look-alikes (and not)

The Grass Lake slough at Snail Lake regional park is teeming with butterflies, especially Monarchs, which may be congregating here for their southward migration.  The Monarchs especially seem to like the meadow blazing star and don’t even mind sharing it with a lot of other butterflies, bees, etc.

The number of flowers present on meadow blazing star is one reason that butterflies congregate on it. The fact that it provides a lot of nectar at a time (late August) when butterflies are migrating is another.

Plenty of nectar to go around for Painted Ladies (also migrating by the thousands in late summer), Monarchs, bumblebees, honeybees, and a few stray beetles.  The orange, black, and white pattern of the Painted Lady is similar to that of Monarchs, but they are not mimics and the two are easily distinguished from one another.

But one of the many Monarch butterflies I photographed wasn’t a Monarch, but a Monarch mimic, the Viceroy, and these two unrelated species ARE difficult to tell apart.  Can you spot the difference(s)?

Both species exhibit the bold orange and black pattern on the wings as well as the pattern of white dots on the black head and thorax of the insect. 

The biggest difference in coloration of the two species is the bold black horizontal (sort of) stripe on the hind wing of the Viceroy, seen from above or below.  The thick black lines on the hind wing of the Viceroy are similar to those of a female Monarch but are much bolder than the male Monarch’s, which also has a distinctive dot on each hind wing.  In addition, Viceroy butterflies are smaller in size, only about 2/3 the size of a Monarch.

But where Monarch caterpillars grow up eating milkweeds containing poisonous cardiac glycosides which they sequester in their bodies (and wings), Viceroy caterpillars eat willow, poplar, and cottonwoods — not at all poisonous. Bird predators find Monarch butterflies extremely distasteful and will regurgitate or spit them out. Viceroy butterflies that most closely resemble their poisonous cousins in coloration are better protected from predation, and thus, the mimics survive to reproduce.

And then there are these two, apparently dissimilar butterflies, flitting around the same plants, often displacing each other from the same flowers.

The Eastern Tiger Swallowtail comes in two colors: yellow and black, and black.

Not at all look-alikes, in fact, color-wise, they couldn’t look more different, except for the pattern of white dots around the margin of the wings and the distinctive iridescent blue splashes of color on the back end of the hind wings.  So, what’s going on here?

Male Eastern Tiger Swallowtail butterflies are almost always yellow with black stripes.  Females on the other hand vary between yellow morphs and black morphs.  And, the black morph is more commonly found in the southeastern U.S. where a similar-colored, poisonous and unpalatable Pipevine Swallowtail butterfly occurs.  These are the “look-alike” models for the Tiger Swallowtail (and other Swallowtail butterfly species) to copy.  Interestingly, the proportion of female black morphs of the Tiger Swallowtail is higher in southern populations because of genetic (sex-linked) process that makes black morph females produce mostly black morph females, and yellow females produce mostly yellow females!

Bottom line:  you have to look closely when identifying a butterfly, because it might be a mimic!

Comparison of three common swallowtail butterfly mimics and their model, the Pipevine Swallowtail. From butterfliesathome.com

A tale of four swallowtails

Black Swallowtail butterflies have frequented the backyard garden every summer, laying their eggs on the dill.  They usually arrive early and stay well into late summer, so there might be as many as two crops of Black Swallowtail caterpillars in the backyard (see an earlier post on this subject).

black-swallowtail-on-dutchmans-breeches

A Black Swallowtail butterfly lapped up nectar from early spring blooming Dutchman’s breeches.  Note double row of yellow spots on the top (dorsal) side of the wing and double row of orange spots on the under (ventral) side of the wing.

I wasn’t surprised to see what I thought was a Black Swallowtail butterfly zooming around the wildflower garden, nectaring on the coneflowers the other day, and went out to get some photos.

spicebush swallowtail

But the markings are not quite right for a Black Swallowtail butterfly.  The large whitish-orange splotches on the outer margins of the wing are only very small dashes of color.  

spicebush swallowtail

A large blue area showing on the hind wing and two rows of yellow dashes means this is not a Black Swallowtail, but the black form of the Eastern Tiger Swallowtail instead.

Black Swallowtail, Spicebush Swallowtail and Pipevine Swallowtail butterflies co-occur along with the black form of the Eastern Tiger Swallowtail throughout most of their ranges in the midwestern and eastern parts of the U.S.  The four species are part of a large Batesian mimicry complex, with the Black, the black form of the Eastern Tiger, and Spicebush butterflies copying the color pattern of the unpalatable Pipevine Swallowtail to protect them from predation. At least three other species, as well as the males of several of the six species mimic the female Pipevine’s color patterns.

PipevineSwallowtail-female-Bob Moul

The model — the unpalatable Pipevine Swallowtail.  Photo by Bob Moul.

It is thought that the more palatable female butterflies, in particular, profit from mimicking the unpalatable species because they spend more time with their wings extended while laying eggs, and thus increase their vulnerability to predation during that time.  Mimicry works well when predators have experienced the real thing — i.e., the unpalatable Pipevine Swallowtail butterfly.  However, since its range does not include Minnesota (yet), the color patterns of the mimics won’t work to their advantage here.

A beautiful villain

I found one of the culprits that can cause cucumber wilt on some milkweed plants in a backyard nearby yesterday — the very attractive orange and black Squash vine borer moth (Melittia cucurbitae).  It’s too bad they are such damaging pests in the garden because they are quite photogenic.

squash vine borer moth sipping nectar from milkweed

It looks like a wasp from a distance with its striped body and long dangling legs, but is actually a member of the clearwing moth family (Sesiidae).

