Milkweeds, Monarchs and More!

  • Monarch butterfly nectaring on Tropical Milkweed
    Monarch butterfly nectaring on Tropical Milkweed

Milkweeds and Monarchs 

Late summer in Marin is when we’re all on the lookout for the Monarchs; these regal butterflies are making their way back to traditional over-wintering sites along the California coast, and the real miracle of this migration is that it is not the same individual returning to the site! There are two distinct populations of Monarch butterflies in the United States, and the Rocky Mountains are the dividing line between the Eastern and Western populations. The monarchs we see in the West are over-wintering along the California coast, and during much of the rainy season, thousands of Monarchs are simply hanging in large groups in these special groves of trees. On warm days they may fly a bit, but mostly they are in a torpor. They are also in a reproductive diapause, waiting for days to lengthen and warm up.  

About twenty years ago, when I first started learning all I could about butterflies, I would see Monarch butterflies and larvae in large numbers in early spring, but these days, Monarchs and their larvae are much more common in the late summer and early fall in Marin. Populations have declined, and much of that has to do with the loss of habitat and the plant most important to the life cycle of this species: Milkweed (Asclepias species).

One of the best milkweeds to support the Monarch larvae is the Mexican (or tropical) milkweed, because it contains very high levels of cardiac glycosides - the toxin that makes the larvae distasteful to vertebrate predators. The Mexican milkweed (Asclepias curassavica), is a tender perennial that can be grown to a good size in a 4” pot, and is therefore the best one for classroom studies on the metamorphosis of the monarchs.

There is some concern that including these plants in gardens close to over-wintering sites could be discouraging monarch females from entering into a reproductive diapause, but the solution is simple! In my garden, Showy Milkweed (A. speciosa) starts to die back in September, but the Narrow-leaf Milkweed (A. fascicularis) is strong until the end of October. So when this native species starts to die back, I simply cut the Mexican milkweed down to the ground. If your garden tends to freeze, be sure to cover the root zone of this frost-tender perennial with a layer of mulch.

The other "Monarchs" on our Hill

In early October, I’m monitoring larvae in all stages on the Narrow-leaf Milkweeds that grow in several large stands on my property. But Monarchs are not the only regal presence on my hillside. It’s the rutting season and a number of very handsome bucks have made themselves visible again. It’s almost comical, the way they follow behind the does with their powerful necks outstretched, upper lip curled back to "taste" the smell of a particular doe - all of whom seem to do their best to ignore or avoid them. I imagine it’s tiring, too, and on one afternoon recently, a huge white-faced buck lay down for a rest under a blue oak not twenty feet away from where I was busy potting up plants – what a treat for me!

Young Buck still in velvet
Young Buck still in velvet

The buck’s "lip curling" reveals a special receptor on the top of the palate called the Jacobson’s organ that determines if the doe is in estrus and receptive to mating. The buck’s antlers at this point are smooth and formidable, all the velvet has been rubbed off, and it’s this process that can cause a lot of damage in a garden. As the mating season approaches, the antlers are fully developed and the velvet (a soft, moss-like skin that has nourished the growing antlers) has done its job. Apparently, the velvet as it peels off is itchy and uncomfortable like peeling sunburnt skin, so the bucks like to rub on all sorts of things to hasten the shedding. One favored "rub" is willows, and it’s quite possible that the salicylic acid (an active metabolite of aspirin) contained in the willow bark provides some temporary pain relief.  

Antlers are solid bone, grown and discarded every year. The antlers are the fastest growing normal cells of any mammalian tissue, and growing a new set requires a lot of resources early in spring. Co-evolution is displayed in yet another wonderful association between plants and animals - Ceanothus species put on a lot of new growth in early spring; this new growth is very high in calcium and thereby provides the perfect browse for the bucks growing their antlers! As the deer browse the plant, enzymes in their saliva act as a hormone spurring on even more new growth!

But during the rut, the bucks will use whatever is around, and so it is important - especially with young trees - to leave some caging around the trunks. The bucks also like to show off by just ripping up other shrubs. Coyote brush (Baccharis pilularis) is one that is often damaged on my hillside, but it is easily pruned back to the base, and fresh new growth (and often a better form) develops during the rainy season.

