In the early part of each season, as the wasps emerge after Winter, they need protein to feed their young. This is why they go for the butterfly eggs, caterpillars and then butterflies themselves. By the end of the season, when their young have grown, the wasps go only for nectar.
We feed sugar water to our nectar-sipping birds (Tui and Waxeye). Later in Summer we notice that many wasps are crowding around our nectar feeders.
In previous years I've tried all sorts of concoctions in the wasp traps, to entice the wasps in, but hardly any of them worked. Right now I'm putting sugar water into the traps and every day they catch dozens. I keep the level of the sugar-water quite low and I also put a small amount of dish-washing liquid into the sugar water, because that breaks the surface tension so the wasps just fall into the liquid.
The wasp we're catching is the German Wasp.