Here in New Zealand they devour the plants and over lay eggs on them massively, always have in my nearly 50 years of watching them. Stripped plants is the norm here. In fact you almost cant grow enough plants. Not sure if ours are the same as what you call Milkweed but ours are called Swan Plants. I took a cool photo of one a few years back, I'll have to dig that up and addd to the thread over the weekend. One of my customers is apparently very well known for "breeeding" them and on a site visit there I was stunned to be sourrounded by hundreds (or more) of them and they have the biggest swan plants I have ever seen, as big as a small car. The only place I have ever seen plants not stripped in season.