Balance the beetles on your native milkweed


Everywhere, beetles pollinate our favorite flowers, fruits and vegetables and serve lunch to the birds. They perform an important service in the food supply of humans, animals, insects and butterflies. How do we know which insect to control and which one to report to the UC Cooperative Extension office? First, examine the nature around us.

Photo by Alice Cason

The ladybug larva looks like an orange-black alligator, but it can eat many aphids while undergoing a complete metamorphosis.

Insects are pollinators and predators. They can also be parasitoids, small insects whose immature stages develop either inside or outside of other insects who then become their hosts. Insects can also be the decomposers in our compost heap. UC Marin Master Gardeners teach the principles of integrated pest management. We identify the pest problem and solve it with the least toxic solution that does the least harm to the environment.

On my narrow-leaved milkweed family (Asclepias fascicularis) I have identified some orange colored aphids known as oleander aphids. This is a plant native to Marin, the western coast of the United States, and Mexico, and is an important host to monarch butterflies. It has a milky latex juice that contains cardenolide, a chemical that, when ingested, makes the monarch’s caterpillars poisonous and resistant to predators.

Monarchs only lay eggs on milkweed and the caterpillars only eat milkweed. Get used to seeing the leaves chewed up by the hungry caterpillars. That’s right, there will be holes in the leaves. This is a sign of success that your host plants are doing their job.

Fortunately, the oleander aphid feeds only on milkweed and oleander (nothing else), and even then, it rarely kills an adult plant. It produces a juice called honeydew, which attracts ants and some beneficial insects. Aphid predators include the ladybug, ladybird larva, syrphide or hover fly, lacewing beetle and its larva, and more.

Photo by Alice Cason

This seed-eating western little milkweed beetle has the same orange and black colors as the monarch butterfly to ward off predators.

Observe and watch nature and choose the best solution. Instead of spraying pesticides, you can wash off the aphids with water early in the morning. Use caution when spraying the underside of the milkweed leaves where you could disturb a monarch egg.

You can also crush them with your fingers. (Wear gloves if you don’t like the orange juice on your hands.)

The most fun is watching ladybugs solve the problem by eating the aphids. That is biological control. You may notice ladybug larvae that look like tiny black and orange alligators, but are also beneficial insects that eat their share of aphids.

Many things prey on aphids. This community of insects is associated with a common breakfast – the milkweed plant. According to Anurag Agrawal, author of “Monarchs and Milkweed”, there are a total of 11 insect species that frequently attack the common milkweed. They create a balance of predators or prey that has evolved over millions of years.

When the milkweed begins to form seed pods, a new insect appears – the western little milkweed beetle, Lygaeus kalmii. This seed-eating bug is red and black and binds cardenolide, the same sap that makes the monarch resistant to predators. By including these and other native plants in your landscape, you provide food sources for insects and a healthy habitat for insects.

Photo by Alice Cason

A monarch caterpillar nibbles on narrow-leaved milkweed.

Plant milkweed in full sun and well-drained soil. Asclepias fascicularis is a perennial with light green narrow leaves on 3-foot stems. Pink and white flower clusters bloom in summer.

Spurge is also toxic if swallowed. Mulch vigorously and water well until it is established. Please do not plant milkweed within five miles of the Pacific coast and wintering sites, as this can interfere with monarch migration.

Native narrow-leaved milkweed plants go into a dormancy phase in winter and faithfully return in the following spring at the right time for the coming monarchs.

Enjoy the associated insect community, the host plants for beautiful butterflies in your garden.

The University of California’s Marin Master Gardeners are sponsored by the UC Cooperative Extension and provide scientific and research-based information for home gardeners. Email questions to [email protected] Include photos if you have any questions about plant pests or diseases. The office is closed for visits. Subscribe to the Leaflet, UC Marin Master Gardener’s free quarterly e-newsletter, at marinmg.ucanr.edu

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