AN EVALUATION OF STRATEGIC AND THRESHOLD CONTROL MEASURES AGAINST THEKAROO PARALYSIS TICK, IXODES-RUBICUNDUS (ACARI, IXODIDAE) IN SOUTH-AFRICA

Paralysis caused by feeding female Ixodes rubicundus ticks is a major problem in large areas of South Africa. As the life cycle of the tick extends over a period of 2 years, it was hypothesized that strategic t reatment of sheep with an acaricide over a 2 year period, timed to kil l most engorging females, should markedly lower the biotic potential o f the tick. Two flocks of sheep grazing in separate paddocks known to be infested with I. rubicundus were treated either strategically or on a threshold basis (i.e. only when tick challenge exceeded a predeterm ined critical level in terms of paralysis) for a 2 year period. The ti ck burdens of untreated control sheep running with the two hocks were monitored over a 4 year period and their seasonal dynamics determined. The times at which peak infestations occurred were similar for both h ocks of sheep, but significant differences in mean tick burdens betwee n the two flocks were recorded. Tick numbers on sheep in the strategic ally treated flock did not decrease during the third and fourth years of the trial as was expected. Possible reasons for this were low stock ing densities, especially during times of peak abundance of adults and the presence of wild hosts which maintained tick populations.