what does it mean that the roaches can be killed off ?

Mama tells Beneatha to watch the way she is spreading the roach poison, inferring she might get some of it on Travis. Beneatha answers that she doesn't believe it would hurt her nephew because it doesn't seem to hurt the roaches..... they can never get rid of them. Mama responds little boys aren't as tough as the roaches.... inferring the poison might not kill the insects, but that it will certainly hurt the boy.