QF stopped spraying more than a decade ago and I haven't seen spraying on my last 100 flights into Oz. QF pre-treats their entire fleet, so there is no need for spraying. Other airlines which don't have a dedicated fleet to Oz and don't want to spend the money to pre-treat all their planes will spray when they use an untreated plane. Kind of an indicator of airline quality and willingness to spend extra for a better pax experience