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Abstract:
The survival of insect herbivores on chemically defended plants may often depend on their
ability to metabolize these defense compounds. However, only little knowledge is available on
how insects actually process most plant defense compounds. We investigated the metabolism of
salicinoids, a major group of phenolic glycosides in poplar and willow species, by a generalist
herbivore, the gypsy moth (Lymantria dispar). Seven salicinoid metabolites identified in gypsy
moth caterpillar feces were mostly conjugates with glucose, cysteine or glycine. Two of the
glucosides were phosphorylated, a feature not previously reported for insect metabolites of plant
defense compounds. The origins of these metabolites were traced to specific moieties of three
major poplar salicinoids ingested, salicin, salicortin and tremulacin. Based on the observed
metabolite patterns we were able to deduce the initial steps of salicinoid breakdown in L. dispar
guts, which involves cleavage of ester bonds. The conjugated molecules were effectively
eliminated within 24 h after ingestion. Some of the initial breakdown products (salicin and
catechol) demonstrated negative effects on insect growth and survival in bioassays on artificial
diets. Gypsy moth caterpillars with prior feeding experience on salicinoid-containing poplar
foliage converted salicinoids to the identified metabolites more efficiently than caterpillars prefed
an artificial diet. The majority of the metabolites we identified were also produced by other
common poplar-feeding insects. The conversion of plant defenses like salicinoids to a variety of
water-soluble sugar, phosphate and amino acid conjugates and their subsequent excretion fits the general detoxification strategy found in insect herbivores and other animals.
