Hi all

I have long suspected that colony density has a lot to do with the difference in mite levels we see and the ability of some people to maintain colonies without mite treatments, while others cannot do so successfully. I just read this (must have missed it)

> We quantified the weekly invasion rates and the subsequent mite population growth from the end of July to November 2011 in 28 honey bee colonies kept in two apiaries that had high (HBD) and low (LBD) densities of neighboring colonies.  

> The average invasion rate per colony over the entire 3.5-mo period ranged from 266 to 1,171 mites at the HBD site compared with only 72 to 248 mites at the LBD apiary. In the untreated colonies, the Varroa population reached an average Þnal infestation in November of 2,082 mites per colony (HBD) and 340 mites per colony (LBD).

> We recorded a dangerously high invasion pressure at the HBD site from mid-August through mid-October. This leads to reinvasion of mites in previously treated colonies as is demonstrated by our largely mite-free experimental colonies. One can assume that the invading Varroa mites come from more highly infested colonies of neighboring apiaries. Invasion, coupled with subsequent reproduction by invading mites, can be a substantial problem for beekeepers who treated their colonies earlier, giving them a false sense of security. It also might explain, at least in part, unexpected winter mortality of colonies belonging to experienced beekeepers who have performed Varroa treatments according to recommendations.

> There were large differences between the HBD and LBD sites for infestations of both brood and bees. At the HBD site, the average brood infestation exceeded 6% at the beginning of September. At the end of September, the average infestation rate exceeded 22%, meaning that a quarter of the hatching winter bees were already weakened. At the HBD site, the infestation level of the adult bees was comparatively low until mid-September but then clearly exceeded the autumn economic threshold of 3-5 mites per 100 adult bees suggested for the United States and Canada

Autumn Invasion Rates of Varroa destructor (Mesostigmata: Varroidae) Into Honey Bee (Hymenoptera: Apidae) Colonies and the Resulting Increase in Mite Populations
Author(s): Eva Frey and Peter Rosenkranz
Source: Journal of Economic Entomology, 107(2):508-515. 2014. 
Published By: Entomological Society of America

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