What is a low-toxicity pesticide that spreads as a thin film over water's surface called?

A low-toxicity pesticide that spreads as a thin film over water's surface is known as a monomolecular film. This type of pesticide works by creating a barrier that inhibits the ability of mosquito larvae to emerge from the water. The film reduces the surface tension, making it difficult for the larvae to break through and transition into adults. This method of control is particularly effective and is favored for its targeted approach while minimizing environmental impact.

The other options represent different types of pest control substances. Microencapsulated formulations refer to pesticides that are enclosed within a protective coating, allowing for slow release and prolonged activity, yet they do not specifically form a thin film on water surfaces. Biopesticides are derived from natural materials and include a variety of pest control methods but are not specifically characterized by forming a film over water. Synthetic pyrethroids are a class of chemical insecticides that mimic natural pyrethrins but do not have the low-toxicity, surface film properties associated with a monomolecular film.