Exterminators have observed that there hydramethylnon-corn syrup baits initially work for attracting cockroaches, but the effectiveness lessens over time. does this mean that the aversion trait evolves in the cockroach populations exposed to the bait?

No, this does mean that the aversion trait evolves in the cockroach populations exposed to the bait. Cockroaches are a diverse species. This diversity is one of the reasons they thrive. Not all cockroaches will be strongly attracted to hydramethylnon-corn syrup. However, the ones that are all died and can not procreate. The cockroaches that are not very attracted to the syrup live on to breed, and pass along the aversion trait. So the trait does not necessarily evolve, but it does become more prevalent.