How do systemic and contact pesticides differ in their mode of action?

• A. Systemic pesticides move within the plant to kill feeding pests; contact pesticides kill on contact with the pest.
• B. Systemic pesticides kill pests by contact with the leaf surface; contact pesticides move through the plant.
• C. Systemic pesticides move through the soil and do not affect the plant; contact pesticides are systemic.
• D. Systemic pesticides require sunlight to activate; contact pesticides require moisture.

Answer: (A)

Systemic pesticides move within the plant after being taken up, so the active ingredient travels through the plant tissues to reach feeding pests. This means a pest must eat or feed on treated parts of the plant for the poison to work. Because the chemical is distributed internally, it can protect new growth and targets that aren’t directly contacted by the spray.

In contrast, contact pesticides act only where the chemical is present on surfaces or the pest itself must touch it. They don’t move through the plant to reach pests that aren’t in direct contact with treated tissue, so effectiveness depends on the pest contacting the treated surface.

So the best choice says systemic pesticides move inside the plant to kill pests that feed on it, while contact pesticides kill pests by direct contact. The other options mix up how movement through plant tissues works or add conditions (like needing sunlight or moisture) that aren’t defining features of systemic versus contact products.