July 1, 2013

Milkweeds vital to monarch butterflies


---- — I have only one peony plant, and each year, as it begins to send up foliage, half a dozen milkweed plants send up their shoots all around it.

I used to pull them up as they broke ground, but now the only ones I pull or cut off at the ground are the ones that are growing right in the center of the peony.

Now that the peony has lost its petals, the milkweeds are beginning to set flower. I don’t think milkweed plants are the loveliest plants in the garden, but they are attractive for more than their blossoms.

You may know that milkweed is the only plant that the larvae, or caterpillar, of the monarch butterfly eats.

The female monarch butterfly lays her eggs on the leaves of the milkweed in spring and summer. When the eggs hatch, the larvae, or caterpillars, eat the egg cases and then the leaves of the milkweed.

Milkweeds contain cardiac glycosides, called cardenolides, which offer a defense to the monarch larvae from predators by making them toxic and bitter-tasting to other insects or birds that may try to eat them.

Interestingly, the monarch butterfly retains the protection of the cardenolides that it consumed in its larval stage. The bright markings, yellow, black, and white on the caterpillar and orange and black on the butterfly are recognized by predators as signals of its unsuitability as food.

The milkweed has its own defense system, as it does not exist just to support the monarch population and has ways of protecting itself from being eaten.

The milky substance from which the milkweed gets its name has a latex-like quality that can gum up the mandibles of the caterpillar, rendering it unable to eat.

There are about 100 species of milkweed native to North America, and the level of cardenolides varies widely. High levels can be toxic to the caterpillar itself.

Research indicates that the monarch butterfly may choose to lay eggs on plants with lower levels of cardenolides. Larvae use a variety of strategies to avoid the effects, including chewing areas that effectively stop the flow of the latex-like material, which is high in cardenolides.

It’s a fascinating series of interactions between monarch and milkweed, with nuances that we have little knowledge of and no control over.

One thing we can control is our part in preserving the habitat of the monarch butterfly.

I think the common milkweed is at a disadvantage because of its name. We think of weeds as something that are undesirable among our perennials or shrubs, on the boundaries of our property, along the roads and in the fields.

I suggest that we leave as many of them as possible where they are. Certainly, if you find them to be an eyesore in your garden, remove them.

If you are concerned about them spreading into areas where you don’t want them, remove the seed pods before they dry and open to release their seeds.

I ask that you consider their importance in the lives of the monarch butterflies before you decide how to proceed when you find them in your yard.

For more information on studies of the interaction between the monarch butterfly and milkweed plants, visit the University of Minnesota website and search for Monarch Lab.

Jolene Wallace is the horticulture program assistant for Cornell Cooperative Extension in Clinton County. Contact her at 561-7450 or