4.4. Estimating damage thresholds
This section describes methods used to measure varroa damage at the colony level and to associate that damage with economic damage thresholds. One of the goals of varroa Integrated Pest Management (IPM) is the reduction of beekeeper reliance on pesticides in the bee hive. Mite eradication is not a necessary goal, as IPM philosophy recognizes that eradication may require practices that are excessively toxic, invasive, or impractical. Varroa IPM places a premium on non-chemical management practices that eliminate mites from a colony (such as drone brood trapping) or slow the rate of mite population growth (such as genetic host resistance). The most promising varroa IPM practices have been reviewed by Rosenkranz et al. (2010). Unfortunately, it has been shown anecdotally as well as in computer simulations (Hoopingarner, 2001; Wilkinson et al., 2001) that few if any of these practices can by themselves or indefinitely keep mites at non-damaging levels. Thus at this point it seems best to think of IPM as a means to delay, not necessarily eliminate, the application of acaricides (Delaplane, 2011). Many benefits accrue if the time between chemical treatments is delayed – namely, reduced toxin delivery to bees and the environment, reduced chemical residues in honey, and relaxed selection pressure for chemical resistance and the conservation of susceptible alleles in the mite population that prolong the commercial life of an acaricide. Since the focus of IPM is not mite eradication, but rather mite management, the whole system hangs on the existence of criteria that can distinguish mite densities that are tolerable from those that are approaching damaging levels. Classical IPM tenets (Luckmann and Metcalf, 1982) identify the economic injury level (EIL) as that pest density at which point the grower is experiencing economic loss. The goal is to prevent this level from happening, in other words, to identify an earlier and lower pest density at which point a treatment could prevent reaching the EIL. This lower pest density is named variously the economic threshold, treatment threshold, action threshold, or damage threshold. Implicit in the use of damage thresholds is the recognition that some pest levels are tolerable and do not warrant the use of a pesticide. A good damage threshold will not only distinguish damaging from non-damaging pest densities, but also accommodate local variations due to geography and biology of host and parasite. Thresholds are simply a form of applied population modelling, and like any model, they are only as good as the data, detail, and specificity applied to their construction.