Sugar maple is in bloom, shaking its anthers in the breeze. This species is supposedly wind-pollinated, but looking closely at the male flowers I saw thousands of bees, wasps, beetles, and even a butterfly working at the blooms. I estimated about thirty thousand insects on the tree’s flowers at any one time. Some were gathering pollen, but many had their heads buried in the bells from which the filaments and anthers hung. Could there be nectar or sugary exudate up there? Why else would a spring azure butterfly systematically work its way across the tree’s flowers? I can find no reference to sugary lures in any of the sources that I’ve read. I’d welcome other observations or insights…
Naturalists from North America will recognize this plant, the maligned, non-native garlic mustard (Alliaria petiolata). I photographed this one in South Queensferry, Scotland. Recognition is likely not the only response to the sight of these leaves and flowers. Hands and elbows may start twitching in anticipation of the pleasure of uprooting the plant. This tugging reflex is organized into gatherings called “pulls” where people mass to yank swaths of the plant from the soil, bag the plants, then consign them to the landfill or fire. Depart from me, you who are cursed, into the eternal fire prepared for the devil and his angels.
This vigorous dislike has a good ecological foundation. The plant invades woodlands and smothers native plants, reducing botanical diversity. Chemical weapons are used in this process: garlic mustard releases chemicals into the soil that sap the vitality of surrounding plants. These chemicals act by suppressing the germination and growth of mycorrhizal fungi whose mutualistic relationship with plant roots helps many forest plants to grow successfully.
In the UK, where garlic mustard is native, the species is known as Jack-in-the-hedge and is fairly common in damp hedgerows and field edges. Unlike their American counterparts, local naturalists esteem the plant for its role as the host plant for caterpillars of several native butterflies. The orange-tip (Anthocharis cardamines) is the most well-known of these. This conspicuous white butterfly with bright orange wing tips often loiters in patches of Jack-in-the-hedge. The green-veined white (Pieris napi) is another species that uses the plant as a primary host for its caterpillars. The small white (Pieris rapae) — sometimes known as the “cabbage white” — will lay its eggs on Jack-in-the-hedge when it can’t find a gardener’s cabbage or broccoli.
In North American, the plant’s relationship with butterflies is not so nurturing. The West Virginia white butterfly (Pieris virginiensis) normally lays its eggs on native toothwort (Dentaria), but the female butterflies are also attracted to garlic mustard. Unfortunately for the caterpillars of these fooled females, the novel chemical mixture in garlic mustard prevents the youngsters from growing. The plant is therefore an ecological trap: drawing in butterflies with the promise of good food, then killing them.
This confused tale has its origins in the close family ties among the species involved. Toothwort and garlic mustard both belong to the family Brassicaceae. North American butterflies are drawn to the presumably familiar scent of the import. But family resemblance only goes so far. American butterflies have not evolved the particularities of biochemical detoxification needed to feed on garlic mustard, whereas their European kin in the same subfamily of butterflies (Pierinae) have mastered these mechanisms. Whether evolution will be fast enough to allow the Americans to adapt remains to be seen.
It doesn’t help that butterflies in Bible Belt states are kept in the dark about natural selection, giving the missionary mustards a boost in their colonial quest.
As we slide down the slope behind the equinox, animals have accelerated their autumnal movements. My backyard now consistently hosts several migrant bird species each day. In the last week: rose-breasted grosbeaks, magnolia warblers, Tennessee warblers, American redstarts, gray catbirds, chestnut-sided warblers, warbling vireos, and a summer tanager. Unlike the songsters of spring, these mostly silent birds can be hard to detect. A flicker of foliage reveals their presence, then a glimpse of their plumage as they prance through the concealing twigs. Grosbeaks are an exception to this crypsis. Although they can be hard to see, their sharp tweek call, given repeatedly through the day, gives them away. The sound is just like that of a sneaker squeaking on a gym floor. Listen for it, then look up.
Last night, as I left the Biology picnic on campus, another migrant bird making a spectacular display over the old building that houses the fire station. About two hundred chimney swifts were scything the air in a tight, fast vortex. They swirled around the brick chimney that protrudes from the station’s roof. One by one, they folded their wings and dropped in. Like hot cinders carried up by the wind, these birds seemed to ignite the dead dusky air with their coordinated vitality. A little tornado of life. The swifts are on their way south to the Amazon where they’ll feast on tropical gnats all winter. I suspect that they are speeding on their way as I write: this morning’s cold rain squalls mean there will be few flying insects in Sewanee today. Time for swifts to get out of here.
Birds are not the only migrant animals making their way through our skies. This week has seen an impressive number of monarch butterflies winging across the treetops. It seems impossible that so slow and delicate a flyer could make it all the way to Mexico, but that is where they are all headed, to a few small patches of dense fir forest in the highlands. The monarchs gather there in the tens of millions to rest in the cool but unfrozen woods. Remarkably, these autumnal migrants are the grandchildren of the butterflies that left Mexico this spring. Somehow their genes guide them to precisely the right location.
