Re. “distant horizontal transmission” of Varroa, I’m currently doing a lit review of the spatial epidemiology of Varroa, with the view to modelling vectors, impacts, and mitigation should the mite ever arrive here in Newfoundland and Labrador.    Well-known horizontal transmission routes include swarming, absconding, inter-apiary colony robbing, and beekeeper biosecurity breaches (e.g., introduction of infested nucs into a previously mite-free apiary).  Peck, Smith and Seeley (2016) also point to Varroa transmission on forage as a possible vector.  Of course, migratory beekeeping and untreated Varroa ”bombs” increase the geographic spread of Varroa and aggravate the severity of infestation.

In 1995, when then-provincial apiarist, Dick Rogers, first discovered Varroa in Nova Scotia (Canada), he found that Halifax/Dartmouth had a ring of Varroa infested hives around the two adjacent cities. I know nothing about the distance between these apiaries or about inter-apiary traffic in nucs, brood comb, etc. at that time. His 1996 report to the Canadian Association of Professional Apiculturists (CAPA, 1996:28-29) notes, “Evidence suggests that Varroa has been in the Halifax/Dartmouth area of Nova Scotia for 3 years. The introduction appears to have been human assisted, but unable to get admissions of involvement or supporting proof. Mite levels in Halifax/Dartmouth were very high and were estimated to be 3000-5000 mites per colony in the 14 colonies that suffered total collapse in October/November. Border closure and surveys have been very effective at preventing natural immigration of parasitic bee mites, however, there is very little that can be done to completely prevent human assisted introductions. Eradication is not being considered because of the distribution of Varroa finds. Movement of colonies out of Halifax/Dartmouth area for blueberry pollination are responsible for the rapid and widespread dispersion of Varroa in 1995. Quarantine of positive apiaries and suppression through treatment are definite actions to be taken in 1996.” A year later, Rogers reported, “Varroa mites have become more generally distributed within a portion of Halifax County. A quarantine was declared for Halifax Co. West, effective January 1, 1997. The purpose is to slow the spread of VM to other areas of the province. This effort is considered a temporary measure that will be modified or lifted as VM distribution changes” (CAPA, 1997:48). 

Varroa’s first appearance in the neighbouring province of New Brunswick dates to 1989.   However, its spread to Nova Scotia is not likely to have occurred across the Tantramar Marsh border region due to rigorous quarantine and other management measures. A possible introduction to Nova Scotia by way of illegal import from Europe is suspected but not proven.

Dennis van Engelsdrop suggests that inter-apiary Varroa transmission can occur when apiaries are within 3 km of one another (in the absence of direct human assistance). “Out in the environment, we have mite mines acting as untreated colonies; they blow up and spread their goodwill to all the colonies with 3 kilometres” (Bee Culture, “Everthing Varroa – Part 3”).   I’m inclined to use 5 km as the inter-apiary distance for Varroa transmission, for my modelling purpose, although I see a range of figures in the literature related to honey bee forage distances (Gary, 1992:319 – 6500 metres; Seeley, 1985:89 – 6000 metres; Winston 1987:171 – 3700 metres).

Any advice about the most appropriate inter-apiary distance to use for Varroa transmission would be appreciated. NB – this is transmission facilitated by robbing, swarming, absconding, not human traffic in live bees or hives.

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