Which of the following describes residual insecticides?

Residual insecticides are specifically designed to have a lasting effect on target pests, meaning they can remain effective for an extended period after application. This characteristic allows for continued control of insect populations without the need for constant reapplication. They work by remaining on surfaces where insects are likely to land or walk, thereby maximizing the likelihood of contact and reducing the frequency of treatments needed.

In contrast, the other options highlight characteristics that are not applicable to residual insecticides. For instance, some insecticides may kill mosquitoes on contact, but that does not define residual characteristics. Similarly, reapplying insecticides every hour is impractical and not a feature of residual insecticides, which aim to provide prolonged protection. Lastly, while some insecticides may not be effective against certain life stages of insects, including adult mosquitoes, residual insecticides are often effective against various life stages, especially if applied correctly. Thus, the defining feature of providing a long-lasting effect is what makes the second option correct.