I’ve protected certain mature shrubs that aren’t so easily salvaged, and deterred the deer by planting really bad-smelling plants next to them. Mexican Marigold (Tagetes lemonnii), a non-native shrub with lovely orangey flowers but stinky foliage, works well to protect a beautiful Mexican salvia (Salvia melissadora) from the ravages of the rut. This salvia produces small lavender blooms almost non-stop throughout the year and is beautiful when contrasted with the bright orangey blooms of the marigold. These plants grow in a border along with shrubby buckwheat (Eriogonum fasciculatum), goldenrods and grindelia - all fall bloomers. These plants bring in the butterflies; hundreds of skippers, painted ladies, buckeyes, field crescents, and even monarchs come to nectar at the flowers. October is one of the best months for high populations of butterflies in Marin.

More Butterflies!

I’m also seeing more California Tortoiseshells (Nymphalis californica) than usual; it’s a high-population year for these butterflies! During the summer months, this species is found in the mountains at much higher elevations; they migrate back down to the lowlands by October. Here they over-winter as adults, and in early March, right around the time of bud break, they mate and lay eggs on Ceanothus while the leaves are still young and tender. By late May the adults emerge and start flying to higher elevations in the Coast or Klamath ranges, or towards the Sierra Nevadas. They breed again on the later emerging mountainous Ceanothus species, and stay in the mountains until late in the summer.

California Tortoiseshell larva feeding on Ceanothus, photo by Nancy Hanson
California Tortoiseshell larva feeding on Ceanothus, photo by Nancy Hanson

The "Torties" that migrate back down to the lowland areas live in the adult form until the spring, when the first new leaves emerge on Ceanothus. The cold days are spent tucked into small crevices in rocks or tree bark, or beautifully camouflaged by the cryptic coloration of their underwings in leaf litter. On warm winter days, they will fly out to sun themselves or feed at flowers. Tortoiseshells and some other species of butterflies also get nutrients from animal dung, rotting fruits, and from the honeydew that many true bugs (like aphids) exude as they feed on plants.

California Tortoiseshell showing cryptically colored underwing, photo by Nancy Hanson
California Tortoiseshell showing cryptically colored underwing, photo by Nancy Hanson

California Tortoiseshells are a monophagus species like the Monarchs, devoted to a particular genus of plants. Like the Monarchs, they will use several different species of plants within that same genera. California wild lilac (Ceanothus species) do not confer any special protection in the form of toxic compounds to the Tortoiseshell larvae that feed on them; the adult female is simply choosing the Ceanothus species that exhibits fresh new leaf growth on which to lay her eggs. Not so with the Monarchs and Milkweeds! There are many different species of milkweeds (sometimes simply known as "Butterfly Weed") growing all over North America, the Caribbean islands, and in Africa; in California alone there are more than ten different species. But milkweeds are not all created equal, and the females know this - but more on that later!

California Tortoiseshell adult, photo by Nancy Hanson
California Tortoiseshell adult, photo by Nancy Hanson

An established drift of Showy Milkweed in my Novato garden
An established drift of Showy Milkweed in my Novato garden

Asclepidaceae: A Family of Plants with Many Uses

The genus name, Asclepias, derives from the Greek god and healer, Asclepios. The Roman god of healing and medicine was named Aesculapius, and there was good reason for naming this genus of plants after these gods. The plants contain organic compounds that have been used to treat various ailments and conditions of the heart and nervous systems, as well as the stomach and intestines. All parts of the plant contain cardiac glycosides and cardenolides, specifically one called oleandrin, which are concentrated in the milky latex sap.  

Showy Milkweed flowers still in bud
Showy Milkweed flowers still in bud

From ancient times these compounds have been used as heart tonics, diurectics, and emetics. Theses toxic compounds are fatal when ingested in large quantities, so they were also used as poisons on arrowhead tips, to kill mice and rats, and to poison people. In modern days these compounds are used to treat congestive heart failure and cardiac arrhythmia because they can increase the output force of the heart and decrease the rate of contractions with actions based on a cellular level.

Showy Milkweed flowers are beautiful and fragrant
Showy Milkweed flowers are beautiful and fragrant

Oleanders are in the same family (Asclepidaceae) as milkweeds and there are famous stories of death by "white oleander". Some other plants that contain cardiac glycosides are: Foxgloves (Digitalis spp.), Hellebores, Lily of the Valley, and some species of Kalanchoe. The Cane Toad (Bufo marinus) also produces this deadly toxin.

Humans have used parts of these plants for medicinal purposes and have found many other beneficial uses as well, so there are many common names for milkweeds; including simply "butterfly weed". 

Asclepias tuberosa, native to the eastern US, is also known as Pleurisy Root; the root was used by early colonists to treat inflammations of the pleura, a thin membrane covering the lungs. The milky sap was also used as an emetic and a diuretic.

