I one hundred percent believe that they helped increase crop yields, as pests are the number one cause of bad crop yields. Does this mean that they yielded healthy or even safe crops? No, absolutely not, but they most definitely yielded more of whatever state of the crop that they were growing.
Personally I believe that DDT is harmful to the ecosystem and the human body if consumed with produce. If insect numbers reached a level where they had to be reeled back in to feed the planet, then an increased use of DDT would be necessary. However, the insect problem has not reached a point currently where we cannot produce enough crops, so I do not support an increased use of DDT.
I believe DDT pesticide usage decreased crop yields. DDT pesticide usage contributed to lower crop gains because use of the poison inhibited crops from growing. Studies were conducted among certain oil-rich seeds such as peanut and mustard seeds which showed that exposure to DDT did inhibit germination, and subsequent plant growth.
I think that the usage of DDT pesticides have decreased crop yields. The reasons for this, I do not know, but this is the case from what I have read. I think there may be some sort of balance that is off-set by the poisons used on all of these crops.