According to federal law, under what condition is it legal to combine pesticides?

The legality of combining pesticides is primarily governed by the stipulations on the pesticide labels. Under federal law, it is legal to mix pesticides as long as the specific label instructions do not prohibit such combinations. This means that if the label does not expressly state that combining the product with another pesticide or substance is forbidden, then it is permissible to do so.

This guideline helps ensure safety and effectiveness in pesticide applications. It allows for flexibility in the use of pesticide products provided that the applicator adheres to all label instructions. Labels typically include safety precautions, application methods, and specific mixtures that can or cannot be used, thus it’s crucial for applicators to read and understand these instructions fully.

While other options might exist as conditions for legality, they do not encompass the broad permissiveness afforded by the lack of a prohibition on the label. For instance, just because a manufacturer specifies something doesn't necessarily mean it’s legal in all cases, and water as a mixing agent doesn't cover all necessary conditions that might apply. Therefore, focusing on label prohibitions is the most comprehensive rule guiding pesticide mixtures.