Got Milkweed?

Then you have an ecosystem.  Usually we think of ecosystems as big areas like a forest, meadow or lake. But a single plant can play an important role in the survival of a single species or several species.

honeybee on butterfly weed (Asclepias tuberosa)

Most gardeners plant milkweed (Asclepias) to support monarch butterflies. The precipitous decline in their population is reason alone to be planting all the milkweed we can in our gardens as milkweed habitats across the country are dwindling quickly. Every year I plant more milkweed in the hopes that monarchs will come. But they don't always show up. Last year, I mourned the absence of these spectacular butterflies as none were seen in our garden during either their spring or fall migration. In previous years we've had up to 12 arrive at the same time, usually in fall when the winds favorably blow our way. Sadly, we haven't had any monarch caterpillars here since spring 2011.

Monarchs visiting in October last year

(For a list of native milkweed species that grow in Georgia here is a field guide)

Monarch butterflies may not be the most abundant insects found on milkweed but they are certainly the most familiar.  There are other insects, who are also dependent on milkweed plants and couldn't survive without it, while some insects simply use milkweed as a major food source.

Eastern Tiger Swallowtails on Asclepias tuberosa

Milkweed blooms provide nectar for an array of pollinators including butterflies [such as the Tiger Swallowtail, Painted Lady, Great Spangled Fritilary, Buckeye, Black Swallowtail, Pipevine Swallowtail, Pearl Crescent, Clouded Sulphur], honeybees, wasps, and bumblebees.

But, did you know that milkweed is also a vital host plant for several other insects?

There is another caterpillar, usually found in late summer, that eats the older leaves of milkweed and unlike the monarch caterpillars who prefer young, vigorously growing shoots, this caterpillar will even eat the leaves that are turning yellow and crispy. The milkweed tussock moth (Euchaetes egle) caterpillars are orange, black and white and are nothing but hair. Just check out those long black lashes!

milkweed tussock moth caterpillars

These tussock moth cats are communal feeders who hang out in groups of 10 or more on the leaves. Plants are often blanketed with more than 50 caterpillars. Should you be alarmed seeing so many caterpillars defoliating your plants? I say no. These caterpillars hatch in late summer and are not really competing with monarch caterpillars in my garden, who would hatch in spring or early summer during their northward migration. Any monarchs visiting our garden during the fall will most likely be one of the super generation monarchs and not be laying eggs at this time.

Milkweed Tussock Moth (Euchaetes egle) caterpillars on butterfly milkweed

If you look closely at the stems and tops of milkweed you will probably find little yellow round things. These are not eggs but milkweed aphids, also known as Oleander aphids (Aphis nerii), which were introduced from the Mediterranean region where Oleander is its native host.

milkweed aphids (Aphis nerii) on butterfly weed

Like other aphids, milkweed aphids, tend to have a negative reputation, mostly because they feed by sucking the phloem of the plant, which can sometimes cause damage when in large numbers. I recently learned that scientists have failed to find any male milkweed aphids and therefore believe all the adult aphids are female. They produce without mating, a process called parthenogenesis, from the Greek meaning "virgin creation". The females don't lay eggs but deposit nymphs that are clones of the adult females. When conditions get too crowded on a single plant some nymphs are created with wings so they can fly off to establish new colonies on other milkweed (host) plants.

milkweed aphids, some with wings

Teeny tiny ants are often seen running around furiously on the foliage and pods of our milkweed. This is because the aphids secrete large amounts of watery honeydew attracting ants to the milkweed.

ants searching for honeydew on milkweed

Like other insects that feed on milkweed, the aphids are toxic to most predators because they ingest the toxic cardiac glycosides from the milkweed plant. Their yellow coloring is a warning to predators not to eat them.

Syrphid fly larvae eating milkweed aphids

But there are several predators and parasites that can tolerate the glycosides and help keep the aphid populations in check. Milkweed aphids are food for lady beetles, especially in their larva stage who devour large numbers of aphids daily.

Syrphid fly (aka Flower Flies or Hover Flies) larvae are insect predators too and crawl all over the milkweed foliage eating dozens of aphids each day. And there are tiny aphid wasps (Trioxys, Diaeretiella, Lysiphlebus and Aphidius) that will lay eggs inside the aphids and when the wasp's larva hatch it feeds on the insides of the aphid. Aphids killed in this way are often referred to as "aphid mummies".

There are two types of Milkweed bugs, Large Milkweed bugs (LMB)  and Small Milkweed bugs (SMB), that are found in large numbers on milkweed. The Large Milkweed Bug (Oncopeltus fasciatus) is the most abundant in our garden with many stages present on the plant at the same time.

Large Milkweed bug with nymphs

Milkweed bugs are a true bug so you can call them "bugs" without being frowned upon by your entomologist friends. These bugs go through a simple metamorphosis (vs. complete metamorphosis of butterflies). They lay eggs in the crevices between the seed pods. Females lay around 30 eggs a day or 2,000 in a lifetime. A lifetime being about one month! Just like caterpillars, the milkweed bug nymphs grow through a series of molts, usually 5 instars which are a week long.

Large Milkweed Bug instar beginning to show markings

These bugs have a long proboscis which they use to pierce the seed pods to feed on the seeds. My experience is that they rarely reach the inner seeds and therefore there are still viable seeds for producing more plants. Keep in mind that milkweed plants also spread through rhizomes and will form healthy colonies.

Large Milkweed Bug piercing milkweed seed pod

LMBs also suck sap from the plant so they too are toxic to predators. The bugs are gregarious and in large numbers intimidating to predators. An inexperienced bird may try to eat them but will soon learn that orange and black are warning colors.

Some adults may also feed on nectar and I do notice them occasionally on blooms using their long proboscis like a butterfly.

Large Milkweed Bug on fennel blooms

So, you may be wondering what role these bright bugs play. Well, back in the day, before humans started destroying milkweed habitats, these insects would help regulate populations of milkweed.

Single-handedly, milkweed plants support a group of insects, who are dependent on this plant. Perhaps not all these insects are desirable to every gardener, but my experience is that mature milkweed plants come back each spring with vigor. Our view on insect-plant relationships is often skewed by a few insect "pests" on a relatively small number of crop plants. My approach in our garden is to let nature do its thing, thus creating the biodiversity in the garden that is essential to make it grow productively and keep the balance between plants and insects healthy. And you can enjoy watching life on your own little milkweed ecosystem.