Which substance has the highest acute toxicity?

Nicotine is known for its exceptionally high acute toxicity compared to the other substances listed. It is a potent neurotoxin, affecting the nervous system even at very low doses. The lethal dose for humans is relatively low, making it one of the most acutely toxic substances encountered in pest control.

In contrast, while substances like triclopyr, diquat, and 2,4-D have their own levels of toxicity and potential health risks, they do not match the immediate and severe effects of nicotine when exposure occurs. The mechanisms by which these compounds operate in pest control, including herbicidal and insecticidal activities, involve more complex interactions and generally require higher doses to achieve harmful effects. Thus, nicotine's rapid onset of harmful effects makes it the substance with the highest acute toxicity of the options provided.