Should I treat the trap with insecticide?
If you are using traps to assess the distribution and abundance of tsetse then you do not need to treat your trap with insecticide. However, you will find that ants will soon discover flies in the cage and eat them. So you need to prevent this from happening. One way of doing this is to spray the interior of the cage with a persistent pyrethroid such as deltamethrin. This will rapidly kill all the insects that make their way into the cage - tsetse and ants included.
In some parts of Africa, people use insecticide-treated traps to control tsetse. The insecticide treatment means that the trap will still kill tsetse even if it is badly ripped. However, if the intention is only to kill tsetse then it is probably cheaper to use a simple insecticide-treated target for control and just use the trap for survey purposes.