Abundance of adult horn flies, Haematobia irritans irritans (L.), was monitored on 25 untreated Hereford cows in Tacuarembó Department, Uruguay, during three consecutive grazing seasons, from October 1999 to May 2002. The population showed a variable pattern of abundance during three years, with peaks in late summer-early fall of each year. Adult flies were continuously present, although in very low numbers in intervening winters. Numbers of flies per cow rarely exceeded a reference level of 200 flies per animal during the grazing season. Degree-day calculations indicated that approximately 12 generations were possible each year. Time series analysis of mean densities among consecutive generations indicated that population growth was governed by simple, direct density-dependence, with additional effects of seasonally varying weather. Response surface regressions confirmed that intergenerational growth was inversely related to mean density, and directly related to temperature. Stochastic simulations with the response surface model suggested that within the range of temperatures observed in our study, horn fly populations on Hereford cattle will tend toward densities of approximately 150 flies per animal in summer, and exceed a nominal level of 200 flies per cow one or more times in about 65 of every 100 grazing seasons.
Bibliographical noteFunding Information:
We are grateful to staff at “Los Álamos” (Caraguatá) farm near Ansina, Tacuarembó, for providing husbandry of their animals and for giving us access to their property, to S. Weisberg for advice on regression analysis, and to D. Andow and W. Hutchison for comments on an earlier draft of this paper. This study was supported by INIA-BID-LIA 25, travel grants from Partners of the Americas, and University of Minnesota Agricultural Experiment Station project MN-050.
- Horn fly
- Population dynamics
- South America
- Weather effects