Save your milkweed plants for monarch larvae
Milkweed (Asclepias) has become increasingly popular to grow as an ornamental plant and to draw monarch butterflies (Danaus
plexippus) into the garden. Invite this magnificent species into your yard to help to ensure their survival on our planet.
Native milkweeds such as common milkweed (A. syriaca), swamp milkweed (A. incarnata) and butterfly weed (A. tuberosa) are host plants for monarch caterpillars. Together with other native and nonnative plants, they provide nectar to the striking orange-black adult monarch butterfly. Monarch caterpillars have alternating yellow, black and white bands and two pairs of “stink horns.”
Milkweed leaves contain cardiac glycosides, compounds which affect heart function, making them toxic to livestock and other animals. But those same compounds make monarch larvae toxic and predators avoid them. There are other insects that can consume milkweed leaves without being poisoned. Some of these pests display vivid orange-black color patterns that suggest to predators that they’re toxic monarchs, an adaptation known as Müllerian mimicry. Pests commonly found on local milkweed include:
• Milkweed bugs (Oncopeltus fasciatus), whose bright orange nymphs are found along stems, leaves and seedpods. • Red milkweed beetles (Tetraopes tetrophthalmus), half-inch red beetles with black spots that aggregate on leaves and flowers in late summer.
• Oleander aphids (Aphis nerii), oval yellow insects with black legs and cornicles — tubes projecting off the abdomen that emit pheromones. Aphid eggs overwinter on milkweeds or plant debris. They are piercing/sucking insects seen in spring-summer that feed on the undersides of young leaves, buds and stems, causing distortion and discoloration. Their sweet, sticky excrement attracts ants. Unsightly black sooty mold often grows on the “honeydew.”
• Milkweed tussock moth caterpillars (Euchaetes egle), native, ravenous leaf-eating insects with orange, black and white tufts of hair that distinguish them from monarch caterpillars, which are hairless. They can be seen in clusters on the under surface of leaves.
None of these pests are cause for alarm. They may impact the appearance of the plant, but chemical control is not recommended as that may impact monarch caterpillars. Employ good cultural practices to ensure plants are healthy and resistant to insect problems. Remove plant debris in the fall, especially if insects were a problem in the growing season. Reduce irrigation, pruning and fertilization to decrease robust new growth attractive to aphids. You can prune out
affected branches to get rid of aphids, or hit them with a stream of water. Or you can leave the aphids and watch for beneficial insects to keep them under control.
Hungry monarch larvae can almost decimate a plant when feeding on the leaves, so you aren’t striving for plant perfection.
If you find tussock larvae on your milkweed and you want to save the leaves for the monarchs, handpick the tussock moth caterpillars and drop them into a dish of soapy water.
Butterfly weeds that have hosted many monarch larvae will be tattered. Plant summerand autumnblooming annuals such as cosmos or zinnias near your milkweed to fill in space left behind from monarch larvae. Then enjoy the beautiful butterflies that will visit their colorful flowers until they take their long journey south in the fall.