I’ve noticed lately that there are piles of these insects on my milkweed plant. Are they harmful to it? Or could they be harmful to other plants in my garden?
They're Milkweed bugs: https://en.wikipedia.org/wiki/Large_milkweed_bug
They feed on the Milkweed seeds, stems and leaves and can also feed on other related plants.
The can become problematic to the plant if there are too many of them. If you're not seeing too much damage and don't care about the plants not producing viable seeds you could leave the bugs be.
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Thanks. We planted the milkweed to attract butterflies. Will these bugs inhibit that?– dpwJul 31 '19 at 19:50
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@dpw If they are doing a lot of damage to the plant they could be. For reference I also have Milkweed beetles on my Milkweed plants but far less than in your picture. I've still had Monarch butterflies in my garden for the last 3 years, every year. Sadly no caterpillars, just feeding butterflies.– GillesJul 31 '19 at 19:56