Are insecticide-treated cattle better than targets?

A single insecticide-treated ox kills about as many tsetse as a single odour-baited target. So if there are already many cattle in an area, then treating cattle appears to offer several advantages over using targets.

First, treating an animal is cheaper than constructing and deploying an insecticide-treated target (Shaw et al., 2017).

Second, cattle owners are strongly inclined to treat their cattle with insecticide, maintain them and protect them from theft. So several problems associated with using targets - such as the loss of targets due to general wear and tear, theft, and animal damage - are less with insecticide-treated cattle.

Third, in some settings (e.g. south-west Uganda) cattle can be reservoir hosts for the pathogen (T. b. gambiense) causing Rhodesian sleeping sickness.  In this situation, treating cattle with insecticide will not only control animal trypanosomiasis but also sleeping sickness (Hargrove et al., 2012). 

So is there any point in using targets?

The problem with using insecticide-treated cattle to control tsetse is that cattle tend to be confined to certain areas. Indeed, farmers will often avoid grazing their livestock where tsetse are abundant, and the imperatives of finding adequate water and grazing further limit their distribution. Cattle are also generally grouped into herds, and a group of ten cattle does not kill ten times as many tsetse as a single animal. In fact, a herd of ten cattle will only kill about three times as many tsetse as a single animal (Torr et al., 2007). The upshot of this is that densities of four cattle per square kilometre are generally not sufficient to control savanna species of tsetse, whereas this density of targets is. In general, it is necessary to have an overall density of about 10 cattle/km2 to achieve effective control.

Mind the gap!

The existence of small areas without cattle (i.e., a cattle-free gap <3 km) in a large-scale operation (e.g. 1000 km2) will not affect the level of control (Torr & Vale, 2011).  If there are larger gaps (> 3 km) then targets should be deployed in areas without cattle. 

Most tsetse control strategies will require a combination of insecticide-treated cattle and targets, with the latter being used to plug the inevitable gaps in the local distribution of cattle.