There are about 2,000 species of wasp in NZ and the five species that are a nuisance to us humans give the others a bad name. If you are encouraging butterflies then you will be well aware of the main predatory wasps: social wasps.
Social wasps build nests whereas most species of wasps are solitary. At the beginning of the summer, social wasps are intent on nest-building, then laying eggs. When the eggs hatch the adults search for protein to feed their young..
Bingo! Caterpillars are a great source of protein. For several months social wasps will steal your eggs and caterpillars of all stages, even pupae. By the end of the summer the juvenile wasps will be adults, and then the wasps will all be seeking out nectar for energy (no longer protein for building better bodies).
Nest of Asian (paper) wasp, thanks Alison Meier
Paper wasps (Polistes spp.) build nests from chewed up wood material, hence their name. Vespula species will often build nests in caves, trees and underground.
These wasps affect beekeeping operations as well as our native flora and fauna, and their stings can genuinely be a health risk. Be vigilant when outdoors around your property, looking for nests. Often they are in trees, bushes, or on buildings such as fences and the outdoor faces of homes and garages.
Tell the neighbours, pointing out what a pest these wasps are. Social wasps usually only forage within 200-300 metres of their nest. If you've located a nest, go out at dusk when the wasps are inactive and spray it or remove it with a plastic bag, tieing it off and putting it straight into the freezer. After 24 hours the wasps will be dead and you can put it into your rubbish.
"Wasp traps" may not be effective. So far I haven't found one that works (traps or baits).
Old curtains can be effective in making it harder for wasps to "get into" your plants to attack the caterpillars. A mosquito net over a frame can also be useful. Monarch butterflies have been known to lay eggs on the outside of the net and the caterpillars that emerge climb through the mesh onto the plant.
For monarchs grow swan plants now for next year and encourage them to bush out by pinching out the new growth. Not only does it mean the plants provide more shelter and protection but also more food for more caterpillars. Plant plants close to bushes and shrubs (or annuals like zinnias and cosmos) to give more shelter as well.
Please feel free to add more tips in the comments below!