caterpillar

The monarch butterfly (Danaus plexippus) is one of our most iconic pollinators, known for its large size, bright orange color, and impressive fall migration to Mexico. Yet the monarch’s recent listing as endangered on the IUCN red list has led many to wonder what steps to take to help the monarch in their own yards. Read on to learn TPI’s advice to those looking to help monarchs and for a guide to noticing the different life stages of monarchs in your garden.

Monarch butterfly (*Danaus plexippus* ) nectaring on goldenrod

1. Plant milkweed

As caterpillars, monarch butterflies can only feed on milkweed (plants in the genus Asclepias), meaning these host plants are necessary for the monarch’s survival. There are several species of milkweed native to the northeast, including common milkweed (Asclepias syriaca), butterfly weed (Asclepias tuberosa), and swamp milkweed (Asclepias incarnata). Butterfly weed and swamp milkweed both perform well in gardens – the former preferring dry soils and the latter preferring wetter soils – and can be found at most garden centers. Swamp milkweed also has shallower roots, which means it can grow well in a container if garden space is limited.

2. Plant nectar-rich flowers

Unlike the caterpillar stage, adult monarchs are not limited to milkweed and feed on a wide array of plants for nectar. These sugary resources extend their lifespan and give them the energy to fly farther and make more offspring. TPI has many resources to help you make your garden even more friendly for monarchs and other pollinators, including plant lists, a guide on how to leave swamp milkweed stems for bees and this blog post!

3. Don’t take eggs or caterpillars off of plants

Although it is tempting to take in any eggs or caterpillars you see to keep them safe and watch them grow, it can actually do more harm than good. Rearing large numbers of monarchs inside can increase the incidence of disease within the population. And, even worse, recent research has shown that monarchs reared indoors without natural environmental cues are less able to migrate south in the fall. While raising monarchs can be a great educational experience, we recommend instead making a daily habit of watching the progress of the wild eggs and caterpillars you find in your garden (See below for our guide to finding monarchs in the garden!).

4. Don’t apply pesticides to your garden

Milkweed plants often become infested with tiny orange aphids (Aphis nerii). Do not worry–this is a totally natural occurrence and won’t hurt the monarchs. Aphid infestations can be unsightly but refrain from using pesticides as this will harm the monarch caterpillars. To remove aphids, use natural methods like a spray of soapy water. Natural predators such as ladybugs, lacewings, and their larva will also take care of the aphids!

Oleander aphids (*Aphis nerii*) on swamp milkweed (*Asclepias incarnata*)

How do you know if you have monarchs using your garden?

Monarchs are easy to spot as adults but we don’t see them as frequently as caterpillars. Here are some tips on identifying monarch presence in your garden:

Adults

Adult monarch butterflies are highly conspicuous––look for a big orange butterfly with black veins. They glide around with their wings held in a “V” and visit gardens to lay eggs on milkweed and drink nectar from flowers. Similar looking viceroy butterflies are smaller and hold their wings flat when they glide. Monarchs like a variety of garden flowers for nectar including purple coneflower, swamp milkweed, joe-pye weed, mexican sunflowers, and zinnias.

Adult female monarch ovipositing (egg laying) on swamp milkweed (Asclepias incarnata)

Eggs

Monarch eggs are sesame seed sized, a creamy white color, and slightly pointy–like a tiny hard boiled egg. These eggs are singly laid, meaning there is only one egg instead of a mass of many eggs. Monarchs tend to oviposit on the underside of leaves, but eggs can also be found on other parts of the plant, especially the flower heads. If you want to find eggs, patiently search the undersides of leaves and flower heads, especially focusing on the tallest areas of the milkweed, where monarchs tend to land. It may take a few days of checking before you see an egg, but don’t be discouraged! Even if you don’t see a monarch laying eggs on your plants, your garden may still have them–an adult monarch can lay an egg in under 5 seconds! Lastly, remember that although monarchs can start appearing in our region (MA) as early as the start of May, they are not widespread until late June, so time your searches for July-August if you want the best chance of success.

Closer look at a monarch egg, PC Jay Cossey

Caterpillars

Monarch caterpillars hatch in 3-5 days and eat their very first meal: their eggshell! These baby caterpillars are very small and difficult to spot. They are vulnerable at this stage to hungry insects like paper wasps. As they eat milkweed leaves, the caterpillars grow bigger and molt through stages called instars. You can tell a caterpillar’s approximate age by identifying its instar – monarch caterpillars go through five instars before pupating. Most of the caterpillars we find in the wild are small (in their first or second instars): due to the high rates of predation on these small stages, it’s less common to see large caterpillars.

First instar monarch caterpillar eating its egg

While these small caterpillars can be difficult to spot, their feeding damage can be more apparent: it takes the form of small ‘windows’ into milkweed leaves, half-chewed holes often carved into a small circle, which the caterpillar uses to avoid drowning in the milky latex exuded by its host plant. If you see frass (caterpillar poop: poppy seed-sized brown pellets) or large missing chunks from the edges of the milkweed leaves, then be on the lookout: these are the telltale signs of a much larger stage 4 or 5 caterpillar! Be sure to check under the leaves, as caterpillars of all stages prefer to eat from beneath the leaf, where they are more sheltered.

Small black frass surrounding a second install caterpillar

Pupa

Monarch pupa are hard to spot as they blend into the surrounding environment. The 5^th instar caterpillars tend to crawl away from the milkweed to pupate on nearby plants. They will remain in the pupa for 7-10 days before emerging as a beautiful monarch butterfly.

