In September I visited soybean field trials across the southeastern U.S. It’s a trip I’ve done every year for the past I don’t know how many years and one that I enjoy immensely due to the opportunities it gives me to see the country, kick the dirt with academic cooperators, sample the local cuisine… and photograph insects. New for me this year was the Carolinas, and in a soybean field in Plymouth, North Carolina I encountered this geometrid larva on the stub of a soybean leaf petiole. Geometrid larvae are known variously as inchworms, cankerworms, spanworms, measuring worms, loopers, etc., depending on the species. Most of the common names refer to the same thing that the family name does—the larval method of locomotion whereby the caterpillar—possessing legs only at the two extremes of its body—”inches” its way along as if measuring the ground it walks on (Geometridae is derived from the Latin geometra, or “earth-measurer”). The resemblance of the larvae of many species to dead twig stumps is nothing short of remarkable, and had it not been for the contrasting coloration I may never have noticed the larva in the first place. I also did not notice until looking at it through the macro lens of my camera the tether attached by the larva to the tip of the twig—invisible to the naked eye but providing energy-saving stabilization for the larva to hold its cryptic position.
I’ve not encountered a geometrid larva in soybeans before, and my impression has been that they are largely deciduous tree feeders (perhaps due to the periodic occurrence in my area of outbreak species such as fall cankerworm). In trying to determine the species, I found no geometrids covered in the Higley & Boethel (1994) handbook on U.S. pests, and when I consulted the Turnipseed & Kogan (1976) and Kogan (1987) global reviews of soybean pests I found reference only to a few minor pests in India and southeast Asia. Hmm, time for BugGuide. Of course, lepidopteran larvae are not nearly as well represented as the adults, but it seemed most similar to species of the subfamily Ennominae, so I turned to Google and searched on “Ennominae soybean.” This turned up Passoa (1983), who reported larvae of Anacamptodes herse as pests of soybean in Honduras (and mentioned references to several other geometrid species associated with soybean in Brazil). Back to BugGuide, where I found the genus Anacamptodes listed as a synonym of Iridopsis, but the species I. herse was not among the list of species represented in the guide. Checking the link provided at the site to a revision of the genus by Rindge (1966) revealed that I. herse is strictly a Central American species. Perhaps another, North American species of the genus also favors soybean, which led me to Wagner (2005) who mentions soybean as a favored food plant for I. humilis. However, the contrasting purple-brown/yellow-green coloration and relatively thickened body of that species are quite unlike this individual. I don’t have Wagner’s book (only his smaller one on caterpillars of eastern forests—no match in there, either), so it may be that my only remaining option is to post the photo at BugGuide and hope that David Wagner encounters it (actually I should get David’s book anyway)¹.
¹ Update 10/5/13 11:30 am CDT—or hope that Brigette Zacharczenko runs into the post via Facebook and offers to pass it along to Dave during their lab meeting on Monday.
Higley, L. G. & D. J. Boethel [eds.]. 1994. Handbook of Soybean Insect Pests. The Entomological Society of America, Lanham, Maryland, 136 pp. [sample pages].
Kogan, M. 1987. Ecology and management of soybean arthropods. Annual Review of Entomology 32:507–538 [pdf].
Passoa, S. 1983. Immature stages of Anacamptodes herse (Schaus) (Geometridae) on soybean in Honduras. Journal of The Lepidopterists’ Society 37(3):217–223 [pdf].
Rindge, F. H. 1966. A revision of the moth genus Anacamptodes (Lepidoptera, Geometridae) (1966). Bulletin of the America Museum of Natural History 132(3):174–244 [pdf].
Turnipseed, S. G. & M. Kogan. 1976. Soybean entomology. Annual Review of Entomology 21:247–282 [pdf].
Wagner, D. L. 2005. Caterpillars of Eastern North America: A Guide to Identification and Natural History . Princeton University Press, Princeton, New Jersey, 496 pp. [Google eBook].
Copyright © Ted C. MacRae 2013