Possible Clues:

• Prohibited bug spray
• One target of Rachel Carson's 'Silent Spring'
• Initially I have the divine to tea - it's poison
• Banned pesticide whose discoverer won a Nobel Prize
• No good for 22 down
• Carson subject
• Bug-spray letters
• Anti-malarial pesticide
• Insecticide, for short
• Insecticide banned in 1972