Final answer:
The forestry department uses aerial spraying to apply pesticide directly to tree leaves, offering a quick and evenly distributed treatment method that minimizes ecological impact when non-persistent pesticides are carefully targeted.
Step-by-step explanation:
The method the forestry department is using to spray pesticide directly on the trees' leaves is aerial spraying. This technique involves dispersing pesticides over large areas from an aircraft or drones, which allows for rapid treatment of forestry areas that might be challenging to reach by ground. Aerial spraying can be more efficient and cover areas more evenly when compared to other methods. However, it is crucial to choose and apply pesticides in a way that minimizes ecological disruption, by using non-persistent products and targeting application as effectively as possible.