SEASONAL-VARIATION OF HAEMATOBIA-IRRITANS (DIPTERA, MUSCIDAE) IN A RECENTLY INFESTED REGION OF CENTRAL ARGENTINA

The seasonal variation of adult populations of Haematobin irritans Lin naeus was monitored at Santa Fe, Argentina from August 1992 to August 1994 by weekly counts of flies on grazing heifers. Each year, flies we re recorded at a density of more than two flies per heifer from August /September to the following June, with peaks of abundance in mid to la te spring and late summer to mid autumn. The seasonal trend of fly num bers was best correlated with the mean temperature 4 weeks prior to ea ch fly count (correlation indices of P<0.01 in six of the nine fly per iods) followed by the previous 4 week mean saturation deficit. Rainfal l and relative humidity correlated poorly with the seasonality of H. i rritans. Temperature appeared crucial at the beginning (late winter) a nd at the end (early-mid autumn) of the fly season while its relevance diminished in the middle of the season. The percentage of heifers tha t were infested decreased from 80-100% in the fly season to 0-55% betw een fly seasons but H. irritans never disappeared from the herds, indi cating that if diapause occurs at the latitude of the study area, part of the fly population was not sensitive to diapause inducing factors.