Why are new pesticides less harmful to ecosystems than DDT and related compounds used in the 1950s and 1960s?

In the 1950s and 60s (and earlier) chemical pesticides were used extensively but untested. After Silent Spring was published by Rachel Carson in 1962, chemical companies including Monsanto hit the author with both lawsuits and personal attacks. President John Kennedy took note and asked for an investigation into claims that led to increased regulations concerning chemical pesticides. 
Some believe that currently there is still a lack of commitment to proper testing to validate the safe use of pesticides. Often, publications note connections between current pesticides and diseases such as Parkinson's, Alzheimer's, cancer, infertility, and others. Some scientists believe a commonly used weed killer - atrazine - could be more harmful to humans than originally thought. An article published in 2009 stated that birth defects, low birth weights, and reproductive problems could be outcomes even when used at concentrations that meet current federal standards. 
Natural pesticides do exist. As an example, by planting garlic with tomatoes a grower can keep red spider mites away. Farmers who practice sustainable farming methods use the least toxic substances in the least amounts possible to avoid endangering both human and animal health. Positive sustainable farming methods include crop rotation, selecting pest resistant plant varieties, and planting root stock that is pest-free. 
So, while more testing is done today for negative effects of chemical pesticides, the focus needs to remain on the continued safe and responsible use of chemicals.