There are confounding co-factors to birth rate in advanced economies. it would be grossly simplistic to ascribe the birth rate to falling sperm motility levels, when advanced societies offer IVF and assisted reproduction often on the public purse.

I suspect the economics here are stronger than biology: it is too expensive to raise and (privately) educate children and in the developed economies, familys delay childbirth into later years and so tend to have less children.

TL;DR I believe the birth rates in Africa and Asia are varying more as a function of economy, than of insecticide.