Population growth of Varroa destructor (Acari: Varroidae) in honey bee colonies is affected by the number of foragers with mites

Authors: DeGrandi-Hoffman, Gloria; AHUMADA, FABIANA - Agscience Llc; Zazueta, Victor; Chambers, Mona; Hidalgo, Geoffrey; Watkins De Jong, Emily

Submitted to: Experimental and Applied Acarology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 2/18/2016
Publication Date: 2/24/2016
Citation: Hoffman, G.D., Ahumada, F., Zazueta, V.E., Chambers, M.L., Hidalgo, G., Watkins De Jong, E.E. 2016. Population growth of Varroa destructor (Acari: Varroidae) in honey bee colonies is affected by the number of foragers with mites. Experimental and Applied Acarology. doi: 10.1007/s10493-016-0022-9.

Interpretive Summary: Varroa mites are a serious pest of honey bees and the leading cause of colony losses. Varroa have relatively low rates of reproduction so it should take more than a year before mite populations are large enough to cause colony losses. However, this often is not the case. There are indications that factors other than reproduction might be causing unexpectedly high mite numbers in colonies. One factor might be that Varroa enter colonies on foragers drifting from other hives. We measured the proportion of foragers carrying mites on their bodies while entering and leaving hives, and determined its relationship to the growth of Varroa populations in those hives at two apiary sites. We also compared our estimates of mite population growth with predictions from a Varroa population dynamics model that bases mite population growth on mite reproduction. We found that mite populations in brood cells and on adult bees were low until September when there was a sharp increase especially at site 1. The frequency of foragers entering and leaving colonies with mites on their bodies also increased in the fall. We captured more foragers with mites at site 1 than site 2, and site 1 had larger mite populations in colonies. The growth of mite populations was significantly related to the total number of foragers with mites (entering and leaving the colony). The proportion of brood cells where mites successfully reproduced was not significantly related to the growth of the Varroa population at either site. The model accurately estimated mite population sizes until November when prediction were much lower than those measured in colonies. These results provide evidence that growth of mite populations might not be due to reproduction alone, and that foragers carrying mites and entering hives might be contributing significantly to colony mite populations especially in the fall.

Technical Abstract: Varroa mites are a serious pest of honey bees and the leading cause of colony losses. Varroa have relatively low reproductive rates, so populations should not increase rapidly, but often they do. Other factors might contribute to the growth of Varroa populations including mite migration into colonies on foragers from other hives. We measured the proportion of foragers carrying mites on their bodies while entering and leaving hives, and determined its relationship to the growth of Varroa populations in those hives at two apiary sites. We also compared the estimates of mite population growth with predictions from a Varroa population dynamics model that generates estimates of mite population growth based on mite reproduction. Mite populations in brood cells and on adult bees remained low through the summer but increased sharply in the fall especially at site 1. The frequency of capturing foragers with mites on their bodies while entering or leaving hives also increased in the fall. The growth of Varroa populations at both sites was not significantly related to our colony estimates of mite reproduction, but instead to the total number of foragers with mites (entering and leaving the colony). There were more foragers with mites at site 1 than site 2, and mite populations at site 1 were larger especially in the fall. The model accurately estimated phoretic mite populations and infested brood cells until November when predictions were much lower than those measured in colonies. The rapid growth of mite populations particularly in the fall being a product mite migration rather than mite reproduction alone is discussed.