Long, highly “feathered” back legs drag behind the body as the moth flies, making it look like the wasp that it is trying to imitate.  But the clear, moth-shaped wings give it away, although these are not really visible to the human eye because they are moving so fast, they almost disappear.

squash vine borer moth on common milkweed flowers

The wings are just a gray blur in this photo, and you can clearly see the long proboscis inserted into the nectary of the milkweed flower — a very un-wasp like head. It looks like those hind legs also have some sharp spikes on them, as well as their feathery covering of scales.

Many of the species in the Sesiidae family are active in the daytime and are brightly colored, yellow and black or orange and black, mimicking bee or hornet coloration. This Batesian mimicry (in which a palatable species mimics an unpalatable or predatory one) presumably reduces their chances of being eaten by predators while foraging on flowers in the daylight.

squash-vine-borer on swamp milkweed

At rest, the mimicry is closer to the real model, since its folded wings do resemble the shape of a wasp’s.  (This is probably the culprit whose larvae wilted my cucumber vines last year — feeding on the swamp milkweed flowers in my backyard.  You can read more about that here.)

Other members of the Sesiidae family are also agricultural pests, infesting fruit tree, vegetable, and timber crops, as their larvae bore into the woody stems or trunk and decimate the interior vasculature system of the plant.

The Clearwing Hummingbird Moth (Hemaris thysbe) is no relation to the garden villain above.  It’s a member of the Sphingidae family of hawkmoths, and although the larvae do feed on honeysuckle vines and some fruit trees, they don’t do nearly the damage that the Sesiidae species do.

Clearwing Hummingbird Moth nectaring on flowers of an Apocynum species

This photo is from a recent post on critters of the northern bogs and forests, but the Clearwing Hummingbird Moth can be found almost everywhere in North America from Alaska to Florida.   Hawkmoths have a stouter, less slender body, shorter legs, and no scales on their legs, compared to the Squash vine borer moths. 

I’m not sure what the significance of clear wings is in these very different clearwing moth species.  Perhaps lack of scales on the wings means less air resistance during hovering flight, which both types of moths use for nectar foraging; or perhaps clear wings enhance their mimicry; or perhaps clear wings just present a smaller target to potential predators, since they appear almost transparent while the moth is flying.  Whatever the reason, the convergence of species from two different moth families is an interesting one.

Look alikes (… or why mimicry works)

Recently, I have misidentified a few critters in the field, only to realize what they really are when I begin editing them on the computer.  For example,

Not a Monarch!

Not a Monarch Butterfly!  Though this Viceroy looks like one, there is a faint black line on the hind wings that is absent in Monarchs, and Monarchs have many more white spots on the leading edge of the forewing.

From:

Viceroy butterflies tend to be smaller than Monarchs, but this is difficult to determine in the field without both present.  From:  Learner.org

Milkweeds have their own set of orange and black insects, some of which are distasteful, but most of which are simply mimicking the color patterns of the noxious individuals (like the Monarch butterfly).

milkweed insects

It’s probably no accident that Box Elder bugs also cash in on this Milkweed mimcry complex, copying the pattern of the Common Milkweed Bug (top right above).  It is also about the same size as the milkweed bug.

Boxelder bug

Boxelder bugs are considered pests, as they congregate by the hundreds in our window sills in the fall.  (Photo from the National Pestcide Information Center)

And then there is the “bee complex”.  Everyone wants to copy the yellow and black pattern of the bees and wasps, cashing in on the threat of a painful sting.

The things that sting have bright yellow and black coloration; some have fuzzy hair and some don't -- even that pattern is copied.

The things that sting have bright yellow and black coloration; some have fuzzy hair and some don’t — even that pattern is copied.

The mimics might even try to act like their models -- hovering in front of flowers (hoverflies) or between perch sites (robberfly)

The mimics might even try to act like their models — hovering in front of flowers (hoverflies) or between perch sites (robberfly)

There are obvious advantages for the mimic — gaining protection from potential predators who have learned that certain color combinations mean distasteful and/or dangerous prey.  So, the nectar-loving, innocuous hoverfly seemingly poses a threat to would-be consumers of these bite-sized flies.  This is an example of Batesian mimicry, where the model is toxic, but the mimic is not.

But in other cases, the mimic is just as distasteful and perhaps as lethal as its model.  Naive Bluejays offered Monarch or Viceroy butterflies (sans wings) didn’t like either one of them and regurgitated their meal.  Many species of bees and wasps similarly utilize the yellow and black warning coloration.  These are examples of Mullerian mimicry, where the mimics and their models are both unpalatable or dangerous.  What is gained with this type of mimicry most likely is reinforcement of the warning to potential predators, who because of one bad experience with a particular individual prey item, avoid anything that looks like that in the future.

There are many websites devoted to this topic; one of the more interesting (from Discover Magazine) can be found here, where you can read about other uses of mimicry in the natural world, like:

Orchid flowers fool flat-footed flies by faking fungus-infected foliage

Is this plant diseased?  Or is it disguising itself with fungal spots to lure in potential pollinators?

Is this plant diseased? Or is it disguising itself with fungal spots to lure in potential pollinators?  From Discover Magazine blogger, Ed Yong.

Not really a Fritillary

Recently, I have posted photos of the Great Spangled Fritillary and the Aphrodite Fritillary found in the Midwest.  In Los Angeles, there is a beautiful orange butterfly called the Gulf Fritillary (Agraulis vanillae), that isn’t a member of the fritillary family at all.  Instead it is more closely related to the Heleconia butterflies found in the tropics.

The range of this butterfly extends from Argentina through South and Central America to the southern U.S., as far north as San Francisco.  In the tropics the larvae feed on Passionflower (Passiflora species), which contain a variety of alkaloids that make the caterpillars toxic to their predators.

The bright orange color of the adult butterflies serves as a warning coloration of their potential distaste; predators beware.