Milkweed attracts many insects and invertebrates, including monarch butterflies. Learn about its fascinating link to two insects that coevolved with it.
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What is A Butterfly Host Plant?
A butterfly host plant provides nectar for species of adult butterflies that we call specialists. Specialists prefer a specific plant for laying eggs.
Did you know that monarchs may travel upwards of 3,000 miles on their migratory route? This makes them especially vulnerable to habitat loss, drought, and logging, which reduces wild stands of their host plant, milkweed.
Not only is it the preferred food source for adult butterflies, the caterpillars dine exclusively on it as well.
There are over 100 species of milkweed native to North America, and I happen to have two of them in my yard: honeyvine milkweed (Cynanchum laeve) and butterfly milkweed (Asclepias tuberosa). Swamp milkweed (Asclepias incarnata) is another popular type for wet areas.
Poison in a Flower: Nature’s Protective Strategy
As this plant’s flowering season winds down, it seems like the action might be done for the year. But for this plant, it’s just getting started!
In late summer, you can spot insects such as monarch caterpillars, milkweed aphids, and large milkweed bugs all feeding on the plant—two of these are good, one not so good. All of them are here to dine on the milky sap that give the plant its name.
Interestingly, these insects have evolved to take in the toxin glycoside that exists within the plant’s juices, which then makes them poisonous to anything that might want to eat them.
2 Surprising Insects that Love Milkweed (as much as Monarch Butterflies and their Caterpillars)
Let’s take a look at two insects that love this plant just as much as monarchs!
1: Milkweed Aphids
Honeyvine milkweed is an aggressive vine, and although it’s native to my bioregion, I would never grow it by choice. Unfortunately, our neighbor allows it to take over our shared fence, and it spreads by seed and roots. I’m constantly pulling the vines that pop up EVERYWHERE and threaten to take over our entire 0.10-acre.
The vine is covered each year with milkweed aphids, which are reportedly non-native and invasive. However, it’s unclear whether they help or hurt the situation for monarchs. I expect that it’s a little bit of both, which is usually the way it goes in nature.
On one hand, the aphids cover the plant and may make it hard for 1st and 2nd instar monarch caterpillars, who are vegetarians and dine strictly on milkweed leaves, to get enough to eat.
Garden experts generally recommend a soap and water spray (when caterpillars aren’t present) to wash the aphids off, since there are no known predators that can ingest the now-toxic insects that have been feeding on the toxic milky sap.
On the other hand, the aphids may be an important protein source for 4th and 5th instar monarch caterpillars, which seek out additional nutrition as they prepare for pupating.
What’s more, some sources report that ladybugs can eat milkweed aphids. This reputes the claim that glycoside toxicity makes the aphids unappealing to species that haven’t coevolved with the plant. However, if ladybugs can ingest the aphids, then it may discourage them from preying on monarch caterpillars, thus making the aphids a positive coevolutionary monarch companion.
2: Large Milkweed Bug
As I was doing my daily walk around the garden yesterday—inspecting everything for unusual signs and making harvest lists—I noticed the immature milkweed seed pods covered in red and black bugs.
I’ve seen this in years past and assumed they were some aspect of the aphid’s life cycle. With a little more digging, however, I discovered that they are appropriately called the large milkweed bug (Oncopeltus fasciatus), and also feed on the milky sap.
As far as I can tell, they’re common and although a nuisance, not a threat to monarchs or the plant.
As you can see, milkweed is an interesting plant that is both important for the monarch butterfly as well as a number of other insects that have coevolved to favor the toxic milky sap.
Are you growing milkweed? Have you noticed any of these insects on your plants? Do you provide habitat for monarchs or other specialist butterflies?