Back in the early days of the South, in the US, people grew a lot of cotton – it was a huge cash crop and for 5–6 decades, was the mainstay of the Southern economy.
The problem was that cotton is very attractive insects, particularly the Boll Weevil. An infestation of Boll Weevils can pretty much wiped out a cotton field in a matter of days.
Farmers figured out how to make an insecticide that was fairly effective against some of the boll weevils, but it had the nasty side effect of leaving some of the older ones alive – it killed many of, but not all, of the younger ones. So it worked, somewhat, but not perfectly.
The farmers on the affected plantations needed to make a decision about whether or not to use the insecticide. The had to make the choice between the lesser of two weevils.