Um, don't want to rain on anyone's campfire, but we have not eradicated the white cabbage butterfly here. They are an extreme pest, to the point where I've given up trying to grow brassicas for 4-6 months out of the year. My horseradish always gets skeleton leaves. Any stray mustard plants out in the paddocks are destroyed. Seedlings sown in autumn for winter and the following spring have to be netted and watched daily.
The article is referring to a second species, the "large" white cabbage butterfly, that was successfully contained around Nelson and is now probably gone. But the small ones are everywhere, thanks to the common planting of fodder brassicas and turnips as animal feed crops. If the farmers in each district would agree not to plant any about one in five years, we could get a handle on it using a combo of neem, Bt, and the UV tricks. But they won't have a bar of it.