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What Are These Bugs on My Butterflyweed?

Question:

It’s September and I’m noticing a lot of bugs on my Butterflyweed.  What are they and should I do anything about them?

Answer:

Your Butterflyweed (Asclepias tuberosa) has a robust population of two types of insects, commonly found on milkweeds – Oleander Aphids and Large Milkweed Bug nymphs.  They are just some of the amazing variety of insects that use milkweed – it’s a micro food web.  A terrific little book was written about these many insects:  Milkweed, Monarchs and More: A Field Guide to the Invertebrate Community in the Milkweed Patch  Here is a short review of the book.

It may look like an army of insects on your plants, but don’t worry, at this time of year, in early fall, these pests are more unsightly than harmful.

Oleander Aphids

Oleander Aphids by Andrew Cannizzaro_Flickr

Oleander Aphids have a fanciful scientific name: Aphis nerii Boyer de Fonscolombe.  These insects are thought to have come from Mediterranean countries and were likely accidentally introduced to North America. Their name refers to their primary host – oleander – which is related to milkweed.  Any species of milkweed and dogbane is susceptible to these pests.

All Oleander Aphid adults are females – they reproduce asexually.  Instead of depositing eggs, they deposit live nymphs that are clones of the adult females.  The nymphs go through 5 nymphal stages.  As adults, some have wings and some are wingless.

The aphids eat the sap from the tissue of the host plant, in this case, milkweed. The resulting damage is mainly aesthetic, depending on the time of year and level of infestation.  As the aphids eat, they poop out what is referred to as honeydew – a sticky, sweet substance that develops a black sooty mold.  Unsightly, but usually not devastating to the host plant.

Oleander Aphids are mostly preyed upon by parasitic wasps –  tiny, non-stinging wasps that parasitize the target insect.  Lady bugs may also find Oleander Aphids to be a tasty snack.  Keep in mind that we live among many food webs – everybody has to eat – so think about keeping nature in balance, not eradication.

Attracting Natural Enemies

To attract and support parasitic wasps, lady bugs and other natural enemies in your landscape, you have to provide the food sources they need at various times in their life cycle.  Some population of pests is necessary to keep natural enemies around, as are small flowered plants that offer nectar and pollen, including Golden Alexanders, Heart-leaved Alexanders, Common Yarrow, Slender-leaved Mountain Mint, etc. Exotic plants in the carrot family, like parsley, dill, fennel and Queen Anne’s Lace are also very attractive to many natural enemies.

For more information on native plants that attract beneficial insects/natural enemies, listen to the EcoBeneficial podcast interview with Dr. Doug Landis.

Large Milkweed Bugs

Large Milkweed Bug Nymphs by Katja Schulz_Flickr

There are several species of Large Milkweed Bugs. The ones in your garden are Oncopeltus fasciatus, in the nymph stage. They are very common in the Northeast. Large Milkweed Bugs feed  primarily on milkweed seeds, but will also eat plant tissue and and nectar.  Since these bugs are part of the milkweed ecosystem, most people just leave them alone and tolerate a bit of damage – these bugs usually don’t decimate milkweed plants.

As they eat milkweed, Large Milkweed Bugs store harmful cardiac glycosides in their bodies – making them unpalatable to insectivores, like some birds.  Their coloration of orange and black is a signal to birds that they are not tasty.  Accordingly, few of these insects are likely eaten by birds.

Large Milkweed Bug adults do not overwinter in cold climates – they actually migrate south.  Having said that, with climate change and global warming, we will likely see more of them overwintering.  If you feel that your milkweed plants are being overwhelmed, the most organic control would be to pick off some (not all) Large Milkweed Bugs and toss them in a bucket of soapy water.  Even better, just leave them alone.

From Kim Eierman at EcoBeneficial!

Photo: Adult Large Milkweed Bug
Photo credit: Andrew Cannizzaro_Flickr