A stomach poison applied to the upper surface of leaves will fail to control an insect if

• A. The insects feed on the underside of the leaf
• B. The insects feed on the upper surface
• C. The plant has a waxy cuticle
• D. The insect is diurnal

Answer: (A)

Stomach poisons work only after the insect eats plant material containing the toxin, so exposure depends on where the insect feeds. If the poison is applied to the upper leaf surface, insects that feed on that surface will ingest it and be affected, but an insect that feeds on the underside won’t ingest much (or any) poison at all. That means the poison fails to control that pest because ingestion isn’t happening. The waxy cuticle or diurnal activity isn’t the deciding factor here; the key is where the insect feeds relative to the treated surface.