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Pipevine Swallowtails and the case for caterpillar gardening

When you think of gardening for pollinators, you most likely think of beautiful flowering plants which attract bees, butterflies, and other insects by offering copious amounts of nectar or pollen. But flowers cannot give all pollinators the food they need. Butterflies in particular can be very picky when deciding what to eat when first starting out their lives as caterpillars. Most butterfly species have evolved to specialize feeding on one or a handful host plants. In a sense, when it comes to supporting butterfly communities, providing ample host plants for these soon-to-be-butterflies could be just as beneficial as planting nectar flowers for adults!

Pipevine swallowtail eggs are laid in small clusters so that when they emerge, these caterpillars work together eating leaves in small groups

The Pipevine swallowtail Battus philenor is one butterfly that benefits tremendously from the gardening of its favorite plant, Pipevine Aristolochia, after which it is named. Both Pipevine and the butterfly with which it shares its name is native across eastern North America, including Massachusetts. The plant itself is highly toxic and could cause severe kidney problems or even cancer if ingested by humans, but Pipevine swallowtail caterpillars could not care less.

These caterpillars have evolved not only to cope with the toxins in the plants, but to actively seek them out. Pipevine swallowtails, like many other butterflies, including Monarchs and their beloved milkweed, acquire the toxic chemicals present in their host plant and sequester them. As caterpillars they build of reserves of this chemical making them unpalatable to potential predators for the rest of their life. This is part of the reason many butterflies display such bright and captivating scales – it can be a signal to predators that nothing good will come from eating them.

In the right conditions Pipevine can scale great heights clinging to trees or other supports

Though these caterpillars cannot get enough of Pipevine, the adult swallowtails themselves do not nectar on the flowers it produces. Pipevine, like the name suggests, produces curved flowers that resemble a smoking pipe which is not conducive for a butterfly proboscis. Dutchman’s pipe Aristolochia macrophylla, is the most common species in eastern North America, and produces small tubular flowers with a small circular entryway. Into that entryway go flies which are actually attracted to the flower’s fragrance. Upon entering the narrow pipe, the flies get a little stuck, catching some of the flower’s pollen and carrying it with them until they end up in another flower which they help fertilize. After fertilization, the plant produces great seed pods which, when rooted and grown, will provide more food for those hungry caterpillars.

Before you start thinking this toxic, fly-attracting plant might not be the right fit for your garden, consider that Pipevine has been an immensely popular ornamental plant since Victorian times. It is an ideal option for trellises, fences or pergolas, providing plenty of shade when mature! If you are interested in attracting these beautiful butterflies to your home or garden in the future be sure to water your Pipevine as it needs lots of water and give it a place to climb. Be sure, too, to get Aristolochia macrophylla specifically, as some species will not support these caterpillars or might even be invasive – moreover macrophylla will provide the best shade (it is given the Greek name meaning ‘large leaf’) and will attract some marvelous butterflies looking for the right place to lay their eggs.

Adult Pipevine swallowtails have a remarkable coloration, with black and iridescent blue wings that have orange and white spots on the underside (Photo credit: John Flannery)

Container gardening for pollinators

In the city, outdoor gardening space can be hard to find. Backyards are replaced by balconies, but this doesn’t mean you can’t still garden for pollinators. You just have to get creative about it. This National Pollinator Week, TPI has tips and tricks for how you can support urban pollinators with container gardening whatever your outdoor space looks like (even if it’s just a windowsill!).

1. Choose perennial native plants. Apart from a few species with deep taproots, many native plants will thrive in containers. Since they’re perennial, your plants will come back year after year, and support pollinators without any extra cost. Here are some good options that TPI has had success with:

  • swamp milkweed (Asclepias incarnata)
  • anise hyssop (Agastache foeniculum)
  • lance-leaf coreopsis (Coreopsis lanceolata)
  • smooth aster (Symphyotrichum laeve)
  • great blue lobelia (Lobelia siphilitica)
  • cardinal flower (Lobelia cardinalis)
  • blazing star (Liatris spicata)
  • virginia mountain mint (Pycnanthemum virginianum)
  • purple coneflower (Echinaecea purpurea)
Blazing star (Liatris spicata), aka gayfeather, is a favorite of pollinators and easily grown in a container! PC: Flickr, CC.

2. Combine plants with complementary bloom times to have the greatest impact. For your first container, try picking three plants: one that blooms in early summer, one in mid-summer, and one in fall.

3. Deep containers are your friend. Choose containers at least 16” deep to allow your native plants to build strong root systems and thrive for years to come.

