Insects are not all the same, some are hundreds of millions of years evolutionarily distant from one another. This leads to quite specialised biology in differe

Insects are not all the same, some are hundreds of millions of years evolutionarily distant from one another. This leads to quite specialised biology in different insect groups, and many insecticides have highly specific action that targets only a subset of insect species by interfering with molecular pathways specific to them. So actually, we really don't want to jump to conclusions. Just because something is an insecticide, doesn't mean it kills or harms all insects. A further relevant fact is that bee decline is related to many factors, possibly including pesticide use, but almost certainly including the modern practise of intensive bee farming and transport for pollination services, which leads to rapid spread of diseases.