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

Methoprene, being a growth regulator, operates through disrupting the normal development of mosquito larvae. Its mode of action targets the hormonal processes necessary for larvae to mature. Consequently, when evaluating the effectiveness of methoprene using standard pre- and post-treatment dipper sampling methods, the results may not accurately reflect the larvae's developmental stages or populations in the short term. This is because methoprene acts more subtly and over a longer period—its effects may not be immediately visible in the field as there might still be live larvae present that will not complete development into adults.

In contrast, the other options, such as Bti, Temephos, and Permethrin, typically kill larvae on contact or through ingestion, thereby leading to immediate observable reductions in larval populations that can be easily assessed with dipper sampling. Therefore, methoprene's specific action and the time required for it to exhibit noticeable effects make it unsuitable for evaluation through conventional dipper sampling in the field immediately after treatment.