Is Rachel Carson's thesis in Silent Spring still relevant today?
Of course it is still relevant. As long as there are as many people as we have now, using as many resources as we do now, the major point Carson was trying to make remains relevant.
You don't even have to go away from the agricultural sector to find the current relevance of Carson's thesis. She was writing about pesticides and birds, but other agricultural chemicals continue to impact the environment. Perhaps the biggest example of this is the huge "dead zone" in the Gulf of Mexico in which essentially nothing can live because of agricultural chemicals being washed down the Mississippi River.
So, there will always be a tension between humans and the environment and Carson's thesis will be relevant for the foreseeable future.
Absolutely, and not just on pesticide use. The overarching idea in her book about how man directly impacts the environment with industrial and agricultural policy could not be more relevant in the modern day, with the effects of climate change becoming more apparent by the year, water shortages and extreme weather events becoming commonplace, along with a alarming degree of food insecurity facing many populations around the globe. The very attitudes and actions of mankind Carson speaks of in the 1950s is indeed catching up with us today.