Which larvicide cannot be evaluated by taking pre- and post-treatment dipper samples in the field?

The reason methoprene, a growth regulator, cannot be evaluated effectively using pre- and post-treatment dipper samples stems from its mode of action and the way it affects mosquito larvae. Methoprene functions by disrupting the normal growth and development of mosquito larvae, specifically targeting their ability to mature into adults. This means that, rather than killing the larvae outright or causing immediate mortality, methoprene alters their life cycle, leading to impaired growth and eventual inability to reproduce.

When using dipper sampling, the intent is to assess the direct impact of a larvicide on the larval population by counting live larvae before and after treatment. However, because methoprene doesn't kill the larvae immediately, but instead prevents them from growing into adults, the dipper samples could show similar larval counts before and after treatment, despite the fact that the treatment is still effective. Thus, evaluating its effectiveness through this method does not yield reliable or meaningful data.

On the other hand, the other larvicides listed, such as Bti, temephos, and permethrin, function through different mechanisms that typically result in immediate mortality of larvae. This allows for clear comparisons using pre- and post-treatment samples, making them suitable for evaluation through that method.