Another migrant butterfly, less celebrated than the monarch, is the gulf fritillary. This species breeds all over the southeastern U. S., but overwinters only in the deep south. Unlike the fluttery monarchs, these butterflies scull their way across the air with seemingly powerful and directed wingbeats. In Florida, where adults linger all winter, huge flocks of them will sometimes stream over fields and scrubby areas. A river of bright amber.
Eileen Schaeffer and Arden Jones’ guide to Sewanee’s butterflies is now available online (as a pdf at Issuu.com). Eileen and Arden are both rising seniors at Sewanee and have worked on this guide for the last two years, collating photographs, information about host plants, and data about abundance. The end result is a fabulous review of the diversity of these beautiful animals. The guide is also available in hard copy at Lulu.com.
This is not the only lepidopteran project that Eileen and Arden have undertaken. Last summer they completed a survey of the butterflies of St Catherine’s Island, GA. This summer they have taken a step into the daunting world of moth identification and are studying the moths of the Valles Caldera National Preserve in New Mexico.
I have also uploaded a very short checklist of the butterfly species found in Sewanee.
“The butterfly counts not months but moments, and has time enough.” — Rabindranath Tagore
Number of species detected: 24 (a little lower than most years)
Number of individuals detected: 414 (way higher than most years)
Average temp: also approx 414
We found almost no butterflies over the majority of the areas surveyed. It is much too dry and baking. But in areas that had been irrigated or that were near bodies of water, we found impressive aggregations of butterflies supping on nectar. By far the most abundant species was the sachem (Atalopedes campestris). We saw 213 of these little skippers; the previous record for the species was 26 in 2001. This species thrives in disturbed areas where its caterpillars feed on grass.
We also came across this spectacular pipevine swallowtail (judging from its freshness, it was newly emerged from its chrysalis):
Thank you to my co-leader, David Coe, and to brave heat-defying participants Louise Kennedy and Tam Parker.
Complete results are summarized below:
|Common name||Scientific name||2012 count|
|Pipevine Swallowtail||Battus philenor||2|
|Eastern Tiger Swallowtail||Papilio glaucus||1|
|Spicebush Swallowtail||Papilio troilus||4|
|Cabbage White||Pieris rapae||13|
|Orange Sulphur||Colias eurytheme||7|
|Gray Hairstreak||Strymon melinus||1|
|Red-banded Hairstreak||Calycopis cecrops||4|
|Eastern Tailed-Blue||Everes comyntas||47|
|Spring Azure||Celastrina ladon||5|
|American Snout||Libytheana carinenta||5|
|Gulf Fritillary||Agraulis vanillae||3|
|Variegated Fritillary||Euptoieta claudia||6|
|Great Spangled Fritillary||Speyeria cybele||1|
|Pearl Crescent||Phyciodes tharos||7|
|American Lady||Vanessa virginiensis||4|
|Red Admiral||Vanessa atalanta||1|
|Common Buckeye||Junonia coenia||1|
|Common Wood-Nymph||Cercyonis pegala||1|
|Silver-spotted Skipper||Epargyreus clarus||74|
|Horace’s Duskywing||Erynnis horatius||1|
|Least Skipper||Ancyloxypha numitor||2|
|Fiery Skipper||Hylephila phyleus||10|
|Little Glassywing||Pompeius verna||1|
This freshly emerged Banded Hairstreak (Satyrium calanus) was perched on a hackberry leaf outside my back door. Butterfly colors come from tiny scales that cover the wings. As these scales gradually wear away, so does the vibrancy of the insects’ colors. Only butterflies that have newly emerged from their chrysalis look so tidy and fresh. The stripey antennae on this hairstreak add some panache.
This species has just one generation per year. Adults mate in early summer, then lay their eggs on oak and hickory twigs, placing the eggs near the twigs’ buds. The eggs remain dormant until the next spring, when the caterpillars emerge and eat the fresh greens sprouting from the trees’ buds. So, the individual in the photograph is nearly a year old, having spent most of last year as an egg.
This humble vine is at the center of a evolutionary tangle of butterflies:
The vine makes poisonous defensive compounds which keep away most chewing insects. But pipevine swallowtails have evolved the ability to not only eat the pipevine, but to sequester its nasty chemicals, taking on a protective mantle. The poisons are stored by caterpillars when they feed on the vine and the chemicals are retained in the bodies of adult butterflies. The butterflies advertise their distastefulness with distinctive blue and black colors. Few birds or other predators are interested in attacking this species.
This bad-tasting butterfly carries a gaggle of other species in its wake. These others lack the defensive chemicals, but mimic the blue and black colors, gaining protection through false advertising. This kind of mimicry is called Batesian mimicry, after Henry Walter Bates, a Victorian naturalist who first described the phenomenon in the rainforests of South America.
Humans also use the chemicals in pipevine as a herbal medicine, but the severe toxicity of the aristolochic acids in the plant make this risky medicine, at best.