Asclepias curassavica, Mexican or Tropical Milkweed, and sometimes also called "Bloodflower", is known by the people in its native range as "soladitos"; it was used to treat scorpion stings and to cauterize wounds. 

Asclepias syriaca (an eastern species) and Asclepias speciosa (the western counterpart) were both used by the native peoples as a food and fiber plant. Both these species are large in stature with big leaves emerging early in spring, large clusters of fragrant flowers in early summer, and large fuzzy seed pods forming later in summer. The tender young leaves and the newly formed seedpods were cooked and eaten as a vegetable.

Later in the season when the leaves had dried and fallen, the bare stalks were harvested for the fibers, from which a very fine twine was made. Early colonists also collected the floss, or silk (the tiny "parachutes" attached to each seed), spinning it to make candlewicks. They called the plants "silkweeds"! During World War II, the US government set up milkweed farms in order to harvest quantities of the silken floss as a substitute for kapok fibers. The "silk" was used as a filling flying suits and life jackets.

Showy Milkweed seeds and silk
Showy Milkweed seeds and silk

In more recent times, Milkweeds have been the subject of eradication campaigns along roadside ditches and field edges in the Midwest and in many grasslands and fields all through the West where cattle are pastured, because the plant is poisonous to most herbivores.

Milkweeds Provide for Many Creatures

Most vertebrates, and the smart herbivores, such as the deer, know to avoid this plant; they never browse it. But I know, from the experience of growing thousands of these plants over the years, that the small seedlings are eaten down to a nub by rats, which usually kills the little plants. Lately I’ve also noticed that rats will eat the seeds, pulling stalks close to the ground to get to the seedpods, and then leaving nothing but the silk and the empty seedpod. Roman snails, (the large introduced garden snails), also eat the plants! How do these creatures manage the toxins? Are they self-medicating, and deriving a benefit from the toxic compounds in the plants?

The toxins protect a number of invertebrate species that associate closely with the milkweeds besides the Monarchs, and they all exhibit some combination of the warning colorations of red, orange, or yellow with black and white. There are large and small red and black seed bugs (Family Lygaeidae) that feed on the milkweed seeds, and a bright red milkweed longhorn beetle (Family Tetraopes) that feeds on the foliage. There’s also a milkweed tussock moth (Family Lymantriidae) whose colorful spiny larvae feed on the plants, and three species of tiger moths (Family Arctiidae) which use Asclepias as a larval host plant.

Besides the creatures that feed on milkweeds there are numerous other arthropods that visit the flowers regularly. The most spectacular are our giant "Spider Hawks" wasps (Family Pompilidae), with blue-black bodies and bright orange wings that they constantly flick while hunting for prey. Spiders, especially crab spiders, position themselves amongst the flower clusters to wait for prey, and predatory arthropods such as mites, beetles, ants, lacewings and some "true bugs" all hunt on milkweed plants.

Many of these creatures also predate on the Monarchs; eggs and early instars are eaten by invertebrate predators, from which the milkweed toxins don’t offer much protection. The benefit of the toxins are more obvious in the later instars, when visual vertebrate predators like birds are a bigger problem. By these later stages, the larvae have eaten enough milkweed to build up the levels of toxins, which are sequestered within their bodies, then passed on to adult through the pupal stage. Even then, the individual doesn’t necessarily benefit, as the distastefulness of their bodies has to be learned, but it does benefit the species, and all sorts of Mullerian mimics who assume the same warning colorations.

Toxic Aphids

The aphid (Aphis nerii, or Oleander aphid) that shows up on Milkweeds is orange, brought to the American continent on Oleanders. It is a host-specific insect, and if it shows up on other plants in the garden, you can be sure that the  plant is a member of the family Asclepidaceae. These aphids are so toxic that ants don’t usually tend them and the common invertebrate predators of aphids such as ladybird beetles, syrphid fly larvae, and green lacewings often can’t control the populations.

Toxic Aphids
Oleander aphids can form colonies on stems and both top and bottom surfaces of the leaves

Aphid species have an unusual and bizarre form or reproduction called parthenogenesis: females give live birth to already pregnant females. That’s why populations increase so rapidly, and with most aphid species there’s simply a lag-time between large populations of a normally important prey species and predators that control those populations. It’s not until later in the season, in most aphid species, that winged forms and males show up in the aphid colonies. Winged forms find new plants to feed on, and expand their range when population density gets really high. Males mate with females, and eggs will be laid that assure next year’s population as the cold weather kills off the old generation.

