Having witnessed the effects of the varroa mite on honey bee colonies across the world and learned from beekeepers about their attempts to control populations, Sebastian Owen, commercial director of Vita Bee Health, has penned three articles on varroa and best management practices. First up, he goes back to basics to assess the nature and impact of varroa on beekeeping worldwide.
No other pest or pathogen of honey bees is known to have such a massive impact as the varroa mite, Varroa destructor. The impact has been so sudden and so large because the parasite jumped from Apis cerana, the eastern honey bee, which had built up defences to the mite over a very long period of time, to the defenceless western honey bee Apis mellifera, the mainstay of global beekeeping.
In a matter of decades, the mite infested colonies of Apis mellifera around the world with devastating results. Only parts of central Africa, some extreme northern territories and some remote islands seem to have escaped the invasion. Australia, varroa-free for so long, is currently attempting eradication after an incursion of varroa mites in New South Wales.
The jump of the parasite from Apis cerana to Apis mellifera probably happened in several places on different occasions, possibly starting early in the twentieth century, reaching New Zealand in 2000 or earlier, having arrived in China (by 1959), India (by 1961), Europe (by 1977) and North America (by 1987). First reports of the presence of varroa mites are usually a significant time after their initial arrival, as New Zealand discovered and as Australia may now be discovering after the finds around the Port of Newcastle.
The mite’s presence is not immediately obvious until populations build up and in small numbers. It is seldom visible without diagnostic tests or a knock-down agent and varroa screens. The ability of the mite to hide its presence has assisted its very rapid spread and has been aided by beekeeper colony movements and bees drifting between colonies.
In every region that varroa established itself, it has taken a huge toll on honey bee colonies. At first it was thought that the colony deaths were a direct effect of the mite, but in the 1990s it was discovered that viruses transmitted and multiplied by the mite were the real culprits. Often top of the list are deformed wing virus (DWV) and acute bee paralysis virus (ABPV). These were not new to honey bees, but with the arrival of varroa their impact soared to unprecedented levels.
Initially it was thought that the mite fed off the bee’s haemolymph (blood), but recent evidence indicates that it feeds off the bee’s fat body. In any event, varroa is a very effective vector of many viruses, and the ‘injecting’ of these viruses has accelerated their spread within and between colonies, turning previously low-impact viruses into colony killers.
As every beekeeper managing colonies with varroa notices, all sorts of changes become apparent, and the general health of the colony declines, with apparently minor viruses becoming more visible. The immune system of the bee is being compromised, resulting in all sorts of manifestations.
Varroa mite characteristics
The tiny mite, oval, flat and red-brown in colour, measures just 1.1mm by 1.6mm and is seldom visible on the body of the bee because it tucks itself away, hidden in the folds of the abdomen. If the mite is visible on bees, even in small numbers, it is likely that the colony is already severely infested.
The female mite enters a cell containing a larva about five-days old, the cell is capped, the mite feeds on the pupa and lays up to six eggs. The developing mites feed on the larva, damaging it and exposing it to pathogens. Adult females leave the cell with the emerging bee while the males and immature mites remain for a time. Drone cells are particularly favoured by the mite because of the additional days that adult drones take to emerge through the capping.
The hidden danger develops
As the season progresses, the varroa infestation can grow at an alarming rate, so that by late summer or autumn the population on adult bees (which are decreasing in number) can increase dramatically. In the UK, a ‘varroa calendar’ has been produced to show the potential risk that population threshold levels can present at any time in the season – even a natural drop of a few mites in spring (as measured by counting the mites on a varroa screen over a fixed period) can presage an existential threat to a colony by the end of the season if treatment is not applied.
Bee defences
Over a very long period of time, Apis cerana has developed defence mechanisms against varroa, alongside which it can now co-exist. For example, its worker brood is highly sensitive to a toxic protein from the mite and this counter-intuitively limits the successful reproduction of varroa in drone larvae because the larvae die and disrupt the varroa mites’ reproductive cycle. Apis mellifera isn’t so sensitive to the toxin, so reproduction continues apace. Apis cerana also appears to have developed several other defences including grooming techniques that inhibit varroa. Generally, its own lifecycle is not so conducive to varroa development.
Unfortunately, Apis mellifera is far behind in its evolutionary response to the mite and how quickly it can respond is debated. Meanwhile, the initial impact of varroa on honey bee populations has invariably been devasting with a large percentage of colonies being wiped out within a few years of the parasite’s arrival. Honey harvests have been very badly hit, as any beekeeper who has lost colonies will verify, and so too has agricultural pollination, although figures on this are difficult to evaluate.
Varroa’s spread through New Zealand
The first reporting of the varroa mite – in Auckland on 11 April 2000 – was immediately followed by a search of the area which indicated that the mite had already been present, undetected, for three to five years having possibly been brought in through an illegal import of queen/s or perhaps accidentally by ship. Three zones – named ‘infected’, ‘buffer’ and ‘surveillance’ zones – helped prioritise the response, but it was suspected that beekeeper hive movements had already spread the mite considerably. Quickly it was realised that eradication was not feasible in the North Island.
By 2004, the mite had been identified in Canterbury and in 2006 in Nelson. Colony destruction and movement controls had no effect in eradicating the mite and, sadly, it was recognised that the mite was in New Zealand to stay, with only a few small islands and very isolated pockets left unaffected.
The impact
Many businesses did recover from the arrival of varroa as they were assisted by the mānuka honey boom, but since that boom the finances of the industry have become stretched and beekeepers have looked for cheaper varroa control treatments, DIY treatments or even no treatment, with many hives being abandoned.
The full economic impact, though a little delayed, was considerable. Both registered hive numbers and commercial beekeeping enterprises have reduced in recent years as New Zealand beekeepers come to grips with a drop in honey prices.
Between 2015 and 2021, the percentage of colonies lost over winter increased year on year and by 2021 beekeepers were attributing losses mostly to the mite, rather than queen problems as previously (as per New Zealand’s Colony Loss Survey).
Although once well established, the varroa mite can never be eradicated from Apis mellifera colonies, its population numbers can be controlled by treatments – so long as manufacturers’ instructions are followed meticulously and IPM (integrated pest management) techniques to alternate treatments from year to year are used to delay or avoid the development of resistance to the active ingredients of the medications.
Therefore, in part two of this topic next month, I will explore the effectiveness of treatments, how to use them effectively, treatment times and how to avoid or at least substantially delay the development of resistance.
