Dogs are not dumb. They have a territoriality threshold within which they automatically switch to alert. They stay at 'Alert' until they get reassurance from their master that an intruder is accepted. They don't have to be called to be alert. They jump to that status without prodding.
After dogs have gone to Alert status there is an expected response, a more cautionary approach. They remain at alert, barking. If an intruder fails to adopt a more cautionary approach, the dog's Challenge status is triggered and there is a good chance they'll become aggressive as the intruder advances too far within the dog's perceived territory, particularly if their Alpha, their master, suddenly appears, too. Without the Alpha's reassurance, the dogs will not step down from their Alert status.
My point being that dogs do not have to be trained to bark at Njerps. They'll do so unfailingly unless trained NOT to bark.