Cockroach taste hairs contain neurons of different classes, including one that responds to sugars and drives feeding and one that responds to bitter compounds and drives rejection. In normal German cockroaches glucose stimulates the sugar neuron. In glucose-averse individuals it also strongly stimulates the bitter neuron, and the aversive signal wins, so the animal recoils from a food it would otherwise eat.
The change spread because commercial gel baits used glucose as a phagostimulant. Any roach that refused the sugar avoided the poison and left more offspring, so the aversion was selected for directly by pest control practice. Glucose-averse populations have since been recorded in multiple countries, and laboratory crosses show the trait is inherited.
The cost is real. Glucose-averse roaches develop more slowly on some diets, and they behave differently during courtship, since males offer a sugar-rich secretion to females and averse females break off the exchange sooner. That trade-off is why the trait had not appeared before baits made it advantageous.