A 5 year old bougainvillea is suffering rapid leaf loss. Close inspection
shows aphids to be the villains. It's planted in the soil of the allotment
greenhouse.

Main concern is preventing the aphids migrating to the newly sprouted
seedlings, in trays of compost, nearby. I need to spend some time today with
soapy water and brush them off.

Q. Can anyone suggest a spray deterrent, to prevent future attacks? I've
tried various insecticides in the past (can't remember their names) with no
luck. TIA.