Application rates of granular insecticides with a horn seeder can be changed by which of the following?

The application rates of granular insecticides using a horn seeder are influenced by adjusting the wand opening and changing walking speed because these two factors directly impact the amount of product released during application.

By modifying the wand opening, the amount of granules dispensed can be controlled; a wider opening allows for more granules to flow out, increasing the application rate, while a narrower opening reduces the flow. Additionally, the speed at which the applicator walks affects the distribution of the product over the targeted area. Walking faster means less product per square foot is applied, while slowing down allows for a heavier application.

These adjustments are critical for achieving the desired pest control while minimizing potential negative impacts on the environment or non-target organisms. The other options involve factors that do not effectively alter application rates in the same manner as wand opening and walking speed.