Collateral advantage: Ingestion of foliar bacteria augments herbivore tolerance of plant defense chemicals

Knowledge of microbial communities, and how they influence ecological processes, has increased dramatically over the past decade.  Insects’ symbiotic bacteria have been shown to degrade lignocellulose, improve nitrogen utilization, and defend against enemies.  Most studies have focused on vertically transmitted microbiota. We have less understanding of how ingested bacteria influence herbivores, and how these effects may arise from attributes bacteria employ to associate with foliage. We investigated how bacterial communities in aspen foliage influence gypsy moth’s (Lymantria dispar) ability to tolerate this plant’s defense compounds. We inoculated L3 larvae with enrichment cultures obtained from aspen leaves, or from midguts of larvae fed non-aspen diet.  In the absence of phenolic glycosides, aspen‘s predominant defensive chemicals, no bacteria affected larval performance relative to controls not administered bacteria.  When 3% and 6% w/v phenolic glycosides were administered in artificial diet, bacterial enrichments from aspen leaves enhanced larval growth relative to controls, but enrichments from guts had no effect. Increased growth by larvae fed bacteria was not accompanied by increased consumption. This suggests that ingested bacteria may contribute to detoxification of phenolic glycosides. Using HPLC, we found that bacteria from aspen foliar communities substantially degrade all three of aspen’s major phenolic glycosides, salicin, salicortin, and tremulacin.  Thus, gypsy moth may be an indirect beneficiary of metabolic pathways that bacteria employ to tolerate plant anti-herbivore and anti-microbial defenses.