Economic threshold for Varroa jacobsoni Oud. in the southeastern USA

Economic threshold for Varroa jacobsoni Oud. in the southeastern USA W. Michael Hbood 0 Department of Entomology, Clemson University , Clemson, SC 29634 , USA 1 Department of Entomology, University of Georgia , Athens, GA 30602 , USA - This research was designed to determine economic thresholds for Varroa jacobsoni mites in mature overwintered colonies under conditions that encourage or discourage mite immigration. Congruent data from the present study and our earlier work suggest that a true late-season (August) economic threshold for mites in the southeastern USA lies within a range of mite populations of 3 172-4 261, ether roll mite levels of 15-38, and overnight bottom board insert mite levels of 59-187 in colonies with bee populations of 24 808-33 699. Overwintering colonies can benefit from an additional early-season (February) treatment. This benefit was realized in colonies which in February had the following average values: mite populations 7-97, ether roll 0.4-2.8, bottom board inserts 0.6-10.2 and bee populations 12 606-13500. Continuous acaricide treatment never achieved colony bee populations or brood number significantly higher than in colonies treated more conservatively. There is evidence that minimizing mite immigration has the benefit of delaying the onset of economic thresholds. © Inra/DIB/AGIB/Elsevier, Paris Apis mellifera / Varroa jacobsoni / integrated pest management / chemical resistance management - 1. INTRODUCTION Synthetic acaricides are the most effective and widely used method for controlling Varroa jacobsoni Oud. But recent evidence of acaricide-resistant mites in Italy [ 18 ], France [ 6, 22 ] and USA [4] has underscored the need to develop management practices that limit chemical resistance in mites. One way to do this is to use economic thresholds [ 19 ], that is, to treat a colony only when the mite population reaches a level at which mites are still tolerable by bees but above which there may be serious and possibly irreparable harm to the colony. A corollary to a research-based threshold is a reliable and practical mite sampling method. A program based on accurate sampling and thresholdbased treatments can be expected to reduce the overall number of acaricide treatments, relax the selection pressure for mite resistance, reduce the risk of contaminated hive products, and reduce production costs for beekeepers. Economic threshold recommendations vary by region [2], owing in part to differences in the length of the brood-rearing season, one of the most important regulators of mite population dynamics[ 14 ]. Thresholds also may vary depending on the risk of mite immigration [ 17 ] and on the management history of colonies. This present paper is part of a project to develop researchbased economic thresholds for the Piedmont region of the southeastern USA under a variety of beekeeping conditions. In earlier work we established a threshold for first-year colonies set up from mail-order packages [9]. In this paper we present experimental evidence of an economic threshold for mature overwintered colonies under conditions that encourage mite immigration (1997) or discourage it (1998). We believe that our design and protocols are applicable to workers developing economic thresholds elsewhere. 2. MATERIALS AND METHODS 2.1. 1997 study: mite immigration encouraged The1997 study was designed to develop economic thresholds for overwintered colonies under conditions that encourage mite immigration. On 17-18 February 1997 we organized 60 overwintering colonies of honey bees in the Piedmont region of Georgia and South Carolina, USA (2 states x 2 apiaries per state x 15 colonies per apiary). Within each apiary we equalized colonies for initial bee populations, amount of brood, and V. jacobsoni mite populations. We did this by shaking all adult bees from one apiary into a common cage, distributing the brood equally (using visual estimates) among the 15 empty hives, and then distributing equally (by weight) adult bees from that state’s sister apiary into the empty hives. By transferring adult bees to their state’s sister apiary, we prevented bees from drifting back to their original hive locations and nullifying our equalizing efforts. We further discouraged bee drift by distinguishing hive entrances with a variety of colored geometric symbols[ 15 ]. We monitored initial colony mite levels immediately after the equalizing procedure with adhesive bottom board inserts[ 10 ]. The number of mites caught per colony in one overnight sampling after the equalizing procedure was 0.7 ± 09 (range 0-3) in Georgia and 6.2 ± 14.3 (range 0-77) in South Carolina. Colonies were requeened, treated with Fumidil B (Mid-Con) to control Nosema disease, treated with Tceriranmy antibiotic (Pfizer) to control brood diseases, treated with vegetable oil/sugar patties to control tracheal mites (Acarapis woodi [Rennie]) [8], and managed optimally for honey production. Each colony within apia (...truncated)