I was very excited to find 2 monarch caterpillars on my milkweed this morning! Then I got to googling and I saw that some people will put mesh bags over their milkweed to protect the caterpillars from birds. Is this a good idea? Or should I just leave them be? I’m also concerned about aphids – last year my milkweed got a lot of aphids and I treated them with soapy water spray. Obviously I don’t want to do that with caterpillars there, but can I move the caterpillar to a different milkweed, spray off the aphids, and then move the caterpillar back after a couple of days? This is the fist year I have had monarch caterpillars and I am so excited, want to make sure they are happy!
While not experts in monarch caterpillars, the Toronto Master Gardeners are very supportive of Torontonians who grow milkweed to help the monarch population survive and thrive. Our organization has given many hands-on workshops on this topic as part of the Toronto Public Library’s “Our Fragile Planet” series – look out for this series beginning again in September.
While aphids may be concentrated on your plants, unless you have a very heavy infestation, they are generally not in sufficient quantities to kill the plant or the caterpillars. As you suggest, spraying with insecticidal soap may inadvertently kill your caterpillar population. One solution that works is to remove the aphids by hand in the areas where no caterpillars are present by squashing them manually and then rinsing the plants with water. This may be a safer option than to try to manually move the caterpillars. It will be a thrill to observe them complete their life cycle!
Here are a few links to websites with information about aphids on milkweed.
https://monarchjointventure.org/resources/faq/aphids-on-milkweed
https://wimastergardener.org/article/common-milkweed-insects/