Which of the following situations would necessitate using a dust pesticide formulation?

Using a dust pesticide formulation is particularly effective in situations where pests are located in hard-to-reach areas, such as behind appliances like dishwashers. Dust pesticides adhere well to surfaces and can penetrate into small crevices and voids, making them ideal for targeting pests that are hidden from view. The fine particles of dust can stay effective longer than liquid formulations, especially in places where moisture may cause liquids to run off or evaporate.

In contrast, the other scenarios listed are more suited for different types of pesticide formulations. For instance, applying pesticides in an open field typically involves using liquid formulations that can cover broad areas efficiently. Similarly, general surface spraying may be better accomplished with liquids that can coat surfaces effectively and provide broad coverage. Finally, when addressing pest issues in crops, liquid applications are often preferred to ensure uniform distribution over the plants and soil. Therefore, the unique properties of dust formulations make them the best choice for targeting pests in hard-to-reach areas, as exemplified by the situation involving cockroaches behind dishwashers.