Tricky trichomes: Chemical defense in geranium and counteradaptations by soybean loopers

Novel interactions are occurring throughout our world as we transport organisms for economic and aesthetic purposes. These novel interactions can reveal evolved plant defenses and insect counteradaptations that may otherwise be hidden by prolonged periods of coevolution. In North America, the African-derived geranium species Pelargonium hortorum is now widely planted in flower bed and gardens leading to novel interactions with American herbivores such as the soybean looper, Chrysodeixis includens, which is found only in the New World. Curiously, final instar soybean loopers often sever leaf veins before feeding beyond the cuts, a behavior that is normally exhibited on plants with canal-borne exudates such as latex; this behavior thereby reduces the outflow of poisonous and/or sticky exudate at the looper's distal feeding site. Surprisingly, soybean loopers not only sever veins of plants with exudates, but also cut the veins of geranium, which lack canal-borne exudates. The goal of this study is to utilize this novel interaction to explore geranium defenses and soybean looper adaptations in a new light.;To explore this interaction between soybean loopers and geranium, 120 potted P. hortorum plants were grown in University of Central Arkansas greenhouses from cuttings obtained from the Ohio Germ Plasm Institute. Additionally, plantain (Plantago lanceolata) plants were established from field-collected seeds in the greenhouses to serve as food sources and control plants. A soybean looper colony was established by collecting adult moths from Conway and Greenbrier, Arkansas.;To determine if soybean loopers will oviposit on geranium and if the eggs will survive, fertile adult C. includens were enclosed on either geranium or on plantain controls. Significantly fewer eggs hatched when laid on geranium compared to plantain. To test whether exudate from tall glandular trichomes found on the geranium leaves was responsible for this low egg survival, eggs were treated with exudate from a single glandular trichome. Significantly fewer eggs hatched when treated with exudate compared to untreated eggs. The exudate was extremely toxic to the eggs, thus providing geranium with a potent defense that prevents egg hatching and larval establishment.;To explore the effects of geranium defenses on early instar soybean loopers, caterpillar survival was observed on intact and excised leaves. First instar caterpillars mostly died within 24 hours when enclosed on either intact or excised geranium leaves. However, survivorship increased substantially when the excised leaves were rinsed with ethanol, thereby removing the orange exudate from the tall glandular trichomes. Reapplication of concentrated ethanol leaf rinses to the dorsal surface of second instar caterpillars resulted in significantly higher mortality than in caterpillars treated only with ethanol solvent. To test if the glandular trichomes were specifically responsible for the high mortality, caterpillars were tested on leaf sites where individual or multiple trichome types were removed. Removal of both tall and short glandular trichomes from the leaf surface was required to deactivate geranium's defense against early instars.;Final instar soybean loopers were tested in laboratory assays to determine if a particular geranium chemical induces vein cutting behavior. Final instar soybean loopers did not exhibit vein cutting when tested with three defensive chemicals found in geranium: quisqualic acid, a neurotoxin that mimics L-glutamic acid and causes paralysis in Japanese beetles; geraniol, an essential oil associated with insect deterrence; and anacardic acid, a long chain phenol found in the glandular trichomes. However, application of pure exudate from tall glandular trichomes did trigger vein cutting, thus documenting for the first time a chemical trigger for vein cutting in a plant that lacks canal-borne exudates.;Exudate from glandular trichomes thus affects three stages of soybean loopers. The exudate kills both eggs and early instar larvae and triggers vein cutting by the final instars. Both the orange tall glandular trichome and clear short glandular trichomes serve as a defense. Soybean loopers feed on diverse plant families, including Brassicaceae, Convolvulaceae, Fabaceae, Malvaceae, and Solanaceae; however, the caterpillars are poorly adapted to attack geranium, which has been recently introduced from Africa. New results from this thesis include the discovery that trichome exudate can trigger vein-cutting, just like exudates stored in secretory canals.