I don’t have examples offhand, but recently did a bit of reading at the behest of a friend whose cats had fleas, and recently developed pesticides were found to be active at levels of just a few nanograms per flea. Many of the newer insecticides are active to this degree, or close to it. Industry started designing pesticides using the principles of Rational Drug Design and quickly came up with a bunch of fantastically lethal (to the target, namely, insects) chemicals which are now being sprayed all over the environment.

Related: if you’ve ever happened to notice that more recently developed medications often require a lower dose, in milligrams, than older ones in the same class - that isn’t by accident. There are a number of principles that can be used to increase the potency of a candidate bio-active chemical, pesticides included.