All it takes is a single (fertilized) female large milkweed bug (Oncopeltus fasciatus) to find a patch of milkweed plants, and a month later — you find a bonanza of developing nymphs cannabalizing the seed in the milkweed pods.
Females lay a clutch of 30 some eggs in a protected crevice in the developing milkweed pods, and the nymphs immediately begin feeding communally on those developing seeds. She doesn’t stop with just one clutch of eggs, but will continue to lay a batch of eggs every few days, eventually producing 2-3000 of them. No wonder there are so many nymphs on this stand of milkweed.
Like other true bugs (Hemiptera), milkweed bugs feed by inserting their slender proboscis into the plant, secreting saliva chock-full of digestive enzymes to turn plant proteins and carbohydrates into a nutrient-rich slurry, and then slurping back the digested mixture.
The large milkweed bugs I find in my backyard are probably dispersers from more southerly populations, perhaps from neighboring Iowa milkweed patches. They arrive late in the summer just as the pods are developing, and may not stick around to reproduce here in the fall (since it would be a waste of effort and energy to have eggs and/or nymphs freeze), but will migrate again to more temperate southern areas. Wherever there is an ample supply of milkweed seed, these bugs can continue to produce new generations year-round. Those that carry that migratory gene will take off next summer and hopefully recolonize my milkweed patch next year.