How Much Honey Does Varroa Cost You? New Research Provides Answers

More than a quarter-century since Varroa destructor was first detected in New Zealand’s beehives, the country’s beekeepers are well aware of the importance of controlling the parasitic mite. Now though, new research from Victoria University of Wellington has correlated varroa infestation levels in spring with honey production, determining even small reductions in populations early in the season can reap significant rewards at honey harvest.

Phil Lester’s research has found that a threshold of two varroa mites per 100 bees in spring is likely the point at which reducing levels will be economically worthwhile for most beekeepers, “but if you can get them as low as you possibly can it is for your benefit”.

For every extra mite per 100 bees in October, beehives in the study spanning North and South islands were found, on average, to reap 2.29kg less honey 18 weeks later.

“That’s what the math tells us and that’s substantial,” says Prof. Phil Lester, who led the project, conducted alongside Rose McGruddy.

Hives with more than three mites per 100 bees in October produced, on average, 40% less honey than those with lower densities, with the highest starting mite count across the 320 hives being 18.2 per 100 bees.

It’s the first time varroa mite populations have been linked to honey output in New Zealand in published research. The project ran from October 2022 to March 2023, with the full paper only published this year – Spring Varroa destructor (Mesostigmata: Varroidae) density predicts honey production losses and indicates an action threshold in New Zealand apiaries.

The project proposes ‘a spring action threshold of approximately two mites/100 bees as a pragmatic and biologically defensible target for commercial apiaries’. New Zealand beekeepers often measure mite populations in samples of 300 bees, in which case the new research advocates considering a six mite threshold in spring time.

 Beekeepers can use mite washes to monitor varroa infestation levels and now have added research to support decision making based off the results.  

“That two mite per 100 bees is the sort of area where, if you are getting eight or nine dollars a kilogram for your honey, it about equates to a treatment cost. So, if you do it then, you will get an economic return,” Lester explains.

At that pricing, of $9/kg, the reduction in just one mite per 100 bees in spring time could assist in approximately $20 in extra honey revenue. When considering higher value honey crops – and potentially much higher spring mite populations – the research puts the impact of varroa and its resulting reduced returns to honey producers in greater context.

“One of the questions one of the reviewers of the paper asked was, ‘is there some sort of density that your models will tell you that you won’t lose honey?” Lester says.

“So, we went back to looking into what happens if, in October, you have one quarter of a mite, one per 400 bees. And there were still effects. You will still lose honey at that rate. But, there has to be a trade-off there between the cost of applying treatment and the return you get.”

The Project

Previously research into varroa’s impact on honey bee colony health and production has focused on autumn populations Lester says.

“How varroa populations in spring influence honey production has been relatively understudied, but I think it is important. What we see in mite populations wherever you go, when reviewing the literature, is exponential growth. So, it makes sense that, if we see that in New Zealand too, it is going to have influence on your honey production,” he says.

Lester and McGruddy’s project benefitted from the university’s research into RNA-interference to control varroa which has led to the now commercially available (in the USA) Vadescana product Norroa. That research provided the 320 hives across two regions, 160 in Halcombe in the Manawatu and 160 in Ashburton in Mid-Canterbury. The mite densities were much higher in the  four apiaries in Ashburton, ranging from 0 to 18.2 mites/100 at trial start as compared to the Halcombe hives where the range was 0 to 1.2 mites/100. By week 12 and mid-summer those populations had risen to 0 to 88.9/100 and 0 to 8.8/100 respectively, further illustrating a much lower population in the North Island portion of the project.

Beekeepers are well aware of the threat of varroa, but new research has sought to put a figure on the lack of production resulting from the parasite’s infestation of honey bee colonies. Photo Peter Bray.

The hives had no treatments for four weeks ahead of the trial start date, and within the trial period there was a mix of no treatments, amitraz and the Vadescana being developed.

Lester says the treatments the hives were subject to is inconsequential to the model they sought to create.

“What mattered most was the end result of those treatments on mite populations. You have a certain density of mites, and here’s how the honey production is influenced as a consequence.”

Honey supers were weighed at 18 weeks to determine colony production.

How Varroa’s Impact Compares

Alongside varroa-loading, colony strength was assessed at the beginning of the trial, through brood area and adult bee abundance. Results determined that the frames of adult honey bees showed ‘a positive but nonsignificant association’ with honey production. Whereas, for every extra frame of brood present in October the result was more marked, with a 2.68kg increase in honey production observed.

Ultimately Lester and McGruddy’s research determined varroa’s impact on honey yield to be 4.7%, highlighting the vast array of factors influencing a honey bee colony. Lester admits he would have liked the relevance of varroa on production to be higher, but when compared to the brood area’s 2.2% contribution it puts varroa’s influence into greater context.

“There are so many other factors coming into play here so there is that variation. Therefore, we are not saying controlling varroa is guaranteed to get you lots of honey at all. But, it is one of the most influential things you can modify in your hive to get you a return,” the professor explains.

“I would love to assure beekeepers that they are always going to see that increased production, but we need to couch it and say, ‘you are much more likely to get increased honey production and colony health due to reduced varroa densities’.

“The critical thing is, varroa grows in exponential fashion. You might have very few mites, but a small difference in spring means a large difference in autumn.”

Experience over a quarter of a century will have taught many Kiwi beekeepers that, especially in recent years where reported hive deaths due to varroa have increased. Now though, they have a more precise understanding of the challenge and, with it, greater assistance in both hive and business management decisions.

“The critical bit is to keep your mites as low as you possibly can in spring. The difference between two mites per 100 bees or one mite, in spring, can result in a lot of money. They are going to grow exponentially and they are going to be a problem,” Lesters says, adding “while it might not sound like a lot of mites, if you can get them as low as you possibly can, it’s for your benefit”.