Wednesday, August 16, 2017

Walnut Caterpillars

Walnut Caterpillar, Datana integerrima  - Mort Shurtz

D. Integerrima in mass - Missouri University Extension
A while ago Mort Shurtz sent me this picture of a Walnut Caterpillar, Datana integerrima, that he found in his back yard.  When a moth species' common name is for the caterpillar, it means either that the cat is beautiful or obnoxious.  D. integerrima falls into the obnoxious category.  In their big years they tend to cover the trunks in large numbers, defoliate the trees and generously spread their frass around. 

Walnut Caterpillar Moth, Datana integerrima - Bob Moul
Although we have planted over 500 walnut trees in our riparian area within sight of our house, I have yet see Walnut Caterpillars on our trees or to photograph a Datana integerrima along Bull Creek.  At first glance it would be hard to guess that it was a moth or even would be capable of flight.  Its color is more handsome rather than pretty, more like a fine piece of wood furniture.  It has an orange fuzzy head and a black spot on its thorax.

Spotted Datana Moth - Datana perspicua
We commonly see its cousin coming to our deck light overlooking Bull Creek.  The Spotted Datana, Datana perspicua, has a similar color and shape but logically enough has a dark spot on it dorsal wing as well as sharper lines on its wings.  It feeds on sumac that is plentiful in the valley.

Yellow-billed Cuckoo - Joe Motto
Texas A&M cites some species of  wasps and flies that consume egg masses and larvae of Walnut Caterpillars, and many other insects and spiders prey upon larvae.  When disturbed, the caterpillars drop to the ground on a silk thread.  Birds don't make most of the lists of known predators and I suspect the fuzzy hairs of the Walnut Caterpillar serve as a deterrent.  I doubt that their large family gatherings would be possible if they were tasty and convenient for birds to eat.  Our Yellow-billed Cuckoo Coccyzus americanus, is the only bird listed as eating them, first reported in 1922.

Eastern Tent Caterpillar
Fall Webworm

Fall Webworms and Eastern Tent Caterpillars are other moth species that form large communal nests early in life that would seem to offer a feast for birds.  They appear to be protected by their web, until we tear it up exposing the individual caterpillars.  The masses of caterpillars that "flock" together can serve as a defense only if a bird eating one will decide that they aren't tasty, either because of chemicals or the nasty effects of the hairs.  But where potential food congregates there is usually some predator that will develop a strategy for getting it.

Lisa Berger sent me some information about one predator, the Black-billed Cuckoo, Coccyzus erythropthalmus that specializes in these species.  The Birds of North America information on the cuckoo's digestion of caterpillars is well worth a read.
"The Black-billed Cuckoo is a notorious consumer of caterpillars, with a demonstrated preference for noxious species, including the eastern tent caterpillar (Malacosoma americanum), fall webworm (Hyphantria cunea), and larvae of the gypsy moth (Lymantria dispar). Observations of cuckoos consuming 10–15 caterpillars per minute are testimony to the great service this species provides in forests, farms, and orchards. Stomach contents of individual cuckoos may contain more than 100 large caterpillars or several hundred of the smaller species. The bristly spines of hairy caterpillars pierce the cuckoo's stomach lining giving it a furry coating. When the mass obstructs digestion, the entire stomach lining is sloughed off and is regurgitated as a pellet." Excerpted from The Birds of North America Online.
This cuckoo species is rare in the Ozarks and therefore not an answer to our outbreaks.  Birds of North America's map shows that we are just barely in the southern edge of its breeding territory.  That still leaves our Yellow-billed Cuckoo.  According to Allaboutbirds, "Yellow-billed Cuckoos forage slowly and methodically in treetops for large, hairy caterpillars.  (They) are among the few bird species able to eat hairy caterpillars. In the East they eat large numbers of tent caterpillars—as many as 100 in one sitting."

Yellow-billed Cuckoo in the treetop - Clark Creighton
When disturbed by a potential predator D. integerrima drops rapidly to the ground.  Since the cuckoo tends to forage higher up, the caterpillar can start the long slow climb back up the tree while the bird has had its fill of its siblings and leaves, ensuring the survival of the species.  But how does the cuckoo handle the hairs?
"Cuckoos and hoopoes (Upupidae) are also able to clean larvae from their setae by rubbing them on the ground (Payne, 1997;Kristin, 2001), but the best adaptation to feed on hairy caterpillars is found on several cuckoo species. In these species, the gizzard inner layer has evolved towards a soft, thick and non-keratinoid structure that allows the larvae setae to be kept inserted in the gizzard wall and to be regurgitated as mixed pellets of mucous membrane and setae (Gill, 1980)"  Birds as predators of the pine processionary moth.
If that doesn't sound very appealing to you, watch an Asian cuckoo in action in this video.  It rubs the caterpillar along the branch to wipe off most of the hairs as well as drain the intestinal contents before eating it.  No wonder we call them cuckoos!