Fertility and Flight in a Winter Moth

operophtera-bruceataIN THE ANGLED DAYLIGHT of November, a moth crossed my path through barren woods. Yeah, a moth, in the cold – a lesson in adaptation, fertility, and feminine sacrifice going by the name Bruce Spanworm.

As a caterpillar, Bruce Spanworm (Operophtera bruceata) is one of the inchworms, a member of the large moth family called Geometridae (which means “earth-measuring”).*  As an adult, Brucie is not particularly showy. His forewings, about 1.5 inches across, are dull gray with dark flecking, and its hindwings are even less dramatic. But who needs pizzazz when you’ve got the audacity to fly around in winter.

By now virtually all our moths and butterflies no longer live as gossamer-winged adults. Instead, they’re overwintering as either eggs, caterpillars, or pupae (a cocoon in the case of a moth). But walk through deciduous woods in November, and Bruce Spanworm or a relative called Fall Cankerworm (Alsophila pometaria) will most likely be your companion, even fluttering among snowflakes into December. These moths are also called Hunter’s Moths because they fly in the company of deer hunters.

We normally think of moths as ectothermic (or, more commonly, by the confusing term “cold-blooded”), which means they depend on environmental heat sources in order to fire up their metabolism, including their flight muscles. It’s the reason most insects fly in summer.

So how is it that Bruce Spanworm is such a hearty iconoclast? Why does it fly when warmth and solar energy are in such short supply? Well, for one thing, insectivorous birds are also in short supply, most having left us by now for the tropics. So a moth that can fly when predators are fewer can presumably go about its business breeding (more on that later) with greater success.

Yet a cold-weather sex drive and avoiding getting eaten, while certainly important, aren’t enough for a moth to take flight in freezing temperatures. As it turns out, the male Bruce Spanworm, while dull in appearance, is buff; he’s a muscle-man. And his flight muscles perform in the cold.

One study by James Marden, a biologist at Pennsylvania State University, published in 1995 in The Journal of Experimental Biology, determined that Bruce Spanworm’s flight muscles can generate greater force at lower temperatures than a comparative summer-flying moth.

But physique also plays a role. Bruce Spanworm has more muscle mass as a percentage of his total body weight compared to the summertime moth used in the study. Sure, he may be small, but like a welterweight boxer, ounce-for-ounce he’s formidable. Perhaps more importantly, Bruce has a low wing-loading, which is basically the total weight of the moth divided by the surface area of his wings. Our pal Bruce gets a lot of lift for each flap. This lowers the energy he needs to flutter around with the hunters in November, a crucial adaptation for an insect that relies on solar energy for flight. (I’ll point out that University of Vemront biologist Bernd Heinrich, my former office mate, did amazing work on insect thermoregulation, including a 1985 paper on winter-flying moths. Bernd has contributed greatly to our knowledge of Bruce Spanworm and the thermodynamics of insects in general.)

Bruce Spanworm (Operophtera bruceata)None of this wing analysis applies to females. That’s because the females don’t fly. They don’t fly because they lack wings. Females, full of eggs, simply sit there on the trunks of trees and waft pheromones, chemical attractants, into the air to lure males for mating. In fact, one of the best ways to locate a female Bruce Spanworm is to look for a cluster of males fluttering over a spot on the tree trunk. It is a certain sign the boys are competing for a copulation with a flightless female. My pal Kent McFarland caught a pair in the act (pictured here). Read Kent’s post about it.

The female Bruce Spanworm is essentially an egg vessel; her body cavity is packed with eggs – an average of 143, according to Marden’s study – rather than large internal organs or tissue. Were she to regain the ability to fly, to develop wings and wing muscles, a female Bruce Spanworm might have to lose some of her eggs. She can’t get something for nothing. She’s given up wings for greater fecundity – a greater ability to reproduce.

In the course of her evolution, the female Bruce Spanworm has traded flight for fertility. To consider it from another angle, what might you trade in order to take flight?

* As it turns out, these moths might also be another member of the genus, Operophtera brumata; we probably need some genetic sequencing to be sure which is which.

Here’s my short indie film about all this called “Walking with Moths.”

Reference:

Marden, J.H. 1995. Evolutionary adaptation of contractile performance in muscle of ectothermic winter-flying moths. The Journal of Experimental Biology. 198, 2087–2094.

Shadows and Sex

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Red Squirrel / © Bryan Pfeiffer

YOU DON’T NEED PUNXSUTAWNEY PHIL to know which way the wind blows. Groundhog Day ain’t about shadows. It’s about sex. Birds and rodents now begin a season of foreplay.

