What effect do insect growth regulators have on mosquito larvae?

Insect growth regulators (IGRs) function primarily by disrupting the normal developmental processes of insects, specifically during their larval stages. When IGRs are applied to mosquito larvae, they do not kill the larvae on contact but instead interfere with their development. This means that while the larvae may appear to be developing normally in the short term, the IGRs inhibit their ability to molt and mature into pupae or adults. Consequently, they prevent the larvae from reaching maturity, which ultimately reduces the population of mosquitoes over time.

By allowing the larvae to develop normally in the initial phases, IGRs ensure that the insects do not progress through their life cycle successfully, ultimately leading to a decrease in adult mosquito populations. This contrasts with the other options, which suggest direct lethality, rapid growth, or effectiveness against adults, none of which are mechanisms by which IGRs operate. IGRs are specifically designed to target the growth and development stages, making option B the correct choice.