title: Herbivore-specific, density-dependent induction of plant volatiles: honest or "cry wolf" signals?
abstract: Plants release volatile chemicals upon attack by herbivorous arthropods. They do so commonly in a dose-dependent manner: the more herbivores, the more volatiles released. The volatiles attract predatory arthropods and the amount determines the probability of predator response. We show that seedlings of a cabbage variety (Brassica oleracea var. capitata, cv Shikidori) also show such a response to the density of cabbage white (Pieris rapae) larvae and attract more (naive) parasitoids (Cotesia glomerata) when there are more herbivores on the plant. However, when attacked by diamondback moth (Plutella xylostella) larvae, seedlings of the same variety (cv Shikidori) release volatiles, the total amount of which is high and constant and thus independent of caterpillar density, and naive parasitoids (Cotesia vestalis) of diamondback moth larvae fail to discriminate herbivore-rich from herbivore-poor plants. In contrast, seedlings of another cabbage variety of B. oleracea (var. acephala: kale) respond in a dose-dependent manner to the density of diamondback moth larvae and attract more parasitoids when there are more herbivores. Assuming these responses of the cabbage cultivars reflect behaviour of at least some genotypes of wild plants, we provide arguments why the behaviour of kale (B. oleracea var acephala) is best interpreted as an honest signaling strategy and that of cabbage cv Shikidori (B. oleracea var capitata) as a "cry wolf" signaling strategy, implying a conflict of interest between the plant and the enemies of its herbivores: the plant profits from being visited by the herbivore's enemies, but the latter would be better off by visiting other plants with more herbivores. If so, evolutionary theory on alarm signaling predicts consequences of major interest to students of plant protection, tritrophic systems and communication alike.
authors.affiliation: Center for Ecological Research, Kyoto University, Shiga, Japan.
authors.forename: Kaori
authors.initials: K
authors.lastname: Shiojiri
authors.forename: Rika
authors.initials: R
authors.lastname: Ozawa
authors.forename: Soichi
authors.initials: S
authors.lastname: Kugimiya
authors.forename: Masayoshi
authors.initials: M
authors.lastname: Uefune
authors.forename: Michiel
authors.initials: M
authors.lastname: van Wijk
authors.forename: Maurice W
authors.initials: MW
authors.lastname: Sabelis
authors.forename: Junji
authors.initials: J
authors.lastname: Takabayashi
mesh_terms: D000818:Animals; D001937:Brassica / Q000737:chemistry / Q000378:metabolism* / Q000502:physiology; D002080:Butterflies / Q000187:drug effects; D004924:Escape Reaction / Q000187:drug effects; D005260:Female; D007814:Larva / Q000187:drug effects; D008297:Male; D009036:Moths / Q000187:drug effects; D036226:Seedlings / Q000737:chemistry / Q000378:metabolism; D013045:Species Specificity; D055549:Volatile Organic Compounds / Q000032:analysis / Q000378:metabolism* / Q000494:pharmacology
chemical_list: D055549:Volatile Organic Compounds
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