A survey by Landcare Research estimates that 6.4 percent of bee colonies are lost due to the Varroa Destructor mite, demonstrating that its destructive effects are escalating. The annual survey attributed nearly half of all colony losses to the mite and related complications over the 2022 winter. This is a 20 per cent increase from the 2021 rate of 5.3 percent – the year that first saw Varroa become the leading cause of bee losses.
Varroa mites are small parasites that live on honey bees. They feed and reproduce on larvae which can spread viruses, cause malformation and shorten bees' lifespans, eventually destroying colonies. Since the mite appeared in New Zealand in the early 2000s, more and more bees have been affected.
New Zealand has done exceptionally well trying to slow varroa's spread over the past 23 years. This was in part by ensuring only effective treatments had Agricultural Compounds & Veterinary Medicine (ACVM) approval for use. But it is now endemic and seems to be building momentum, making it more destructive for hive populations. The density of hives can exacerbate varroa's spread, and failing to treat is fatal.
Bee with varroa riding on its thorax
Bee colonies in the upper North Island were historically more likely to be affected due to hive density. Regional loss rates over winter 2021 exhibited enormous variation, but losses over winter 2022 hovered close to the national average, ranging from 11.5 percent in the middle of the South Island to 15.5 percent in the upper North Island and lower South Island.
One positive trend from the survey is that varroa management rates have markedly improved. Beekeepers who reported not treating for varroa at all in the 2021-2022 season dropped to 1.5 percent, compared with 4.4 percent the year before. Informal record keeping among some small beekeepers may make varroa management more challenging, according to the report.
It is essential that beekeepers remain highly vigilant for signs of varroa in their hives. Treating varroa with miticides - at the right time and with the right dose - then becomes vital for the survival of the hive.
The chemical treatment for the varroa mite involves placing miticide strips into the hive. They are highly toxic for mites yet totally safe for honey, beeswax, brood, and queen. They don’t leave any residue in hive products and will keep the hive healthy. The bees walk on the strips and distribute the miticide around the hive. Eventually, the mites will be unable to cling to their bee hosts and will fall to the bottom of the hive. Varroa mites often hide in the brood cells of developing bee larvae, where they feed on the larvae and reproduce. Some miticide strips are designed to penetrate the wax cappings of the brood cells, allowing the chemicals to come into direct contact with the mites inside the cells. This helps to target mites that are reproducing in the brood cells and can be more challenging to control using other methods.
It is recommended that beekeepers alternate treatment between the different chemical families to manage the potential for chemical resistance by the varroa mite.
It’s also important to get the dose and timing of the treatment right - generally spring and autumn. A good rule of thumb is to follow the manufacturer’s instructions and beekeeping best practices.
Animal and Plant Health NZ and its members are committed to supporting the New Zealand bee population by providing effective treatments for Varroa and continuing to support best-practice farming. Bees are extremely good pollinators of crops and contribute substantially to New Zealand’s $6.7 billion horticultural sector. It is vital that beekeepers continue their essential work in protecting bee populations, producing honey, and helping to contribute to farming.
