During which larval instar is larvicide ineffective for killing larvae?

• A. First instar
• B. Second instar
• C. Third instar
• D. Fourth instar

Answer: (D)

Larvicides work best when larvae are actively feeding and taking in the product. In the earlier instars, mosquitoes are feeding vigorously, so ingestion or contact with the larvicide leads to death. By the time larvae reach the fourth instar, they slow down their feeding and are preparing to pupate. With feeding reduced and the imminent transition to the pupal stage, they are less exposed to larvicides, making these products much less effective. So, the fourth instar is the stage where larvicide tends to fail to kill larvae. If only late-stage larvae are present, alternative approaches or timing adjustments are needed to control them.