A farmer was working with dairy cattle at an agricultural experiment station. The
population of flies in the barn where the cattle lived was so large that the animals’
health was affected. So the farmer sprayed the barn and the cattle with a solution of
insecticide A. The insecticide killed nearly all the flies. Some time later, however,
the number of flies was again large. The farmer again sprayed with the insecticide.
The result was similar to that of the first spraying. Most, but not all, of the flies were
killed. Again, within a short time the population of flies increased, and they were
again sprayed with the insecticide. This sequence of events was repeated five times:
then it became apparent that insecticide A was becoming less and less effective in
killing the flies.
The farmer noted that one large batch of the insecticide solution had been made and
used in all the sprayings. Therefore he suggested the possibility that the insecticide
solution decomposed with age.

The farmer’s suggestion is that the insecticide decomposed with age. Briefly explain
how this suggestion could be tested.