Specificity of induction responses in Sinapis alba L. and their effects on a specialist herbivore

The glucosinolate-myrosinase system of Brassicaceae is known to hold a defensive function in both a constitutive and an inducible fashion. Glucosinolates are sulfur- and nitrogen-containing metabolites that are hydrolyzed upon tissue disruption by myrosinase enzymes. The resulting products are toxic for most herbivores. Nevertheless, some insects evolved detoxification mechanisms that enable them to feed exclusively on Brassicaceae. Induction of plant chemical defenses that deter or poison generalists might be ineffective against adapted specialists. We investigated the specificity of short-term induction patterns of chemical defenses in Sinapis alba damaged by a glucosinolate-sequestering specialist herbivore (turnip sawfly, Athalia rosae), a generalist herbivore (fall armyworm, Spodoptera frugiperda), or mechanical wounding (cork borer), and their effects on the behavior of A. rosae. After 24 hr of damage to young leaves, local as well as systemic changes in glucosinolate and myrosinase levels were analyzed. The intensity of the resulting changes was highest in damaged leaves. Induction responses in S. alba were dependent upon the attacking herbivore and were distinct from a mere wound response. Specialist feeding and mechanical wounding evoked up to threefold increases in levels of both parts of the glucosinolate-myrosinase system, whereas generalist feeding induced up to twofold increases in glucosinolate levels only. The majority of constitutive and induced myrosinase activity was found in the insoluble fractions. Possible consequences for the plant-specialist interaction were examined in behavioral tests with larvae and adult females of A. rosae on induced S. alba plants. Larval feeding and adult oviposition patterns were not modulated in relation to plant treatment. Thus, specificity was found in S. alba responses in relation to the inducing agent, but it was not present in return in the effects on the behavior of an adapted herbivore.