Pesticides are extremely persistent. Since most have never been studied (out of 80,000+ chemicals, only 200 have been studied) we know nothing about their lifespans, only that they continue to kill.
DDT was banned 40 years ago, 10+ years after Rachel Carson detailed it's devastating effect on ecosystems. DDT is dtill killing birds today.
The newer class of pesticides, neonicotinoids, are 5,000 to 10,000 times more toxic than DDT. For over 10 years now the neonics have been poisoning the land and have been implicated in CCD and bird deaths.