Every strain of German cockroach that we've brought in from the field in recent years has had extremely high pyrethrin resistance. Nymphs are very difficult to kill with pyrethroids. It only takes 30 times more to kill males and 20 times more to kill females. With the PBO additive, the pyrethroid is 20 times more effective. Just as with ants and termites, the most fortuitous baits used for German cockroach control are slow-acting.
Secondary kill is extremely important.
Secondary kill is when insects that did not feed directly on the bait still die. The mechanisms used by German cockroaches are coprophagy and cannibalism. With coprophagy, adults eat the bait, return to the harborage and defecate. The feces is contaminated with toxicant from the bait. Relying on cannibalism for secondary control is also effective.