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Monarchs are migratory, so 20% loss of their food source at critical spots in their migration path could be deadly.

Edit: not food source for the adults! Sorry, it's been a long time since I studied Monarchs!

For butterflies, the food source is nectar from flowers. I don't know that flower populations are in decline.

The milk weed dependency is because the butterflies will only lay their eggs on milk weed plants. The caterpillars will only eat milk weed. They tolerate the toxicity, which in turn makes them unpalatable to birds that would otherwise eat them.

callmeed's post gives excellent source material [https://news.ycombinator.com/item?id=7866154]

The butterfly's migratory cycle is longer than its life cycle. The species needs milkweed throughout its migratory path in order to survive.

Crap, my mistake.

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