Have you found monarchs in your garden? Share your photos to @pollinatetufts!

Silvery blue caterpillar. Photo: Atticus Murphy

What are these ants doing, clustering around a caterpillar? If you guessed eating, you’d be right, but probably not in the way you imagined.

These ants are engaged in what’s called “tending,” and far from being harmed by the interaction, the soft and vulnerable caterpillar is likely a beneficiary. In fact, the caterpillar has a suite of complex adaptations that seem aimed at keeping ants nearby. Most striking among these is the dorsal nectary organ, a gland that secretes a nutritious liquid high in sugar. Foraging worker ants eagerly consume the food and bring it back to their colonies. The cost to the caterpillar is only the cost of producing these little nutrition packets.

A less attractive ant and a silvery blue caterpillar. Photo: Atticus Murphy

But why would a caterpillar want a murderous cadre of ants clustered around it? The answer is protection. For one thing, when you manage to get the bullies on your side, they won’t bully you anymore: that is, the pacified ants are no longer a threat to the caterpillar. And in general, being a caterpillar is very dangerous. They have soft bodies, often feed in the open, and are not known for their quick movement, making them easy prey. In addition to being eaten directly, there are a huge diversity of parasitoids in the insect world, who lay eggs inside caterpillars’ bodies and eat their way out. This kills the caterpillar. A standing guard of ants, who generally protect their food sources and each other, lowers the caterpillar’s risk of being parasitized. Thus, because this interaction is often mutually beneficial, we call it a mutualism, meaning that both the ants and the caterpillars do better because of it: ants get food and caterpillars get protection.

Ants tending a silvery blue caterpillar, who is releasing a droplet from the dorsal nectary organ (the tiny glimmer in the center of the photo). This is located at the rear end of the caterpillar. Photo: Atticus Murphy.

In order to keep their attendants friendly, the caterpillar can also release a potent cocktail of chemicals that mimic ant pheromones, encouraging the ants to stick around, and hopefully keeping them from trying a bite of caterpillar. This cocktail is so effective that sometimes the ants can’t distinguish the scent of the caterpillar from their own kind. If the ants are absent and a predator approaches, some caterpillars also make use of specialized organs that produce noises or fragrances, attracting ants from farther away.

*An adult female Silvery Blue lays an egg on lupine: within 3 days the egg will hatch, and within a week it will be old enough to attract ant attendants. Photo: Rachael Bonoan.*

The butterfly species in the pictures above is the one I worked with this summer, the silvery blue (Glaucopsyche lygdamus). It’s common across the U.S., but this interaction is a global phenomenon, occurring in hundreds of butterfly species that can be found on every continent except Antarctica. And with a diversity of species comes a diversity of interactions: many different ant-caterpillar pairings have emerged, and unique quirks abound. Perhaps the most captivating variations on the theme are the parasitic blue butterflies. These dastardly caterpillars have taken the usual mutually beneficial interaction and tilted things decidedly in their own favor by truly pretending to be baby ants. After spending some time feeding on a host plant like most caterpillars do, these species use their unusually effective chemical mimicry to induce ants to take them inside the actual nest, where the caterpillars are either fed alongside the real ant young, or more sinisterly, the caterpillar devours the ant young, growing fat by pillaging their hosts until they’re ready to emerge as adults.

*The Large Blue butterfly, a parasitic relative of the Silvery Blue. Photo: Ann Collier.*

The ant-tending of these butterflies is not just an interesting quirk of natural history, but for some species may be the key to their continued existence. The classic example of this possibility is the large blue butterfly (Phengaris arion) of Britain, which is a parasite of Myrmica ants. This butterfly was on the decline for decades in the British Isles and was an early beneficiary of an intensive conservation campaign. Unfortunately, this campaign failed, and by the 1970s, the species teetered on the edge of extinction in spite of years of efforts. The conservationists were perplexed. They had carefully cultivated healthy patches of the host plant, Thymus, and there looked to be plenty of ants in the area, so why were the butterflies still declining?

It took a careful reexamination of the already well-known dependence on Myrmica ants to understand what had occurred. The large blue was an unrecognized specialist, a butterfly who relied not just on Myrmica ants to survive, but on a particular species of Myrmica ant. This species was so crucial that even close relatives were totally unsuitable and could not successfully “raise” caterpillars to adulthood. While there were indeed plenty of Thymus plants and plenty of Myrmica ants, the ants were of the wrong species! The large blue tragically went extinct in Britain before this new knowledge could be put in practice, but it has since been successfully reintroduced.

So, the next time you see a blue butterfly, remember that it might well have relied on an unruly bunch of ant nannies to survive into its winged form. Remember also that these butterflies provide still another example of the myriad ways in which our pollinators are dependent on an entire healthy ecosystem and its component parts, not just on their host plants.

Further Reading:

Pierce, N. E., M. F. Braby, A. Heath, D. J. Lohman, J. Mathew, D. B. Rand, and M. A. Travassos. 2002. The ecology and evolution of ant association in the Lycaenidae. Annual Review of Entomology 47:733–771.

Thomas, J. A., D. J. Simcox, and R. T. Clarke. 2009. Successful Conservation of a Threatened Maculinea Butterfly. Science 325:80–83.

Tag: caterpillar

How to help the monarch butterfly