Bti primarily targets which larval instars?

• A. Instars 1-4.
• B. Instar 5.
• C. Adults.
• D. Eggs.

Answer: (A)

Bti is a microbial larvicide (Bacillus thuringiensis israelensis) that must be ingested by mosquito larvae in water to work. It’s most effective when larvae are actively feeding, which is typical of the early growth stages. The first through fourth larval instars are feeding vigorously and are exposed to the toxin in the water, making them the most susceptible to Bti. As larvae reach the later instars, feeding behavior and gut physiology change, reducing their susceptibility and thus lowering Bti’s impact. Eggs and adults don’t have a larval gut or feeding stage in water, so they aren’t affected by Bti. That’s why the best answer is the early instars.