Insects may grow used to the effects of pesticides over time (natural selection), causing their genetics to be stronger than the pesticides, while eagles (having nothing to do with pesticides, because they typically live in the air) would be affected because the presence of the chemicals are new to its genetics.
Through the process of natural selection, insects will become immune to the effects of chemical spray, while an eagle that hasn't ever been exposed to such chemicals would be harmed significantly more.
