Are sheep tick-mops effective in Scotland?

Key findings

• The use of sheep as 'tick-mops' may reduce tick biting rates on grouse chicks where deer densities are lower than five per 100 hectares. 
• Red deer densities of 10 per 100 hectares appear to be too high for 'tick-mops' to be effective.

Two studies by us, one in Yorkshire and the other in Morayshire, have been used to support increased tick control on sheep (four treatments with insecticide and vaccination against louping ill virus where appropriate) on many estates in Scotland. But there are few data on how effective this is in relation to other tick hosts, particularly red deer.

We tracked the effects of using these intensively-treated sheep on tick biting and grouse productivity on three moors in Glen Truim, Strathspey between 2002 and 2005. Tick biting rates on grouse on these moors vary between three and 30 ticks per chick. Between 8% and 65% of mountain hares on these sites had been bitten by ticks carrying louping ill. Treated sheep at a density of 50 per 100 hectares were put onto the 'high deer' moor in 2002 where deer numbered 10 per 100 hectares. Treated sheep were put on to the 'low deer' moor in 2003 where deer were at a density of five per 100 hectares. The 'control' moor was an area where sheep treatment was not intensive and where host densities were stable.

We saw a 25% reduction in tick biting rates on grouse chicks on the low deer moor since the introduction of the treated sheep in 2003 (see Figure 1). Accepting that 2004 was a year of overall low grouse productivity in this area, the trend on the low deer moor was for grouse productivity to increase by 30% during the period of sheep treatment (see Figure 2).

Figure 1. Mean ticks (all stages) on grouse chicks caught on the study moors 2002-2005

Low deer moor
High deer moor
Control

Figure 2. Average number of red grouse chicks per hen on the Glen Truim study moors 2002-2005

Low deer moor
High deer moor
Control

There was generally low grouse productivity on the two other moors (see Figure 2). The apparently low tick burdens and high productivity in 2005 on the control moor and low tick burdens 2004 on the high deer moor may reflect that in each case only two broods were caught for tick biting assessment. On the control moor there were only two broods found in 300 hectares of moor; in the case of the high deer moor only two broods of chicks were found in 200 hectares. Overall it appears that large tick burdens and the continuing presence of louping ill has prevented the treated sheep flock, introduced in 2003 on the high deer moor, from reducing tick burdens on grouse chicks (see Figure 1).

Tick burdens may not be reduced if the treatment of the sheep is ineffective. We monitored the efficacy of the sheep treatment regime on the high and low deer moors by counting ticks on both treated sheep and a population of sentinel sheep purposefully left untreated. With the exception of one treatment period on one moor, the acaricide-treated sheep had lower numbers of ticks than the untreated sheep. The continuing presence of ticks on the treated sheep draws attention to the need for high standards of application and for a realistic assessment of the length of time that the treatment gives effective coverage.

We think that low numbers of deer and hares are unlikely to prevent treated sheep from slowly 'mopping up' ticks on grouse moors, but that high densities of deer may do so. We still need more experiments to find out which are the key hosts that must be controlled to make 'tick mopping' with sheep effective.