L O N G TERM STUDY OF HAEMATOBIA IRRITANS (DIPTERA: MUSCIDAE) SEASONAL DISTRIBUTION

2 0 Holstein cows not treated against ectoparasites. The analysis was mainly focused on winter fly abundance. Two peaks of abundance were found from spring to autumn. Adults of H. irritans were consistently found on cattle during winter, with increasing numbers from the end of July to late August. The only climatic parameter soundly correlated with the four week period, before each winter fly count, was the mean air temperature with the exception of year 1998. N o significant relationship was found between level of cow infestation and relative humidity, saturation deficit and rainfall. Horn fly infestation on cows was lower than 2 0 % from end of June to end of August only in 1995, when mean air temperature was consistently lower than 1 ° C during the four week period previous to most fly counts. Conversely, the mean temperature was higher than 1 2 ° C previous to fly counts in 1998, when most cows remained infested. The results indicate that a proportion (unknown) of immature stages of H. irritans were insensitive to diapause inducing factors and developed through winter