Camo caterpillars and the birds that eat them

 

A Red-eyed Vireo (Vireo olivaceus) in a characteristic hunting posture. The bird is searching for its major prey, the caterpillars of moths. It has its head cocked upwards, searching the lower surfaces of the leaves above, as that's where many caterpillars hide during the day. Lepidopteran larvae - and those are OVERWHELMING moth caterpillars (not butterfly caterpillars) - are the vireo's major food source. During a long summer day, the roughly 130 million Red-eyed Vireos breeding in the U.S. and Canada consume something on the order of 4 billion caterpillars. Hard to believe but based on what is known of their foraging rate and dominant prey items, that's how the numbers shake out.

A Prothonotary Warbler (Protonotaria citrea) with a freshly harvested caterpillar (unsure of species). Almost all of the 38 warbler species that breed in eastern North America eats scads of caterpillars, and so do most groups of songbirds. So do some nonpasserine species, most notably the cuckoos. Many of these birds are migrants that winter in more southerly haunts, often in Central or South America, and migrate northward during the breeding season to exploit the eastern deciduous forest region's vast bounty of caterpillars. As this is crop is only seasonally available, many of these bird species must travel back to warmer climes for the winter - where they undoubtedly also consume great numbers of caterpillars.

If you are a caterpillar, you don't want to be grabbed and eaten by a bird. Evolutionary warfare between caterpillars and birds is probably the main driver of some incredible caterpillar camouflage. If you are a plant-eating caterpillar and can become ever more like the appearance of the plants that you feed on, the odds of surviving surely goes up. Birds are endowed with incredibly keen eyesight and are quite adept at spotting larvae. They still find plenty, and between the avifauna and all of the other predators that take out caterpillars, the survivorship can be as low as 1% in some moth species. So, every advantage that a caterpillar can get to thwart predators becomes very important.

Following are some photos, most from recent forays, that showcase examples of incredible botanical (and gall) mimicry by members of the larval crowd.

Late summer and fall foliage become dappled with color as the chlorophyll fades and leaf tissue begins dying. This plump Blinded Sphinx Moth (Paonias excaecata) caterpillar displays reddish patches that mimic autumnal foliage, in this case Hop-hornbeam (Ostrya virginiana).

This is a sister species of the caterpillar above, the Small-eyed Sphinx (Paonias myops), which also sports rusty dots that resemble necrotic leaf tissue.

This is a personal favorite of the leaf-edge mimicry crowd, a Double-toothed Prominent (Nerice bidentata). It is a specialist of elms, and in our part of the world that's almost entirely American Elm (Ulmus americana) and Red Elm (U. rubra). We find them on both of this species with roughly equal frequency. The back of the caterpillar is scalloped, much like the rough marginal serrations of elm leaves. As it eats into the leaf, the caterpillar becomes the leaf.

This is another elm specialist (mostly at least, it is also said to eat basswood), and its moniker is apropos: Elm Sphinx (Ceratomia amyntor). When at rest on the underside of an elm leaf, as above, the caterpillar typically aligns itself with the prominent midrib of the leaf. The caterpillar also has a raised stripe down the center of its back. The cat's lateral lines are arrayed at acute angles to its pseudo midrib and mirror the angled venation of the elm leaf. In effect, the caterpillar becomes a continuation of the leaf's veins.

Few can match the Checker-fringed Prominent (Schizura ipomoeae) caterpillar's ability to become one with the leaf. This one (the head is to the bottom) is consuming an old dogwood leaf and as is typical, the caterpillar is eating inwards from the leaf's edge. In essence, it becomes the leaf and spotting one of these can be quite difficult even when it is right in front of you. Sharp-eyed botanists might note the cat's scientific epithet: ipomoeae. That references a genus of morning-glories and is probably a misnomer, as this caterpillar probably never eats those plants.

Few do twig mimicry better than the caterpillar of the Dot-lined White Moth (Artace cribaria). They are virtually impossible to see as they lay plastered to twigs, despite being a hefty cat.

Another brilliant example of twig mimicry is the Yellow Slant-line Moth caterpillar (Tetracis crocallata). This is one of the inchworms in the massive Geometrid family, and many inchworms are twig mimics. When not feeding, they attach themselves to a twig of similar diameter and color, and often adopt an acute outward angle. In the case of this species, the head is jagged and pale yellow, exactly as a broken-off branch would appear.

A Redbud (Cercis canadensis) leaf, with what appears to be a raised reddish gall. Such leaf galls are commonplace, and can be formed by many organisms: bacteria, fungi, aphids, midges, wasps and others. Birds don't eat them, insofar as I know.

But wait! Closer examination shows that our leaf "gall" is not a gall at all. It is a Red-crossed Button Slug caterpillar (Tortricidia pallida). Slug moth caterpillars are fascinating as a rule, as are the "cute" little moths that they become. As caterpillars, many species appear to mimic the look of leaf anomalies such as galls, and they can be hard to spot.

This is just a tiny tip of the lepidopteran iceberg when it comes to caterpillar mimicry, much of which is presumably driven by sharp-eyed birds that attempt to feed ravenously on the caterpillar crowd.

