People like to think they are irreplaceable, and nobody ever is - at least for the inner group. The wipeout will happen by viruses that detect predetermined DNA sequences to trigger the payload. This way, the inner group can sterilize one selected group after another, or shorten their life span more or less drastically. They will also use such viruses to change the DNA of selected groups, for example to create a guard caste, or to make their group less susceptible to attacks from other groups.