What will be the environmental impact of treating cattle with insecticide?
Treating cattle with insecticides to control tsetse can have major effects on other species of insect. These effects can be beneficial or, potentially, harmful.
The beneficial effects include reductions in the numbers of biting flies such as Stomoxys. This fly can transmit diseases mechanically and it is also a major nuisance pest of cattle. Treating cattle with insecticide will not eliminate Stomoxys but there are frequent reports that their numbers are greatly reduced in tsetse control operations (Leak et al., 1995).
Insecticide treatment of cattle can also affect tick populations. This is a mixed blessing. On the one hand, ticks can cause serious physical damage to cattle and thus reducing their numbers will prevent this. On the other hand, indigenous breeds of cattle can develop a natural immunity to several tick-borne diseases (anaplasmosis, babesiosis, cowdriosis). The development of this immunity relies on calves being bitten by infected ticks; the calves develop relatively mild and transient symptoms of disease and thereafter they are unaffected by further infection. If, however, cattle first become infected when they are adults, then the disease can be very debilitating and potentially fatal. For this reason, we need to be careful that the mass treatment of cattle with insecticide to control tsetse does not reduce the numbers of ticks to such an extent that cattle become vulnerable to tick-borne diseases (Torr et al., 2002
...and the ugly.
Treating cattle with insecticides can contaminate the animal's dung and, as a result, kill large numbers of the beetles and flies associated with cow dung (Vale & Grant, 2001). These insects play a major role in the natural incorporation of dung into the soil, a process that is crucial to the maintenance of soil fertility. Disrupting this process could be disastrous for farmers. The dung becomes contaminated through the cow ingesting insecticide as it grooms itself and through insecticide accumulating around the anus. This contamination is generally most marked with pour-on formulations of insecticide.
How can we avoid these bad effects?
We can largely stop the insecticidal contamination of dung by treating just the legs of adult cattle with a dip formulation of insecticide. Fortunately, most tsetse feed on the legs of older and/or larger cattle whereas many important vectors of tick-borne diseases feed on the ears, udders, and perianal regions. Thus by treating just the legs of larger animals we can avoid the harmful effects of pyrethroids yet still control tsetse.
This is a current area of research and it is not yet clear whether the selective application strategy will work for all species of tsetse. Results to date indicate that it is effective for Glossina pallidipes and we are currently carrying out studies to identify appropriate strategies for other species of tsetse.