Which factor indicates high toxicity in a pesticide?

In the context of pesticide toxicity, LD50, or lethal dose for 50% of the test population, is a critical measure. A low LD50 value indicates that a relatively small amount of the substance is required to cause death in 50% of the test organisms, which signifies high toxicity. Therefore, when considering the safety and potential hazards of pesticides, a low LD50 is a clear indicator of a substance's capability to cause harm or lethal effects at lower dosages.

In contrast, a high LD50 would imply that a larger quantity of the pesticide would be needed to achieve a lethal effect, suggesting lower toxicity. Chemical durability refers to how stable a pesticide is in the environment, and low persistence indicates how quickly a pesticide breaks down. While these factors are relevant to environmental impact and safety, they do not directly reflect the inherent toxicity of the pesticide itself to organisms. Thus, the correct answer highlights the significance of LD50 as a primary measure of a pesticide's toxicological profile.