There often is controversy over the use of pesticides. And yet, many people don't have a good understanding about exactly what pesticides are, and why they are often necessary for plant production.
A pesticide is anything that destroys pests or suppresses or alters their life cycles. We normally think of pests as bugs, but they also can be invasive plants, fungi, bacteria, and other organisms.
Pesticides are not necessarily synthetically produced. Many pesticides occur naturally. Pesticides are used to control bacteria, fungi, and insects; to control rodents and other animal pests; to control pest-category plants; and to repel pests.
In this suggested science fair project, you'll seek to discover how close to a plant a pesticide must be in order for it to be effective. You could choose any pesticide that you wish, but you'll need to use the same type of plants, growing conditions, and so forth.
You could do this experiment with potted plants set outside, but it would work better if you could plant four groups of the same plant at short distances apart directly in the soil.
Apply pesticide directly on one group of plants, according to manufacturer's instructions. Then, place pesticide near, but not on, the other groups of plants, varying the distance from group to group.
Observe how the different groups of plant fare as far as insect damage is concerned, in addition to their general health.