Toxic geranium trichomes trigger vein cutting by soybean loopers, Chrysodeixis includens (Lepidoptera: Noctuidae)

Abstract

The Africa-derived ornamental geranium, Pelargonium × hortorum, is now widely planted in North America, leading to novel interactions with native herbivores such as the soybean looper, Chrysodeixis includens. Soybean looper eggs and larvae survive poorly on P. × hortorum. High mortality can be attributed specifically to glandular trichomes. Individual eggs treated with exudate from a single tall glandular trichome were significantly less likely to hatch than untreated eggs. Few early-instar larvae survived on either intact or excised leaves, but survival increased when trichome exudate was removed by rinsing leaves with ethanol or when the tall and short glandular trichomes were both plucked from the leaf surface. Surprisingly, final instar loopers often severed the veins of geranium leaves before feeding beyond the cuts, a behavior normally exhibited on plants with canal-borne exudates such as latex. To identify the cue that triggers vein cutting, loopers were tested with solutions of two known defensive compounds found in Pelargonium (l-quisqualic acid, 22:0 anacardic acid); both were inactive in eliciting vein cutting. However, exudate collected from the tall glandular trichomes triggered vein cutting, thus documenting for the first time the chemical stimulant for vein cutting in a plant species that lacks canal-borne exudates. Loopers severed leaf veins more frequently on plants previously fed upon indicating that the chemical trigger for vein cutting increases with damage. Vein cuts sever the major supply arteries in the leaf, thus potentially blocking inducible geranium responses to herbivory.