patchy rather than regional. The same review found resistance emerging as isolated pockets within apiaries rather than spreading uniformly, which means a neighbour's experience is not evidence about your own colonies and only measurement is.
• United States losses in 2024-25 were the highest recorded since the annual survey began, and — unusually — commercial operations lost more than smaller-scale beekeepers. Respondents managing over half of US colonies most often named varroa as the cause.
• The evidence on Tropilaelaps, the other brood mite, is almost absent by comparison: the same review located a single field study of amitraz against it, reporting no effect.

Australia, four years into living with the mite, added a sharper case in 2026. By 23 April, New South Wales government testing had confirmed a population in New South Wales and southern Queensland, unrelated to the 2022 incursion, resistant to both pyrethroids and amitraz; in May, the Australian Government's key messages reported treatment failures at one apiary in north-east Victoria and in hives linked to South Australia's Pooginook detection zone. The table records each finding with the populations it covers — and records that the Victorian and South Australian findings were read second-hand.

The practical consequence for a beekeeper is the same as it has always been, only more so. Monitor rather than assume, treat on a measured mite load rather than a calendar, rotate active ingredients rather than leaning on one, and use the non-chemical methods — brood breaks, drone brood removal — that resistance cannot erode. A treatment that worked last year is a hypothesis about this year.

Where the map is still moving

Most of the world's beekeepers have lived with varroa for decades, which makes it easy to forget that its map is still being drawn. Australia was the last major honey-producing country free of it. The mite was found in surveillance hives at the Port of Newcastle in June 2022; in September 2023 the national management group judged eradication no longer feasible; and the national transition-to-management programme closed on 6 February 2026. The table below is the position by state as the official sources state it.

The Australian case compresses thirty years of European experience into four. Queensland's own account records not only varroa but resistant varroa across many council areas within about a year of its first detection there, and the national programme stopped publishing its New South Wales heat map in September 2025 because falling reports had made it unreliable. That second fact is the more instructive: surveillance data weakens exactly when a pest becomes ordinary, which is when a map stops meaning what it appears to mean.

The small hive beetle is the other pest whose European map is moving. An African hive scavenger, it has been circulating in Calabria since September 2014 and in Sicily since 2025, and in June 2026 it turned up on the Italian mainland outside Calabria for the first time — in hives moved illegally about 300 kilometres to Lazio, carrying beetles genetically identical to the 2014 isolate. Its larvae tunnel through comb and ferment stored honey, which is why it is a honey problem as well as a colony one.

The other pressures, in proportion

Varroa is the largest single problem but not the only one. American and European foulbrood are serious bacterial brood diseases, notifiable in many jurisdictions. Nosema is a gut parasite. Small hive beetle is a spreading pest, and Tropilaelaps — a second brood mite, established in southern Russia, officially reported in Türkiye in August 2026 and notifiable in the UK — has its own guide. Pesticide exposure, particularly to systemic neonicotinoids, causes sublethal harm to foraging and navigation that is well documented. And forage scarcity — the monoculture problem — leaves colonies nutritionally stressed and less able to withstand everything else.