No, spring is not around the corner – at least not here in Vermont. Songbirds don’t rely on the vagaries of weather to calculate their breeding cycles. Instead, they schedule mating and nesting to take advantage of a reliable abundance of food for their offspring, mostly insects, which happens in May and June here at our latitude. As the days grow longer, birds do get ready to, well, um, make more birds. It’s why we’re starting to hear Black-capped Chickadees, Northern Cardinals, House Finches and other birds erupting into song on sunny mornings.

Black-capped Chickadee / © Bryan Pfeiffer

Black-capped Chickadee / © Bryan Pfeiffer

Day length is a far more reliable calendar than weather. It is not entirely clear how birds measure day length, but we do know that photo-receptors in bird brains sense increasing light. It triggers the production of hormones that act like birdie Viagra. Their sexual organs revive from a state of dormancy. So when the food is there in May, songbirds will be ready … you know, physically.

February 2 is indeed significant. It falls about halfway between the Winter Solstice and the Spring Equinox, a period celebrated in various ways in human traditions from Paganism to Christianity. And early February is when we start to get 10 hours of daylight – February 6 this year. It seems to be a turning point for wildlife.

But why the groundhog? Couldn’t we have picked a loftier critter to represent the coming of the light? As it turns out, this rodent is indeed a worthy messenger of spring. In February, woodchucks begin to emerge from hibernation on the prowl. They need to breed soon so that females produce litters during greater food abundance in April and May. Males emerge from their burrows to find and visit with females. But many of these early encounters are merely courtship visits, which pay dividends, research suggests, when it comes time to breed a bit later. It’s sort of like another February ritual – Valentine’s Day.

Squirrels aren’t so tactful. Female red squirrels are in estrus, receptive to males for breeding, for about eight hours on only a single day during this season. And male squirrels outnumber females in the wild by as much as five to one. The consequence of this skewed gender ratio and hard-to-get females is that life during the breeding season can be, to say the least, challenging for the male. He’ll spend lots of time following her in the days before she is in estrus. Should the male be too forthcoming, too eager before she is ready, she will rebuff his advances with a swat to the face or a painful bite. (I hate it when that happens.)

And when those precious eight hours finally arrive, a male is hardly alone in this drama. He often must compete with or fight other males for her affections – actually for a copulation that might last only about 20 seconds. Out there in the trees, it’s a free-for-all. “To the casual observer, what ensues is probably best described as pure and unadulterated chaos,” write biologists Michael A. Steele and John L. Koprowski in their fantastic book, North American Tree Squirrels.

So let’s recognize the real significance of Groundhog Day. This isn’t a holiday about six more weeks of winter. It’s a celebration of romance, even if it turns out to be unadulterated, chaotic rodent romance.

Bryan Pfeiffer teaches writing in the Field Naturalist and Ecological Planning programs.

A Blackpoll Warbler’s Daring Trans-Atlantic Flight

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By Bryan Pfeiffer

Two wings and a prayer carry a Blackpoll Warbler on a remarkable journey to South America each autumn. Well, actually, two wings and the audacity to pull off one of the most amazing feats of migration on the planet: a non-stop, trans-Atlantic flight lasting up to three days.

With most of us only speculating for decades about this amazing journey, my colleagues at the Vermont Center for Ecostudies (VCE) today announced proof. Blackpoll Warblers fitted with miniature tracking devices took off from points in either Nova Scotia or the northeastern U.S. and flew south over the Atlantic, with no safe place to land, until reaching Caribbean islands roughly 1,600 miles away.

“This is one of the most ambitious migrations of any bird on earth,” said VCE’s Executive Director, Chris Rimmer, co-author of a research paper published today on the warbler flights. “We’ve also documented one of the longest nonstop, overwater flights ever recorded for a songbird.” Continue reading

Kiss This: Mistletoe’s Higher Calling

By Bryan Pfeiffer

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Great Purple Hairstreak (Atlides haleus) / © Bryan Pfeiffer

GET YOURSELF UNDER SOME WILD MISTLETOE this Christmas. Your gift might be a shock-and-awe butterfly called Great Purple Hairstreak.

