Bacteria are able to develop immunities to antibiotics that because their life-cycles are really, really short - they evolve really fast. Also, because huge populations of them are constantly being bomboarded with huge quantities of antibiotics due to Industrial Shit I Do Not Actually Understand, so there's plenty of opportunity for one bacteria to come up with a helpful mutation to resist a specific antibiotic and reproduce a lot.
Bugs don't live long, but they live at least a few days, which means population-wide genetic changes are going to take orders of magnitude longer than they do for bacteria. And they've got other places to live than on crops and domestic animals, and we're not spraying pesticides in the forest or giving flea pills to deer.
So I feel like if bugs are "learning" anything on a genetic level, the majority of survival-conducive mutations that actually get passed on are going to be the ones that tell them to stay away from humans and our stuff, rather than tiny changes that make their exoskeletons a little tougher in the face of something corrosive.
Evolution likes path-of-least-resistance shit like that, right? Even if a flea all of a suddem mutates up a total resistance to pesticide X, and that mutation doesn't impact them negatively otherwise, and that flea survives contact with said pesticide and breeds, it's not going to do as well reproductively as another flea on a deer in the forest that's got another mutation that tells fleas to slightly prefer deer/raccoons/bears/etc over dogs/cats/horses/etc. The forest flea has plenty of potential mates because no one's given its deer any Frontline-or-generic-equivalent-thereof; the house flea, in a house where most of the other fleas just got massacred, obviously has fewer opportunities.
(I feel bad for the mutant flea now...)
The only way I can see the "fleas get used to pesticides" narrative making sense is if some of the pesticides actually are antibiotics, and they function by messing up a symbiotic bacteria that lives on/in fleas, and that bacteria is actually the thing that mutates...?
But even then, it would have to be slower, because so few fleas ever come in contact with flea killer in the first place.
Anyway, I refuse to buy still-under-patent flea killer anymore because the generic stuff is fine.