How does BTI affect mosquito larvae?

BTI, or Bacillus thuringiensis israelensis, is a specific strain of bacteria that is used as a biological control agent against mosquito larvae. The correct answer is C because BTI produces a protein that, when ingested by mosquito larvae, is toxic to them. This bacterium produces a crystal protein that disrupts the digestive tract of the larvae, ultimately leading to their death.

When mosquito larvae consume BTI, the protein forms crystals that are activated in the alkaline environment of the larvae's gut. This process creates holes in the gut lining, causing the larvae to stop feeding, and subsequently leading to their demise. This mechanism is specifically targeted and does not harm other beneficial insects or organisms, which is a key advantage of using BTI in pest management.

The other options misrepresent the effects of BTI on mosquito larvae. For instance, while it is true that BTI is not a repellent and does not prevent larvae from breeding, its primary function is as a lethal agent upon ingestion, which is illustrated in option C. Additionally, it certainly is not harmless; rather, it is specifically designed to target mosquito larvae effectively.