Which of the following larvicides must be reapplied frequently due to its limited residual activity?

The choice of bacterial pathogens as the correct answer highlights their unique mode of action and application characteristics. Bacterial pathogens, such as Bacillus thuringiensis (Bt), are biological control agents that target specific insect larvae. They are effective due to their ability to produce toxins that disrupt the larvae’s digestive system. However, one limitation is their short residual activity; they break down quickly in the environment and need to be reapplied more frequently to maintain effective control of mosquito populations or other target larvae.

In contrast, organophosphates, pyrethroids, and insect growth regulators generally have longer residual activity. Organophosphates are known for their effectiveness and longer-lasting properties, pyrethroids create a protective barrier that can persist for extended periods, and insect growth regulators disrupt the development of insects over time, also offering a level of residual activity that can reduce the frequency of applications.

Therefore, the need for frequent reapplication of bacterial pathogens reflects their specific role within integrated pest management, where biological control methods can complement other practices but require careful timing and management to ensure effectiveness.