Epic caterpillaring results in many cool finds

 

Cherry Dagger moth caterpillar (Acronicta hasta)

Shauna and I spent last weekend at the fabulous Highlands Nature Sanctuary in Highland County, Ohio, which is owned by the Arc of Appalachia. A bunch of us were there for our 12th annual "caterpillar safari", which is just a bunch of larvally-minded friends of ecology gathering to seek out the always fascinating caterpillars of moths (mostly) and butterflies (far fewer in numbers and diversity). Now is peak season for caterpillar abundance and diversity, and we prowled into the wee hours on Friday and Saturday nights, exploring interesting habitats. Most caterpillars are active nocturnally, so hunters of them must adjust their search hours accordingly.

Dozens of species of caterpillars were found and photographed, including this Cherry Dagger (Acronicta hasta). We brought lots of livestock back to the Arc's forest museum - our base camp - and from 10 am - 2 pm the public is invited in. Dozens of people got to see crazy larvae that they likely never knew existed. One should not underestimate the value of moth larvae (which the overwhelming number of caterpillars are). They are the primary organisms that convert plant matter into a digestible form of protein for birds and myriad other animals and underpin food webs.

Gray Hairstreak caterpillar (Strymon melinus)

We certainly do not ignore the occasional butterfly caterpillar that turns up, and this was a particularly interesting one. It is the caterpillar of the Gray Hairstreak (Strymon melinus), which specializes on feeding upon species in the Pea Family (Fabaceae). These cats resemble Mike & Ike candies and are tiny and easily overlooked.

This one is feeding on a tick-trefoil in the genus Desmodium. Many hikers know this group of plants by their triangular loments (fruit) that are thickly beset with stout hooked hairs. They evolved for mammalian dispersal, and that includes the clothing worn by humans. Those loments can be tough to get off and will even survive trips through the washing machine.

Here's what that Gray Hairstreak caterpillar in the previous image will (hopefully) morph into - one of our showiest butterflies. I say "hopefully" because the survivorship of caterpillars is astonishingly low. Just about everything wants to eat them and for some species of moths, at least, probably only one percent or so of caterpillars make it through the complete life cycle and to the mature reproductive stage. The rest become part of a great food chain. To compensate for such high mortality, butterflies and moths lay enormous numbers of eggs; a carpet-bombing strategy if you will.

Finally, a cool little-known thing about Gray Hairstreak butterflies. John Howard showed this to me a while back. Hairstreaks habitually rub their hind wings together, and the "tails" resemble antennae. Perhaps this pseudo face with its twitching antennae fools birds and other would-be predators who then lunge at the wrong end of the butterfly. While the hairstreak may lose parts of its hind wings, it might still escape to live another day and find a mate and reproduce.

Anyway, as John pointed out, if you can get directly behind the hairstreak and under it (that can take some effort), the lateral fringes form the illusion of orange eyes capped with long antennae, and even a frowning mouth below. Whether all of that evolved to spook visual predators I do not know, but whatever the case it certainly looks cool. And once a butterfly photographer knows this, they may find themselves spending much time attempting to get into position to get this shot.

Blanchard's Cricket Frog

 

As always, click the image to enlarge

Blanchard's Cricket Frog (Acris blanchardi) can be tough to see. This is Ohio's smallest frog, measuring a wee 0.6 inches in length and weighing but a gram. They are also cryptically patterned and blend well with their surroundings. While exploring an Adams County (Ohio) prairie last Saturday, August 31, we saw dozens of them around a small pond. The section of muddy shoreline that we explored had many dozens of frogs. The elfins make one work for a good photo, I can tell you that.

These tiny frogs, which inhabit small ponds and stream corridors, are easy to miss if one is not attuned to them. Cricket frogs are most conspicuous in late spring/early summer, when males begin to vocalize. They create a series of surprisingly loud, metallic clicks that are quite un-frog-like. But the vocalizing doesn't last all that long and by now the cricket frogs have fallen silent. The little pond where I made this image probably hosts thousands of individuals, at least at this season. The adult's ranks are supplemented with scores of juveniles, but many/most of those probably won't make it till winter. Lots of potential predators, especially for such a small amphibian. The cricket frog lifespan, if all goes well, is said to only be a year.

PHOTO NOTES: I made this image with my workhouse Canon 100mm f/2.8L macro lens, with the Canon R5 and auxiliary light courtesy of the 600 EX II speedlite. Settings were f/14, ISO 400, and 1/200 second. Camera settings are the least of the cricket frog photographer's issues. As you can surmise from the photo, the tiny frogs blend extraordinarily well with their substrates. That coupled with their small size makes them incredibly easy to overlook. Also, when alarmed, they can leap fantastic distances. No frog that I have firsthand familiarity with leaps as far as a cricket frog in a single bound. We figured they were catapulting themselves some 30x the length of their bodies, maybe more.

The trick is to find a possible subject, slowly ease closer, then slowly drop to the mire and ever so gently worm your way towards the frog. Done with delicacy, one can get quite close - I was probably only a few feet from the animal in the photo. It's best, in my opinion, to have your camera on the ground. Eye level is usually best with small ground-bound organisms.