The common classes of insecticides typically have a broad spectrum - so that they can devastate biodiversity of harmless and beneficial insects.
These insecticides include:
Organochlorines (such as DDT) that targets sodium channels in insect nervous systems
Pyrethroids (synthetic and natural pyrethrins) that also target the sodioum channels
Organophosphates that target Acetylcholine esterase (again a protein acting at neural synapses)
Neonicotinoids (such a simidacloprid) that target the nicotinic acetylcholine receptors (as does nicotine)
Various 'Insect Growth Regulators' that targets of some are still unknown!
Insect hormone disruptors (eg. methoprene which acts upon the juvenile hormone receptor)
Anthrallic Diamides (such as the chlorantraniliprole) that are relatively new and target the Ryanodine receptor at neuromuscular junctions.
Many of these insecticides are being de-registered because of the harm they do to humans, the environment or because they de-value market value of commodities.
Ideally new insecticide will control the pest insects without harming other components of the ecosystem such as harmless insects or beneficial insects.
For a recent account of threats to insect biodiversity see: