On feeding hungry monarchs

A migrating monarch feeding on seaside goldenrod in Gloucester, MA. PC: Rachael Bonoan.

Fall is a time for migration in the North, as animals flee the bitter cold of winter in favor of warmer regions. If you’ve been outside in New England in the past few weeks, you’ve probably been in the presence of these migrants. Birds are some of the best known long-distance migrants, but plenty of insect pollinators embark on similarly stunning journeys each fall, in spite of their diminutive size. Some are inconspicuous and elusive, like the various species of hover fly whose migration through our region has only been recorded a few times in the past hundred years; others, like the monarch butterfly, are beloved signs of the closing of summer.

Each fall, millions of these large orange and black butterflies form a remarkable caravan, as monarchs from breeding grounds across a range that stretches as far north as Nova Scotia embark on the long flight to the overwintering grounds in central Mexico. These migratory monarchs need to reach dwindling undisturbed patches of oyamel fir forest, where they spend much of the cool winter perched in massive clumps on the trees, largely quiescent. There they wait for warmer temperatures that signal the arrival of Spring, when they can depart and gradually begin their multi-generational recolonization of the breeding range.

Surviving the 3000+ miles of flight and the subsequent months of cool winter in Mexico requires energy, and lots of it. The butterflies – weighing about a fifth of a penny – bulk up on nectar by gorging for hours on fall-blooming flowers. So efficient are their digestive systems that it’s thought that the sugars in the nectar can be converted into energy-rich fat stored in a special organ in a matter of minutes. Monarchs most likely need to more than double their regular fat stores during migration in order to survive the winter, meaning that stunningly, these butterflies actually gain weight during their multi-thousand-mile-journey. 

Two migrating monarchs on an aster species (Symphyotrichum sp.). In the fall, males and females are non-reproductive; they happily feed on the same plants and often roost together during migration. PC: Rachael Bonoan

You may have heard that monarch butterfly populations have suffered steep declines in recent years, with most estimates putting the losses in the Eastern population over 80%. A broad set of human actions have conspired against the monarchs, including deforestation in Mexico and agricultural herbicide use in the Midwest. Recently, ecologists have raised the specter of a third challenge: increased mortality during migration.

Migration is a dangerous undertaking, and biologists speculate that perhaps as many as 95% of monarchs attempting to migrate perish each year. As meadows and prairies have been converted to asphalt and crops, native grassland habitats that support large numbers of fall-blooming flowers are a dwindling commodity. Some ecologists posit that a lack of nectar along the migration route could be leading to starvation during the flight or in the quiescent overwintering period.

This is where we come in. Through planting gardens that can continue producing flowers all the way through October, we can provide valuable resources not just for monarchs but for all the other pollinators that are still flying. Surviving and thriving at this time of year in New England can be a tenuous proposition for animals that eat nectar and pollen: nighttime temperatures flirt with freezing and many of the flowering plants in our region have already gone to seed or begun senescing by September. In a summer with a drought like the one we’ve had in Massachusetts this year, wildflower outlooks can be even grimmer by the time October swings around. 

But where there is a tenuous situation, there is also an opportunity for our interventions to make a bigger difference. Much of the land in New England would, given the chance, burst into late-fall bloom every year in a riot of goldenrods and asters, pumping out nectar and pollen through until frost.  These native, pollinator-friendly species are conveniently often exceedingly drought-tolerant and some grow aggressively when given the chance, like the goldenrod species in the picture below:

A vacant lot in Rhode Island, September 2020, brimming with goldenrod. PC: Atticus Murphy.

Crucially, these patches can arise in the middle of neighborhoods on vacant lots or road verges, providing unexpected oases for fall-flying insects. Likewise, by carefully planning gardens that bloom continuously until the end of September, gardeners can provide nectar not only for migrating monarchs, but the hundreds of other pollinator species that require pollen during the fall. 

Adding autumn-blooming flowers amounts to starting a food bank for pollinators like monarchs, and by planting easy-to-grow native perennials like goldenrods and asters, the food bank will return every year, essentially for free! Migration is hard enough without pesky human interference; let’s try and help these iconic butterflies along their way, if we can.

Lazy lawnmowers and landscape mullets

By Atticus Murphy

Maybe you’re the type of person who’s interested in nature but don’t know you can help. You live in a city or a suburb, with not much space and just a typical grassy lawn covering what space you have. Maybe you’ve also recently learned that insect pollinators like bees are crucial for our ecosystems but may be experiencing significant declines caused by habitat loss due to humans. What can you do to help, without breaking the bank or spending too much of your time on gardening?

 A lawn with a more and less frequently mowed portion on left and right respectively, showing the diversity of flowers that can emerge when given time to grow (Photo: Sue Pranskus). 

If this sounds like you, there’s a surprisingly simple option available to you: do less! Mowing grass lawns frequently (more than twice a month) is a good way to keep the grass cropped low and cut down on weeds. But this low, flowerless grass lawn is essentially a food desert for bees and other pollinators. Happily, studies have shown that if you simply mow a bit less, your once barren yard can become a buffet for bees in just a few short weeks. Simply shifting towards cutting your lawn every two or three weeks instead of once a week can greatly increase the number of bees and other pollinators that are able to forage there, as quick-blooming “weedy” flowers sprout up from the soil. Instead of treating weeds like unwanted invaders, to help save our native bee populations, treat them like welcomed guests! They’ll beautify your lawn for a few days at a time all throughout the growing season and they’ll do it for free. All it takes it giving them a bit more time between mowing to put up their flowers. 

If you enjoy using your lawn for outdoor activities or are required to maintain it at a certain length by local authorities, reducing your mowing frequency as much as you can within these limits is still a great way to help pollinators by giving flowers more of a chance to bloom before cutting them. Remember, this “lazy lawnmower” strategy for helping pollinators does not require drastic lifestyle changes: instead, it might be totally compatible with whatever uses you get out of your lawn now. Green grass can remain on your property for sitting, lying, and playing, and you can see significant numbers of new flowers all while still keeping your lawn looking well-kempt. If you’re concerned about appearances and don’t want to give over the front lawn to being taller, then consider only letting your back lawn grow out a little. Scientists have nicknamed this practice the “landscape mullet” because like the famous haircut, it’s longer at the back. Because pollinators are able to move around to locate flowers, they will be served just as well by a back lawn with flowers as they would a front lawn.

A monarch butterfly feeding on a mustard flower (family Brassicaceae) sprouting from a grassy lawn (Photo: Rachael Bonoan).

What types of flowers will spontaneously recruit in your lawn? It’s hard to say, and that’s part of the fun! In Massachusetts you might expect to see pollinator-friendly flowers like dandelions (Taraxacum species), clovers (Trifolium species), butter-and-eggs (Linaria vulgaris) and other quick-blooming, low-growing species, like the mustard pictured above. Many of these species may not be native to the U.S., but they are nevertheless great sources of nutrition for native pollinator species. So if you’re interested in helping pollinators but don’t want to make the leap into pollinator-friendly gardening just yet, getting a little lazy with lawn management and welcoming some weeds into your life is a great first step towards making your property a refuge for native insect pollinators. 

The butterflies who are raised by ants

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.