Which type of habitat alteration would be most effective against the American cockroach?

Removing woodpiles can be the most effective method of habitat alteration against the American cockroach. Woodpiles provide essential harborage and breeding sites for cockroaches, along with the moisture they need to survive. By eliminating these potential nest sites, you reduce their opportunity to thrive in an area.

The other methods, while potentially beneficial in different contexts, do not address the immediate needs of cockroaches as directly as removing woodpiles does. Adding more vegetation could inadvertently create more hiding spots, while introducing natural predators may not directly control the cockroach population effectively in urban settings. Sealing off entry points is a great preventative measure but does not eliminate existing populations in the immediate environment. Therefore, removing woodpiles directly disrupts the cockroach's habitat and minimizes their chances of survival and reproduction.