The biggest threat to Justin Bieber's safety is little girls, according to the singer's security boss.

Mikey Arana has been in charge of the Baby singer's security for the past three years and in a new documentary, the bodyguard opens up about his biggest challenges.

In the film Bodyguards: Secret Lives From the Watchtower, Mikey reveals he has come to hate crowds of young fans.

"It may sound crazy, but three or four little girls have so much strength that they would literally rip his shirt," Arana says, "and if it's a big crowd, you could lose him in the crowd."

Bieber also appears in the film, and heaps praise on his security team for keeping him safe, while admitting he prefers his older fans - because they're less likely to scream in his face.

"As the fans get older and knowing themselves more, they start understanding how to act," the pop star explains. "They actually want to act a little bit more sane so that maybe they get my attention, because if you're screaming and super overwhelming, it's hard for me to kind of connect with you."

Bodyguards: Secret Lives From the Watchtower, which is produced by Bieber's dad Jeremy, is being released on iTunes early next month (Dec16). The documentary also features 50 Cent and Lil Wayne's security boss.