Often neglected IPM Strategies against Varroa
By Dr. Ewan Campbell
The mantra of pest control in any livestock / cultivated crop or invasive species is integrated pest management (IPM). It is a holistic approach that draws on a range of different control methods to keep the numbers of a pest at a minimum. We, as beekeepers should be well aware of IPM when considering how to manage that most persistent of bee pests, the Varroa mite.
I am sure nearly all the members of ADBKA use some form of Varroa control. Whether it’s using thymol based treatments, oxalic acid or harder chemical treatments such as pyrethroids (apistan) and amitraz (apivar). These treatments can be combined in IPM with other strategies such as drone trapping. Mites prefer to grow in drone brood by a factor of ten to one! Removing a drone comb every month will knock down mites significantly. Other IMP strategies include queen trapping and the more long-term solutions of hygienic bee breeding.
When done correctly IPM is extremely effective at controlling Varroa. I highly recommend reading Randy Oliver’s fantastic online blog (scientificbeekeeping.com) who has a great mini-series on IPM as well as a host of other great information. Warning….you can lose days in there reading about bees!
More recently a lot of research has focused on other parts of the IPM pyramid that are often overlooked (feature image above). We all know about the chemical acaricides and I’ve mentioned drone trapping, which comes under “mechanical” methods. There is some minor research ongoing into biological control of Varroa but there is no effective agent yet isolated that doesn’t bring with it a host of new problems (although do look up videos of pseudoscorpions battling with Varroa on youtube!). “Educational” is pretty self-explanatory, leaving us with “isolation”….
Did you realise that up to 40% of a hive in your apiary could be full of bees from another colony? Scientists Pfeiffer and Crailsheim carried out research in 1998 to investigate the phenomenon of drifting between colonies. They marked bees individually and then followed their progress and the amount of drifting between colonies over the period of their lifetime. When they carried out the analysis they showed that bees from different colonies made up a large proportion of any hives population. In hives placed less than 26cm apart in straight lines, drifting bees made up to 41% (!) of the hive’s total bees.
Now what’s this got to do with Varroa and IPM? Well, quite a lot! Bees are not drifting on their own….they are carrying a range of pathogens and pests, including mites. Every time a bee drifts between colonies is another chance for a mite to hitch a lift across the great divide. This is where “isolation” and cultural IPM comes in. If you can reduce drifting then you reduce the chance of mites mingling and spreading.
In February 2017 a new piece of research from Keith Delaplanes lab in the US was published. They placed pairs of mite-free hives in apiaries at distances of 0, 10 and 100m between the paired hives. They then sprinkled mites into one of the pairs (the donor) in each apiary and monitored the spread of mites into the clean hives (the recipient) in the same apiary. In apiaries where pairs of hives were close together (0 and 10m apart) there were significantly higher mite levels in the recipient clean hives than when hives were further apart (>100m) and that drifting played a significant role in this. So if you are closer to a neighbouring hive then you are more likely to give them mites. Not a great surprise! Interestingly the research also showed that in apiaries where they were close together (0m), both hives in each pair (recipient and donor) had increased mite levels over time. This unexpected result suggests that mites are competing for brood to infest and that this competition is most fierce where hives are very distant as there is less drifting and movement of mites. When hives are close together the competition is very low as there is enough brood for all mites. It’s an interesting study but how does it actually help us as beekeepers. Well I am not sure how practical it is for beekeepers to keep hives 100m apart or if many people have that kind of room in a garden but it probably does show that if you have the space then you should place your hives more distant from each other than you currently do!
However there are a few more simple ways to reduce drifting that are practical. Firstly, you can paint or mark your hive entrances a different colour and pattern from each other – the theory being that bees will more readily recognise their own hive and not drift into a neighbouring colony by mistake. Secondly, don’t place all your hives in a big line. Bees will drift down the line and you’ll find the end hives will have big populations of bees…and mites! Hives should, where possible, be positioned with their entrances facing in slightly different orientations or in a circle facing outwards. Another suggestion, by Ted Hooper, is facing the hives towards bushes or screens in order that the wind is less turbulent in front of entrances to stop bees being blown off course. Finally, if you have space, then consider moving hives further apart from each other. In nature hives are on average ~500m apart !
Hopefully we all practice some form of IPM to control Varroa mites, but I am sure, and I am guilty of this, we probably concentrate on the chemical control aspect and don’t pay much attention to some of the easy cultural / isolation measures to stop mites moving about so much. With the chemicals proving more and more ineffective it’s probably a good idea to to move the hives further apart when the opportunity arises or in the colder months.
Dr. Ewan Campbell
Guide to Bees & Honey, Ted Hooper (2010)
Pfeiffer & Crailsheim (1998) Drifting of honeybees. Insectes Sociaux 45
Nolan & Delaplane (2017) Distance between honey bee Apis mellifera colonies regulates populations of Varroa destructor at a landscape scale. Apidologie 48:1