Milkweed or monarch observation details

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We purchased a dying Milkweed off the discount table at Lowes in early June and planted it in a sunny part of our yard, next to a year round creek. Yesterday, to our great surprise, we spotted two 5th instars, munching away on the leaves. This morning (6am PST), Aug 7, 2022, both instars were still present, but by 10:30am, one had left....presumably. How on earth did these two find our singular milkweed? Were they on our plants originally as eggs?

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