4. If you only have space for shallow planters (<12” deep), annual or biennial native plants are a great option. These plants don’t invest in a deep root system and can survive in shallower soils. Black-eyed susan (Rudbeckia hirta), spotted horsemint (Monarda punctata), and partridge pea (Chamaecrista fasciculata) are three good choices.

5. Flowering herbs are also a great (and tasty) option for small spaces. TPI previously waxed the benefits of letting your herbs for pollinators, and the same applies here. Dill, lavender, thyme, mint, and cilantro are all popular herbs that do well in containers and favorites of pollinators.

Container herb gardens can be a win-win! Flowering mint feeds all sorts of pollinators, like this metallic green sweat bee (Agapostemon sp.), and you can enjoy mint leaves in sun tea and watermelon coolers! PC: Nick Dorian.

6. Water often. Soil in containers dries out faster than those growing below ground, and as your plants become root bound, they will need water more frequently. As always, water the soil rather than the leaves.

7. Don’t fertilize. Native plants are adapted to soils that are low in nutrients, and adding fertilizer will result in many big leaves and not many flowers. Amend store-bought potting mix with perlite and sand to create a well draining medium for your plants. Leave the manure and kelp fertilizer for your veggies!

8. Protect your container plants over the winter with 3-4” of leaf mulch, by moving your planter to a less exposed area, and potentially covering with a tarp. The key is to keep the soil warm enough so the roots don’t freeze through.

The Feathered Bees’ Spring Arrival

RTHU in flight. PC: Michael Janke.

The smell of spring permeates the air as the grass begins to poke through the sodden dirt and buds emerge from the tips of the trees.  Concurrent with the change of seasons is another great event – hummingbird spring migration!  While there are more than 330 different species of hummingbirds (Trochilidae), only around 12 to 15 species routinely nest in the U.S.  For us resident Bay Staters’ we can expect to see even less!  The Ruby-Throated Hummingbird (Archilochus colubris; RTHU) is the only breeding hummingbird in eastern North America.

Twice a year, these tiny powerhouses undergo an 805 km migratory route—some individuals coming from as far away as the Pacific slope of Costa Rica and flying up into Central Alberta, Canada.  Despite their small size (females averaging 3.5 grams and males averaging 3.0 grams), most fly nonstop across the Gulf of Mexico during spring migration.  In Massachusetts, you can expect to see these sparkling gems beginning in mid-April.  In fact, the first sighting was just reported on eBird—an online database of real-time bird observations—in East Falmouth, MA on April 27th.

Before embarking on their migratory routes, RTHU will gain about 25-40% of their body weight, fattening up on nectar and insect.  Once they have arrived in North America, RTHU get right to work replenishing their depleted reserves.  While hummingbirds are birds, they feed similarly to insects and have appropriately been dubbed “feathered bees” by the National Audubon Society.  Like insect pollinators, hummingbirds fly from flower to flower in search for nectar.  As a hummingbird dips its bill into flowers, pollen grains stick to their feathers and bill which get carried and distributed to the next flower.  It is estimated that nearly 8,000 plant species throughout the Americas rely on hummingbirds as their primary pollinator.

Female RTHU resting on flower. PC: Nick Dorian.

Hummingbirds and plants represent a classic example of a plant-pollinator relationship and demonstrate coevolution—the phenomena of intimately linked organisms influencing each other’s evolution.  Hummingbird-dependent plants have evolved several “pro-bird” traits including sucrose-rich nectar, brightly colored, unscented flowers (smell is important for insects to find flowers, while birds rely on vision), and tubular flowers that allow easy access for hummingbirds.  Like other pollinators, when native plants thrive, so do hummingbirds.  To sustain a heightened metabolism (their hearts beat as fast as 1,260 beats per minute after all), hummingbirds eat once every 10 – 15 minutes and visit between 1,000– 2,000 flowers per day!

Unfortunately, the preferred habitats of RTHU are changing along their migratory routes due to habitat conversion and climate change.  There has been an increase in the concern about the conservation of pollinators in urban environments.  A known way to mitigate the impact of urbanization is through native plants gardens which provide both habitat and resources for pollinators.  Such efforts can also encompass native plants with specialized hummingbird-pollinated flowers in urban landscaping.  In the northeast, planting red cardinal flower (Lobelia cardinalis) or trumpet honeysuckle (Lonicera sempervirens) in your garden may contribute to community restoration and ecosystem functioning by attracting RTHU—and who wouldn’t want these cuties flying around their yard?

Female RTHU drinking from L. cardinalis. PC: Max McCarthy.

Do you want to start your own pollinator garden this year, but don’t know what plants to get? Come to our Grab-n-Grow Native Plant sale next month to stock up on all thing native plants!  More information can be found at: https://sites.tufts.edu/pollinators/cultivate/native-plant-sale/.