Host Plants: Feeding Caterpillars = Building Butterflies
Many homeowners and landscapers don’t realize the futility of just planting exotic nectar plants to support butterflies. Caterpillars of butterflies have evolved with specific host plants on which they depend. Some caterpillars use a very limited number of native host plants, some have the ability to use a wider range of plants. Caterpillars typically eat a small portion of the host plant’s leaves. It’s an important food web relationship in our ecosystems. No host plants mean no adult butterflies.
Monarch butterflies are often used as an example of this relationship, as their caterpillars have evolved to feed only on Milkweed species (Asclepias). You would think that planting any member of the plant family Asclepiadaceae would be great for Monarchs. Not so. It gets tricky with introduced, non-native species.
In the late 1800s Black Swallow Wort (Cynanchum louiseae) was introduced from Europe as an ornamental plant. It is a member of the plant family Asclepiadaceae. Black Swallow Wort is thought to have escaped from a botanical garden in Cambridge, Massachusetts. It has now invaded natural areas in Pennsylvania, New York, Connecticut, Rhode Island, Massachusetts, New Hampshire, Maine, Michigan, Wisconsin, California and several other states.
It turns out that Monarch caterpillars which feed on Black Swallow Wort have a mortality rate of about 100%. Why is Black Swallow Wort so bad for Monarchs, given that it is a member of the Asclepiadaceae family? Monarch caterpillars simply have not evolved with the toxins in the plant’s leaves. Adult Monarch butterflies are “tricked” by the plant believing that they are laying eggs on an appropriate caterpillar food source, not realizing that their caterpillars are doomed.
Lesson learned? What you plant matters!
Plant regionally native plant which have evolved in our ecological food webs from Kim Eierman at EcoBeneficial!
Photo: Tim Spira from the Dept of Biological Sciences at Clemson showing a Pipevine Swallowtail and the native host plant it has evolved with, Pipevine (aka Dutchman’s Pipe) Aristolochia macrophylla.
Photo by Kim Eierman
Check out Tim Spira’s book: “Wildflowers and Plant Communities of the Southern Appalachians.”
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Any ideas on getting rid of st Johns wort? (It’s an invasive here in California.) I am weeding a 40×40 foot section, most on a steep slope, full of the stuff. I had thought to cook the plants under plastic this summer but think that might cook the roots of a native oak tree in the center of all the wort. Alas, the wort is frequently used as a ground cover and has persistent “stringers” that pop up all over the place. I want to preserve the tree and create a rock garden with natives of the Sierra foothills.
Thanks for the great question! This is a great example of how properly identifying a plant and understanding how it grows will help you eradicate it or reduce its population.
Although we have many species of native St. John’s Wort in the US, there are some introduced species which are problematic, including Hypericum perforatum (Common St. John’s Wort or Klamath Weed), which is likely what you are encountering. This plant was introduced to the US from Eurasia and Africa in the 1700s as an ornamental plant and medicinal herb. It often invades farm lands, and grazing lands, including old fields.
Why is it problematic? Because, as you have seen, it is highly aggressive and crowds out many native species. It is also causes gastric distress to mammals which eat it, and is toxic to them cumulatively over time. The plant is particularly bad for lightly pigmented mammals which eat it, causing a photosensitive reaction in these animals which then develops into painful blisters around their mouths, eyes, ears, nose, etc.
On the good news side, Hypericum species, including this one, are great sources of pollen for bees (they do not provide nectar). Although this invasive species has some benefit to pollinators, it would be far better for your ecosystem to replace the exotic invasive Hypericum with a diverse selection of native plants, including one or more native species of St. John’s Wort. The California Native Plant Society (like many others) provides a native species list by region: http://www.cnps.org/cnps/grownative/lists.php
Hypericum perforatum propagates vegetatively by underground stems or runners making it difficult to eradicate. As you have correctly pointed out, putting plastic on soil is not advised as it will cook the soil, destroying many millions of beneficial organisms in the soil. It would also be a very bad idea for the existing oak tree, whose roots probably extend to 2 to 2 1/2 times the diameter of the tree’s canopy. Most tree roots are found in the top 12″ of the soil, so it would be impossible to prevent root damage if you used plastic sheeting.
Here are some earth-friendly measures which you can take:
– There are some biocontrols which have been successful in controlling Hypericum perforatum. In 1946 two species of Chrysolina beetles were introduced into California to control the Klamath Weed. They worked with great success, and reportedly, the plant has been controlled quite well at elevations under 1500 meters, where the beetles can survive. Check with your local cooperative extension to see what the status is for such bio controls in your area.
– Do not let the plant go to seed. One plant can produce 15,000 to 30,000 seeds in a single season, significantly adding to its spread. Note the the plant’s seeds are especially long-lived and can remain viable in the soil for 6 to 10 years.
– Constant mowing to weaken and eventually kill the plants can be useful. Note that this will take quite a bit of time, potentially years, and you will stimulate some vegetative growth, which of course you will mow over again!
– Using a lasagna, or layering approach (aka sheet composting) can smother the plant over a period of time. Again, this is not a quick method and can take a year or more, but it is much kinder to the soil. This method does not involve tilling (which encourages seed germination), but rather is a layering of mulch, compost and cardboard in layers which disintegrate over time. Eventually, you will be able to plant directly into the decayed organic matter. Here is a description of the method: http://organicgardening.about.com/od/startinganorganicgarden/a/lasagnagarden.htm
– When you replace this invasive plant, use a diverse range of regionally native plants which are competitive and can “hold their own.” Delicate, temperamental species will be hard pressed to stand up to “invaders.” When you plant these native species, plant thickly and quickly. Don’t leave soil bare for very long or the bank of weed seeds in your soil will germinate. In terms of what plant format to use, I suggest trying native “plugs” if you can find them. These are small live plants with robust root systems which will take far less time to establish than seed, and will have a greater net survival rate. Plugs are far more economical to use than potted plants. Don’t forget that even native plants must be watered regularly for at least a full growing season to get them well established.
The reality is that tough invasive plants take time to beat back. There is no quick magic bullet. Some may believe that a good dose of glyphosate (the key ingredient in Round Up) is a quick solution. But, we are now learning that this product/ingredient has unexpected consequences which are far more harmful than we first thought.
Invasive plant eradication may well be the least appealing part of gardening! Take comfort in knowing how much healthier your ecosystem will be when you fight the good fight and replace a highly invasive plant with a robust diversity of regionally native species, all supporting the food web in your landscape. Good luck and let us know how your efforts are going in future posts!