26 September 1997

Pour-ons may increase scab resistance

RISKS of scab mite resistance to Bayticol could be higher on farms using pour-on tick treatments in the spring.

MAFF parasitologist Peter Bates says that like Bayticol, all pour-ons used for tick control in the spring are pyrethroid based.

When scab mites are present, use of both pyrethroid pour-ons and dips will increase the speed of selection for resistant mites, according to Mr Bates.

He explains that when scab mites are present and subjected to pyrethroid treatments, all susceptible mites are killed. This allows a population of resistant mites to breed and thrive over the summer.

To minimise resistance risks he suggests keeping stock close to the holding for 24 hours prior to treatment and observing them closely for signs of scab. Any sheep with scab mites in the spring should then be dipped with an alternative such as amitraz.

However, producers should continue using pour-on tick control when scab mites are not present because the procedure is less stressful than dipping at a time when ewes are still pregnant on many upland farms.