Submitted to: Journal of Chemical Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 4, 2007
Publication Date: February 7, 2008
Citation: Carroll, M.J., Schmelz, E.A., Teal, P.E. 2008. The attraction of Spodoptera frugiperda neonates to cowpea seedlings is mediated by volatiles induced by conspecific herbivory and the elicitor inceptin. Journal of Chemical Ecology. 34:291-300. Interpretive Summary: Plants respond to insect attack through the increased production and release of induced volatiles from damaged tissues. These volatile odors benefit the plant by attracting natural enemies (i.e. parasitoids and predators) of insect herbivores to the site of damage, where they can locate and kill the pests. However, these same volatiles might be used by the insect herbivores themselves to better locate host plants because damaged plants are more apparent to the olfactory senses. Insect herbivore attraction to damaged plants and their associated induced volatiles was examined in neonates, newly-hatched caterpillars that must find appropriate plant material to feed upon or starve to death within several hours. Scientists at the Center for Medical, Agricultural, and Veterinary Entomology in Gainesville, Florida have discovered that fall armyworm neonates are strongly attracted to cowpea plants damaged by caterpillars. Researchers further determined that this attraction behavior was mediated by the elicitor inceptin, a peptide found in the oral secretions of caterpillars that triggers plant defenses. Neonates moved less often off caterpillar-damaged or inceptin-treated plants than mechanically-damaged or undamaged plants. In choice tests, neonates were specifically attracted to damage odors and induced volatiles from inceptin-treated plants or plants damaged by caterpillars previously fed leaf material (required for inceptin formation in oral secretions). Neonates were also attracted to DMNT, the main volatile induced in damaged cowpea, both as an individual odor and a supplemental addition to plant odors. Researchers attempting to improve recruitment of predators or parasitoids to damaged plants by genetically engineering plants with stronger induced volatile responses should be aware that such plants may also inadvertently attract insect herbivores in greater numbers.
Technical Abstract: The fall armyworm [FAW; Spodoptera frugiperda (Smith)] often encounters herbivore damage as it disperses as a neonate from an egg mass to an individual feeding site. We investigated orientation responses of dispersing neonates to herbivore damage in cowpea seedings, specifically examining whether neonate responses were mediated by inceptin, the primary elicitor of FAW-induced responses in cowpea leaves. Inceptin contents of damaging first instar FAW were controlled through their diets, with leaf-fed FAW producing inceptins and root-fed or starved FAW lacking inceptins. In a bioassay designed to evaluate neonate dispersal off a host plant, neonates remained more often on herbivore-induced plants than undamaged plants, mechanically-damaged plants, freshly-damaged plants, or plants damaged by FAW lacking inceptins. Further investigations of neonate responses to plant odors with a four arm olfactometer demonstrated that neonate attraction to older damage odors was strongly dependent on the damaging larvae’s prior intake of plant material. Neonates were attracted to older odors from FAW-damaged plants over fresh FAW-damage or undamaged plants, provided that the damaging larvae had previously ingested leaves. Neonates preferred older damage odors from inceptin-treated or leaf-fed FAW-damaged plants over mechanically-damaged or undamaged plant odors. GC-MS analysis indicated that [E]-4,8-dimethyl-1,3,7-nonatriene (DMNT) was the major volatile induced by neonate feeding 4 hrs after initial damage. DMNT was evaluated as an individual volatile and a supplemental component of undamaged plant odors. While both DMNT and undamaged plant odors were more attractive than an air blank, neonates preferred DMNT-supplemented plant odors. These results suggest that there are minor volatile components in cowpea leaves in addition to DMNT that are attractive to neonate FAW.