Miticide resistance is a hot topic among Kiwi beekeepers. So, in the second of three articles looking at varroa and best management practices, Sebastian Owen, commercial director of Vita Bee Health, considers the success of first-generation treatments and the necessity of alternating treatments in an integrated pest management regime to avoid the development of resistance.
When varroa first invaded countries in Europe, no treatments were available and the parasite quickly took a toll, killing colonies once mite levels had reached a dangerous threshold. Previously benign viruses became killers as their transfer within the colony became quicker and more impactful because the mite opened up new transfer pathways by biting into the fat body of the bee. Before varroa, the pathogen transfer was from bee to bee and was much slower, occurring perhaps through feeding. With varroa present, the transfer was, in effect, by inoculation by the mite biting the fat body of the bee.
First-generation treatments usually contained synthetic chemicals, such as pyrethroids, as the active ingredient and were generally very effective in controlling varroa populations, although there was never any possibility that varroa could be totally eradicated in this way.
The ever-present risk of developing resistance
However, as with any medication, the development of resistance to the treatment was an inevitable threat. Mites surviving the treatments would breed and pass on their particular genetic traits which meant that the treatment they had survived would be less effective against their future generations. So, a population of mites resistant to a specific medication would over time multiply and thwart beekeeper attempts to control varroa populations.
The time taken for resistance to develop could have been very long. Unfortunately, because beekeepers did not always follow manufacturers’ treatment instructions precisely, in some areas resistance developed much faster than could have been the case. Entire regions were sometimes affected by resistant mites while in other areas resistance appeared only in relatively small pockets. In some areas, evidence of resistance has yet to be recorded, but this should never lead to complacency.
To delay or even avoid the build-up of resistant mites, it is necessary for beekeepers to follow instructions precisely and to look to alternative treatments so that one class of active ingredient is not used year after year.
Second-generation treatments and IPM
As approved and greener second-generation treatments with different active ingredients began to appear, the possibility of alternating treatments become feasible and beekeepers soon became accustomed to a term, long established in other arenas – integrated pest management (IPM).
Today, IPM is widely practised with varroa treatments in Europe and North America and is proving to be the best way of keeping mite populations under control. Any IPM strategy must involve rotation of treatments with active ingredients from different chemical classes. Tau-fluvalinate (Apistan) and flumethrin (Bayvarol), for example, are both pyrethroids and there is evidence of cross-resistance to the two active ingredients. A successful IPM strategy must therefore alternate between one active ingredient and another of a different class. So, for example, alternating Apistan or Bayvarol with alternative treatments such as those containing amitraz (a different chemical class) or thymol is necessary to avoid resistance.
Often the second-generation treatments require more careful application, but nothing that should prove too onerous to beekeepers. For example, timing of treatments and ambient temperatures often become significant at the time of treatment. The benefits of using many of these second-generation treatments are considerable, however.
Is resistance forever?
Even when resistance to a treatment does develop in mites, that need not be the end of the line for that particular treatment. A break of several seasons from a particular treatment can rectify the situation and make the mites vulnerable to the treatment once more. This has been demonstrated in the case of Apistan in Italy. Over use of the product had allowed resistance to develop over a wide area but, after the product has been withdrawn for a few seasons, it became effective once more. Even today it remains effective through IPM techniques which ensure that the product is alternated with others.
Treatment planning for New Zealand
In New Zealand, where first-generation treatments have been very effective and popular for many years (and still are), resistance can be expected to develop unless IPM practices are adopted promptly. As suppliers of varroa-control treatments, Vita Bee Health strongly recommends that New Zealand beekeepers review their treatment strategies and consult with experienced beekeepers and product suppliers, including Vita Bee Health distributors.