milkweed

Asked September 14, 2020, 4:18 PM EDT

This is my first year growing milkweeds. When blooming I was delighted to see Monarch butterflies flitting among the blooms. I also found orange-colored aphids on the leaves. Water spraying did not dislodge them, so I resorted to just crushing them between my fingers. And yet, the next day there would again be aphids present. Where are they coming from? As the blooms went to seed, milkweed bugs arrived. You folks said I could ignore them because they were only eating the seed pods which would not affect the return of the flowers next season. As of today there are 8 Monarch caterpillars among the 6 milkweed plants. BUT they are being denuded of leaves. Is all this to be expected?

Baltimore County Maryland

3 Responses

Yes, Milkweed does have its own little community of specialist herbivores. As damaging as some of them seem to be, they don't tend to cause the plant any permanent harm, and the Milkweeds should re-emerge next spring fresh and healthy from their roots. The most common leaf chewers are caterpillars like the Monarch and Milkweed Tussock Moth (which feed in groups when young, which speeds-up their denuding). However, even deer and rabbits have been known to chew some leaves off now and then, despite their general distastefulness. Caterpillars usually leave the midrib of the leaf alone because it's tougher to chew, so if this is missing then mammals are suspect. If deer or rabbits are the problem and you need to preserve the plants to keep the caterpillars from starving, then a physical barrier like fencing or netting might be needed; use caution with netting so you don't ensnare birds or other wildlife, and don't trap emerged butterflies underneath.

Aphids are literally born pregnant, so can breed quite quickly from individuals that are missed in any given squishing. (Most aphids on plants, for most of the year, are female. For this species, there are no known males.) Some years, predators and parasitoids keep them in check quite easily, but other years they seem to struggle to keep up (this was one of those years).
https://content.ces.ncsu.edu/oleander-aphid

Milkweed Bugs do indeed only feed in any numbers on milkweed seeds within the pods and don't cause trouble unless you are counting on harvesting all of a plant's seeds and need as many viable as possible. Since this is rarely the case, they don't warrant control, especially since any insecticide sprays threaten the health of Monarch caterpillars and other beneficial insects (including those preying on the aphids and bugs).

As the Monarch caterpillars mature, they will leave the plants and wander off (usually). Caterpillars do this to form their chrysalis elsewhere so predators, which use the scent and sight of chewed leaves to find their prey, are less likely to find the very vulnerable chrysalis. It would be best to delay any cutting-back of perennials in the vicinity of the Milkweeds until at least early winter (better yet, early spring) to prevent harm to chrysalids (which will hatch in fall at the latest) and overwintering beneficial pest control insects.

Common Milkweed Insects - https://wimastergardener.org/article/common-milkweed-insects/

Miri

Thank you, Miri, for this fascinating information! I've made a hard copy of your response so, come next year, I can refer to it to remind me what this activity in the Milkweeds is all about.

Beth Marrus