Making the cut: Behavioral adaptation by soybean looper to a novel geranium host

Herbivorous insects on plants that emit latex or other exudates commonly sever leaf veins before feeding, thereby reducing the outflow of poisonous, sticky exudate at their distal feeding site.  Surprisingly, soybean loopers (Chrysodeixis includens) not only sever veins of plants with exudates, but also cut veins of geranium, Pelargonium hortorum, which lacks canal-borne exudates. In laboratory assays, 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.  The trichomes were extremely toxic to soybean looper eggs.  Significantly fewer eggs hatched when treated with fluid from a single glandular trichome. First instar caterpillars mostly died within 24 hours when enclosed on intact or excised geranium leaves.  However, survivorship increased substantially when the excised leaves were rinsed with ethanol, thereby removing the orange exudate from trichomes. These results document the effectiveness of geranium chemical defenses against a generalist folivore.  It remains to be determined if vein cutting actually benefits soybean loopers on geranium or if it is simply an inappropriate response to a nonnative plant.