Comparing various counting methods used to estimate individual horn fly (Diptera: Muscidae) populations on cattle

Horn flies have been established as one of the most influential ectoparasites affecting cattle. Effects of this pest are often manifested through decreased animal performance measured by weight gain, milk production, and feed efficiency. Often, managerial intervention is recommended when horn fly infestations exceed an economic threshold which is based on visual population estimates.  More recently, digital photographs taken of animals infested with horn flies have been suggested to provide similarly accurate results as compared to visual estimations.  Incorporating digitally based horn fly population estimates requires more details in order to standardize data collection and ensure accurate results.  The objectives of this study were to determine the effect of animal orientation and time of day on visual and digitally based horn fly counting methods.  Four Angus x Hereford cross yearling heifers were artificially infested prior to evaluation. Population estimates were taken visually by experienced (VE1) and inexperienced (VE2) technicians, as well as digitally with pictures taken on both lateral sides of the animal with an east to west (DEW) and a west to east (DWE) orientation.  Counting methods were conducted at three time points the day following infestation.  Average visual population estimates were higher (P < 0.0001) when compared to digital assessments.  Animal orientation and time of day appeared to have no effect on horn fly population estimates regardless of the method used.  The potential impact of the discrepancies between population estimates using either visual or digital counting methods is discussed below.