Mistletoe is a plant that grows on trees or shrubs. And it’s a bit of a leech, a hemiparasite, which means mistletoe draws minerals and fluids from its host. But mistletoe is also photosynthetic, generating some of its own energy demands from sunshine. Oh, by the way, there is no one mistletoe. The world has more than 1,000 mistletoe species. I’ve got one of them in a cottonwood next to my cabin here in New Mexico. Mistletoes grow flowers, have pollinators, and produce fruits like many other plants.

In Arizona this past weekend, among thousands of butterflies, I encountered a single Great Purple Hairstreak. That’s usually how we find them, sluggish and alone, about the size of your thumbnail, crawling among flowers and lapping up nectar. But the business side of this butterfly, at least when it comes to mistletoe, is the caterpillar. Great Purple Hairstreak caterpillars eat mistletoe species in the genus Phoradendron (and maybe others). Continue reading

Zombie Aspen Leaves

populusleaves-550x410By Bryan Pfeiffer

Rotting and fallen to earth, they might appear dead. But they are not quite dead. They are the undead: zombie aspen leaves.

Find them as you walk the brown autumn paths – yellow leaves with a patch of green tissue radiating from the base of the midrib. Here in Vermont, these are mostly quaking aspen (Populus tremuloides), but I also find the green on big-toothed aspen (P. grandidentata) and, rarely, eastern cottonwood (P. deltoides).

When a friend and I first encountered these some years ago, I collected a few and queried a handful of smart botanists for answers. Many had theories; none had an explanation. It wasn’t until I put a leaf under a dissecting microscope that I found the explanation to be less zombie than something from the film “Alien.” The beast lies within.

populus-caterpillar-moth-550Residing in a tiny pocket of tissue near the base of the green patch is a translucent caterpillar not much more than 2 millimeters in length. It’s feeding in there; I could see the frass (caterpillar poop). With help from Dave Wagner, the renowned entomologist at the University of Connecticut, our critter turns out to be a moth in the family Nepticulidae, probably Ectoedemia argyropeza or most certainly a member of that genus.

“The really cool thing is that the larva secretes an anti-senescent substance that keeps part of the leaf alive – probably a cytokinin,” Dave wrote in an email. Cytokinins are plant hormones that promote cell division. In this case, it seems, the caterpillar keeps part of a leaf alive so that it can keep eating.

This moth is also parthenogenetic; females can produce fertile eggs without help from males, which, as it turns out, are quite rare.

For now, however, the caterpillar will continue to dine in the verdant patch of an otherwise dead leaf. It will pupate for winter. And the tiny adult will emerge to fly in spring. Many species in this genus are black and white with orange scales around the head. But don’t expect to find one. Your best bet for discovering this animal is to watch the trail for patterns in poplar leaves this fall.

And if you’re raking them up, please note that some of those leaves, well, they could be saying, “I’m not dead.”

The Nuclear Option for Dragonflies

By Bryan Pfeiffer

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A Common Green Darner (Anax junius) / © Bryan Pfeiffer

On a crisp, sunny day in September, after what was probably a typical summer for a dragonfly (which involves flying around, killing things and having sex beside a pond), a Common Green Darner took off and began to migrate south. As it cruised past the summit of Vermonts Mt. Philo, with Lake Champlain below and the Adirondacks off in the distance, the dragonfly crossed paths with a Merlin.

The Merlin, a falcon that kills in flight, swerved, plucked the dragonfly from the sky with its talons and began to eat on the wing. As the falcon and its prey continued southbound, all that remained in their wake was a single detached dragonfly wing, falling like an autumn leaf toward fields at the base of Mt. Philo.

Eagles, hawks, falcons and Monarch butterflies aren’t the only migrants moving south past mountains this fall. Joining them are dragonflies. Although biologists know plenty about the fall raptor and Monarch migrations, we are only beginning to discover, with some creative chemistry, where these dragonflies go and how migration figures in their conservation.

Fly or Die

Most dragonfly species do not migrate. In fact, most are now dead, having already mated during the summer season, leaving behind eggs or larvae to survive the winter. The killing frost will finish off much of what’s still on the wing. But some survivors will leave.

Wandering Glider (Pantala flavescens) / © Bryan Pfeiffer

Wandering Glider (Pantala flavescens) / © Bryan Pfeiffer

Among the 460 or so dragonfly and damselfly species native to North America, at least five are classic migrants: Common Green Darner (Anax junius), Black Saddlebags (Tramea lacerata), Wandering Glider (Pantala flavescens), Spot-winged Glider (Pantala hymenaea) and Variegated Meadowhawk (Sympetrum corruptum). Each species is on the move this fall.

