What is the most likely reason a customer experiences poor results when using herbicide intended for insects?

The most likely reason a customer experiences poor results when using a herbicide intended for insects is that the pesticide used was not meant for the intended target. Herbicides are specifically formulated to control unwanted plants (weeds), while insecticides are engineered to target insect pests. If a customer applies a product designed to manage weeds in an attempt to control insects, it simply will not work because the active ingredients and modes of action are tailored for entirely different organisms. Therefore, using a herbicide for control of insect pests leads to ineffective results since the product does not affect insects as intended.

The other options, while they can lead to poor results, do not directly address the fundamental issue of mis-targeting. Applying a herbicide late can affect timing but would still be ineffective if the target is incorrect. An incorrectly prepared mixture may cause suboptimal effects but it doesn’t negate the primary target designation of a herbicide being for plants, not insects. An expired pesticide can lose potency but again, it won't change the fact that the product is intended for a different group of organisms.