Black flies, no-see-ums, deer flies, gnats and mosquitoes were instituted by the devil to force people to live in cities where he could get at them better. if it weren’t for them everybody would live in the bush and he would be out of work. it was a rather successful invention.

Citronella oil

Explanation:

In Ernest Hemingway novel Camping Out he tells in a short snippet about the vacation of people who camp to cut monthly expenses and have fun outdoors, but nothing goes as planned. People are plagued by mosquitoes and other insects that make the experience unpleasant. He satirizes that the devil created mosquitoes to make it difficult for people to stay out of town, just to be able to continue to influence them. Fortunately this can be bypassed with citronella oil.

answer; a cost benefit analysis;