Why is larvicide ineffective during the 4th instar phase?

• A. The 4th instar does not feed
• B. The 4th instar has thicker cuticle
• C. Larvicide cannot reach the 4th instar
• D. The 4th instar is immune to chemicals

Answer: (A)

The key idea is that many larvicides work by being ingested by actively feeding larvae. In the fourth instar, the mosquito larva is nearing pupation and typically stops feeding. Without feeding, there’s little opportunity for a larvicide that must be eaten to enter and affect the larva, so the product is ineffective at this stage. Timing matters because agents that rely on ingestion target larvae during their feeding periods, not when they have ceased feeding. The other ideas—thicker cuticle, inability of the product to reach the larva, or innate chemical immunity—don’t explain why ingestion-based larvicides fail here as well as the simple fact that the larva isn’t feeding.