The killing and pest control decimating populations doesn't really effect the largest phyla on the planet making up about 75% of the animal kingdom. There are negatives to killing, spraying, and treating insects. They adapt to every environmental stress they can. Through natural selection, survival of the most fit, and genetics insect generations change and evolve at an alarming rate. Ants for instance are found on every inhabitable landmass on the planet, and have 12,500 documented species with 22,000 or more unknown species.
What my mind is constantly wondering is what happens when they master immunity to all pesticides, and so on. All I am trying to say is that killing insects might not have a immediate effect on the ecosystem, but where one insect fails another will rise. This repeats itself indefinitely. The next insect better than the one before it.