Toxic Aphids
Oleander aphids will also form colonies within the flowers clusters

Not so, however, with the Oleander aphid. This species never produces males and therefore does not overwinter as eggs. They do produce winged females that can fly off and infest more milkweeds. And as the weather gets colder, the living, pregnant females overwinter as "stem mothers" nestled on the remains of stems at the very base of the plants until the weather warms up again, and the milkweeds put out new growth. If the developing aphid populations are not kept under control early in the season, they completely enervate the plants, making them undesirable for the Monarch larvae which typically feed on the milkweeds much later in the season.

The basic problem that is evident here is that of a non-native organism imported to a new environment without its normal biological controls.

Organic Methods of Controlling Aphids

First, a word of warning: if purchasing milkweeds from a commercial nursery, beware of mature plants without any aphids at all on them! Be sure to inquire if the grower has used neonicotinoides, a relatively new systemic insecticide that obviously renders the plant poisonous to larvae as well.

Since I’ve been growing Milkweed plants for many years now, I’ve been trying to find a really effective organic method of controlling the Oleander aphid populations, so far with only moderate success. Here’s what I do: when Showy Milkweed stalks start to grow in early spring, I remove all those that have large, dense populations of aphids, and immediately discard them in the garbage. In established drifts fresh new stalks emerge from the root again. Later in the season when the Narrow-leaf Milkweed puts on new growth, I keep a watch on the aphid populations, and also cut out any heavily infested stems.

Keep monitoring closely; aphids are obvious when they are all over the stems, but they’re harder to see and control when they are in smaller dense colonies on the undersides of the leaves and amongst the flower buds. I kill small populations by either rubbing them off with my fingers (the remains will be a mass of squishy orange pulp and obviously dead bugs), or spray them off the plants with a strong stream of water. Once removed from the plants, these small insects usually can’t get back onto the plants, and even if they do, their feeding tubes are often broken and they won’t survive to reproduce.

If these methods aren’t effective enough, I’ll use a soap spray. The soap kills the aphids by coating their bodies, and the spray knocks them off the plants. An insecticidal soap spray can be purchased or home-made; the best soap to use is castile, an olive oil-based soap. I’ve also tried a neem oil spray, but did not find it any more effective than the castile soap. When using a spray like this I’m really working with each plant individually, getting it on the leaves both top and bottom, and also into the flower clusters. I leave it on the plants for a short while, perhaps ten to fifteen minutes, and then spray off the soap and drench the soil with fresh water.

All this has to be done early in the growing season; if left too late you’ll be endangering Monarch eggs and larva and a host of other beneficial creatures while trying to control the aphids. I’ll also be checking closely during the dormant season to try to find and kill the "stem mothers". I may also try a soil drench, either of castile soap or neem oil spray.

Milkweed Flowers have a Unique Form

Milkweed flowers have a very unusual form
A closer look reveals the unusual structure of Milkweed flowers

Lots of insects visit the milkweed flowers for nectar, but the flowers have a unique form and can sometimes become a death trap for smaller insects. The stamens are fused and create a center column in the flower, surrounded by five cup-like structures called "hoods". The pollen is not in loose grains, but contained within a small sac. These "pollinia sacs" are in pairs and are located between the hoods. An insect has to reach within the hoods for nectar and to get to the pollinia sacs; that’s easy enough for large insects like the Monarch butterfly, but smaller insects can get trapped if their feet get stuck in between the hoods.

Monarch females spend a good bit of time flitting about various plants, taking quick chemical readings with the tarsi to determine if this is the proper plant on which to lay their eggs. They make decisions on which milkweed species to use for their offspring based on the chemical readings they take, because the toxicity of milkweeds varies from one species to another, even among plants of the same species. The females often will avoid ovipositing on plants with serious infestations of the Oleander aphids.

The most benefit comes from the milkweed plants that contain intermediate to high levels of the cardiac glycosoides. One of the best - just for the Monarchs - is the Tropical Milkweed (A. curassavica), and in my experience there does not seem to be any danger of this species invading our wildlands. It does "seed itself around", but only in a tended garden where it can get some occasional water in the summer months. And since it’s a tropical and therefore frost-tender species, the plants are often killed off in the colder months.  

Another reason that this non-native milkweed species is so important in a habitat garden is that this is the milkweed that the Monarch evolved with. Monarchs in California are a fairly recent phenomenon; there are no historic records of over-wintering sites on the coast before the twentieth century!