Dragonflies migrate for the same reasons other animals migrate: to avoid inhospitable conditions, in this case habitats that freeze or become too cold for the dragonflies themselves or their insect prey. Monarchs go to Mexico. Broad-winged Hawks leave for wintering grounds stretching from southern Mexico into South America. Dragonflies head south to who knows where.

Having studied birds for two centuries, biologists know well their breeding and wintering distributions , even to the point of learning the destination of a particular warbler or sparrow after it leaves us in the fall. Ornithologists catch lots of songbirds in nets and place around one leg a tiny silver bracelet embossed with a unique number – an avian social security number – and then release the birds to the winds. A small percentage of them, still sporting their bracelets, are later recaptured while in migration or on wintering grounds thousands of miles away. Better yet, we’re putting small electronic transmitters on large birds, such as Bar-tailed Godwits and American Woodcocks, and tracking their movements real-time with satellites.

On of my tagged Monarchs before heading off toward Mexico.

A tagged Monarchs before heading off toward Mexico.

We can even track the movement of a single butterfly. I myself have placed little stickers, each bearing a unique alpha-numerical code, on the hind wings of more than 1,000 Monarchs here in Vermont and elsewhere in North America, and then set each one free to fly toward Mexico, where many are later encountered by conservationists searching for the buttterflies once they arrive in Mexico. With each recovery, we learn more about Monarchs and how they migrate.

The “Heavy” Hydrogen

Dragonflies aren’t so obliging. For one thing, we’re clueless about where they go. Monarchs concentrate each winter in stands of Oyamel Fir in mountains west of Mexico City. So we know where to find them and how to protect them. Tagging or somehow marking a dragonfly would be like putting a message in a bottle and tossing it out to sea. Actually, I suspect we’d find the bottle before the dragonfly.

Yet it turns out that we need not tag or otherwise mark these migratory dragonflies because they themselves carry clues about where they have been. If the Merlin doesn’t get it first, we can catch any migrating dragonfly, analyze trace elements in its tissue and determine roughly how far it has flown.

Our marker is water, more to the point the two hydrogen atoms in water. Recall from high school chemistry that hydrogen nucleus normally contains a single proton and no neutron. But a tiny fraction of hydrogen atoms around the world carry one proton and one neutron. We call it “heavy hydrogen,” or deuterium. And unlike other such atomic variations among elements (which can be radioactive), deuterium is stable in the environment – a “stable isotope”– and stable in the wing of a dragonfly.

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Credit: Migratory Dragonfly Partnership

The amount of deuterium in water varies somewhat predictably in North America. You can map it. The ratio of deuterium to hydrogen in water falling as rain or snow changes on a gradient corresponding roughly with latitude. Water in Alberta, for example, carries a different deuterium-to-hydrogen ratio than water in Alabama.

Because dragonflies grow up as nymphs in water, they incorporate the local deuterium ratio into their tissue. It’s like a dialect or an accent that a dragonfly bears for life – whether as a nymph in water or a free-flying adult in migration. A Common Green Darner on the wing over Mt. Philo or Miami unwittingly carries a particular deuterium ratio, a birth certificate that tells us generally where it grew up. You are what you eat – or drink.

This science isn’t perfect. We can’t pinpoint a dragonfly’s natal waters in the way we know where a banded bird hatched or a tagged Monarch emerged. But stable isotopes are helping us track the range of migrating dragonflies. It’s “better living through chemistry.” After all, we can’t really know a bird or butterfly or a dragonfly – and what it might need in the way of conservation – until we know all the places it lives or wanders.

By the way, you need not be a chemist to help track dragonfly migration. We’re counting dragonflies in the same way we count migrating raptors during hawkwatches each fall. Learn how to do it and report what you find with help from the Migratory Dragonfly Partnership.

And while we’re out there counting, if a Merlin happens to catch a dragonfly first, we can still make a difference … by catching one of those dragonfly wings floating toward Earth.

Bryan Pfeiffer is a writer and field naturalist who specializes in birds and insects. He teaches writing in the University of Vermont’s Field Naturalist and Ecological Planning Programs.

Hobblebush on Fire

Viburnum alnifolium

 

By Bryan Pfeiffer

An entire season of fall foliage flares from a single plant. Find your fireworks on Hobblebush (Viburnum alnifolium). This understory gem may be the perfect shrub. It adds food and habitat diversity – for nesting birds and other wildlife – beneath forest canopy. And its blooms play a crafty game of deception each spring. Continue reading

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