they used DDT wantonly before they learned how toxic it was or that it was going to destroy egg shells, and the bird population

 

it was cheap and effective at killing pest insects... but house flies? Those damn things got used to it, and immune. 

https://www.proxibid.com/Standard-Insect-Spray-w-DDT/lotInformation/75722006

http://npic.orst.edu/factsheets/ddtgen.pdf