Toward Effective Long-Term Varroa Control

Being able to kill or reduce the fecundity of varroa mites “under the cap” is crucial to reducing mite loadings long-term according to NOD Apiary Products Ltd (NOD) lead honey bee researcher Dr. Heather Broccard-Bell. The Canadian scientist explains the issue of varroa reproduction and fecundity, citing several small-scale research studies funded by NOD and carried out in Europe under Good Laboratory Practice (GLP) conditions which demonstrate long-term effectiveness of formic acid treatments.

BY HEATHER BROCCARD-BELL

Ever since Varroa destructor emerged as a new honey bee parasite, the world has been on a quest to figure out how to deal with it. At NOD, our mission is to create effective varroa control technology that is safe for the environment and for beekeepers—and that is backed by sound science.

Heather Broccard-Bell.

We often think of scientific research as being simple and straightforward, but in the real world, scientific progress rarely happens at a constant rate. Years can go by without a significant breakthrough. Such has been the case for varroa control research.

Most tests of varroa control techniques have focused on measuring only short-term, immediate effects on varroa numbers in colonies: mite falls, alcohol wash counts, and under the cap kill levels shortly after a treatment has been administered. The data from several longer-term studies indicated that this does not give us the full picture of what was happening to the varroa. So, we started to explore what happens to the mites over a longer term and found varroa population rebound was slower after treatment with our extended-release formic acid products

It led us to the conclusion: that treatment had potentially impacted the ability of the surviving varroa to reproduce. So, what form did the studies take and how is this information pertinent to Kiwi beekeepers? Allow me to explain…

Long-term monitoring: three studies conducted in Europe.

A 2010 study from the UK [1], which began in autumn, tracked mite-fall across three treatment groups of 10 hives each, two which were given a single dose of MAQS and one control group with no initial varroa treatment.

Sixteen days after applying MAQS™, a critical treatment was administered using two strips of Apivar® to capture the varroa the test treatment had missed. This is how efficacy of a test product is calculated.

The results were consistent with what we expected: higher mite falls in the two treatment groups than in the untreated control hives (see Figure 1). That pattern shifts following the critical treatment at Day 16 and by day 20, mite falls in the controls were greater than in either treatment group. Thus, indicating there were more mites left in the untreated colonies relative to the those treated with MAQS. Even 37 days after MAQS treatment, which corresponded to 21 days or one brood cycle after the Apivar treatment, there was still a significantly higher mite fall in the untreated colonies than in either MAQS treatment group.

Figure 1: Higher mite falls were observed in both groups treated with MAQS three days after treatment. The critical treatment (Amitraz) was applied at Day 16. By Day 20, mite falls in the controls were greater than in either treatment group, indicating there were more mites left in the untreated colonies to be killed relative to the those treated with MAQS. Up to 37 days post treatment, mite falls continued to be elevated in the control colonies. Error bars are 95% confidence intervals.

In spring, a full 119 days after the initial treatment with MAQS an additional assessment was performed on the colonies showing (Fig.2) that mite populations were not rebounding in the two groups treated with MAQS, but were in the control group.

So, what could be going on? We did not know for sure. But we had some ideas, such as that our data showcases the importance of treating more than once during the active beekeeping season. Treatment with MAQS and then Amitraz led to a lasting varroa reduction, compared to a sole Amitraz treatment which led to a rebound in numbers.

Two additional studies illuminate an additional potential explanation for long-term varroa reductions in hives treated with MAQS, but first, we need to cover a little bit of background.

Figure 2: At the end of March, the controls had significantly higher mite falls than the MAQS treatment groups, following a critical treatment with oxalic acid. Error bars are 95% confidence intervals.

Targeting varroa where it counts.

“Fecundity” refers to an organism’s ability to reproduce – specifically, how many offspring they produce. The higher an organism’s fecundity, the more offspring they produce, and the faster the population grows.

One of the quirks of varroa biology is the fact that they produce offspring solely under sealed honey bee brood caps. Like honey bees, female foundress varroa mate only once, upon reaching maturity, and mating occurs only under the brood cap with the sole male mite present, her older brother. The male develops from the first egg laid by the foundress, and it takes six or seven days for the male to reach sexual maturity. Although mated female varroa can go through several reproductive cycles, all of the offspring produced will be fertilised with the sperm from this one mating window, which is stored inside the female’s body.

Any treatment that interferes with the activity of the varroa under the brood cap also has the potential to affect varroa fecundity directly, depressing varroa populations over the long run – even in the absence of an obvious, immediate dispersal-phase (phoretic) varroa die-off. We believe the vapours from MAQS are able to penetrate the brood cap, causing high levels of mortality to the varroa males and developing females, as well as to a significant portion of the foundress mites.

Capturing under-the-cap kill.

In September, 2015, another GLP study was conducted in France [2], with a similar methodology to the 2011 study. However, this time varroa numbers were assessed in two ways: sticky board mite falls, and by uncapping brood cells within the colony. Although Formic Pro™ did result in a greater mite drop that peaked four days post treatment, the difference was not particularly large.

Uncapping the brood cells four days post treatment tells a story of more subtle effects – the type that could produce a longer-term reduction in varroa numbers within colonies. When we uncapped the cells we saw that, although the percentage of living mature mites was similar across the two groups, the percentage of living immature mites was quite different (see Figure 3).

Figure 3: Although the proportion of living mature varroa found per 100 uncapped cells was similar across the control and Formic Pro groups, the proportion of living immature varroa was greatly reduced in the Formic Pro® group.

If immature mites were dying in greater numbers when the colony was treated with Formic Pro, it suggests that immature mites are more susceptible to formic acid. It is reasonable to suspect – although these data cannot provide a definitive answer – that of those immatures left alive, they might have been weakened. A final experiment supports this hypothesis.

Penetrating the brood cap.

To Spain and a 2014 study [3] looking at how MAQS performed, once again compared to a control group with no initial treatment. All three groups were subject to a CheckMite® (Coumaphos 10%) 16 days in. (Editor’s note: this product is not registered for use in New Zealand).

Again, sticky boards showed a small, but significant spike in mite falls 4 days after treatment with MAQS. When CheckMite strips were introduced on D+16, the mite fall in the untreated colonies spiked, whereas it remained low in the colonies treated with MAQS and Formic Pro. The absence of a spike after the critical treatment in the MAQS and Formic Pro groups strongly suggests that the dispersal-phase (phoretic) mite populations were already reduced at this point, and uncapping of the brood at day 4 supported that hypothesis – finding the proportion of living mites under the brood cap was much lower in the MAQS and Formic Pro groups than in the untreated groups.

The application of either MAQS and Formic Pro produced an especially pronounced reduction in the percentage of living immature and male mites. The ability of MAQS and Formic Pro to impact the populations of both immature and male mites is almost certainly what led to the reduction in dispersal-phase mites we saw in our sticky board counts.

Take-home messages.

The best way to determine long-term effects of treatments is to monitor mite populations for extended periods. It is important to keep in mind that sticky board counts (and similarly, alcohol washes) can tell us only about the dispersal-phase mite population. As we saw in the uncapped brood cells, what is happening under the cap can be quite different from what is happening on the colony floor. Ultimately, targeting varroa under the brood cap with MAQS or Formic Pro will affect dispersal-phase (phoretic) populations, but these changes are difficult to detect using dispersal-phase assessments alone – especially if we only look a few days after our treatment.

Heather Broccard-Bell is lead scientist at NOD Apiary Products Ltd and holds a PhD in Evolution and Behaviour.




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