Disease? on my milkweed plants last summer
I am sorry that I do not have a photo. For the past four years or so, I have grown milkweed in my backyard. Two summers ago, I actually saw two monarch caterpillars on my plants, which was super exciting. Last summer (2017) I was hoping to see caterpillars again, but never did. What did happen, though, is that many of the leaves seemed to get covered in a dark brown substance that seemed a bit sticky, and rather than attracting caterpillars, there instead seemed to be a bunch of wasps that liked to hang out on the leaves (not together). It started off with only a few leaves having this appearance but by the end of the summer, pretty much all of my milkweed plants looked like that. I wasn't sure if I should remove the leaves or not, and I ended up doing nothing. This time, I would like to get ahead of the game and know what to look out for. Is there any way, based on what I described, for someone to tell me what was the source of the disease or infestation on my milkweed plants, and how I might be able to prevent it or treat it if it happens again this year? Thank you,
Based on your description it is possible that you may be referring to possible insects on your milkweeds, such as the oleandar aphid and/or the milkweed bug.
The aphids are sucking insects that suck sap and exude a honeydew that fosters the growth of a black sooty mold. Wasps can be attracted to the honeydew.
Milkweed bugs are more of a nuisance than a threat to milkweed plants. The milkweed bugs and the plants have coevolved together and usually the plant can tolerate the damage. Otherwise sweep the bugs into a container of soapy water.
Take a look at our website and the Missouri Botanical Garden for more information and what to look for http://extension.umd.edu/hgic/earth-friendly/aphids-got-your-milkweeds
This season monitor your plants for signs of possible insects and symptoms. You can send us some photos this growing season once you notice symptoms.