There is not an awful lot that can be done, but here are a few tips:

• Avoid having anything around that may attract wasps (e.g. remove rotten, damaged fruit on the ground and on the trees themselves).
• Put up traps ( e.g. softdrink bottles filled with a strong sugar syrup, to be hung up in the branches).
• Cover trees with a finely woven fabric such as 'Microklima' to keep the wasps out.