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.

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 feeds primarily on the bee's fat body, an organ analogous to a liver, rather than on haemolymph as was believed for decades — which explains why the damage to immunity and overwintering ability is so much greater than the volume of fluid taken would suggest. Second, a colony that looks strong in autumn can be dead by February, because the mites damaged the long-lived winter bees the colony's survival depends on. Untreated colonies commonly die in their second or third winter rather than immediately.

How it is managed

• Monitoring first. Alcohol wash, sugar roll or drop counts on a sticky board give a mite load per hundred bees. 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.

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 and 2025 in ways worth stating with a date on them. This section describes the position as verified on 20 August 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.

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.

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, now established at the eastern edge of Europe 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.