Complex plant quality - microbiota - population interactions modulate the response of a specialist herbivore to the defense of its host plant.

1. Many specialist herbivores have evolved strategies to cope with plant defenses, with gut microbiota potentially participating to such adaptations.

2. In this study we assessed whether the history of plant use (population origin) and microbiota may interact with plant defense adaptation.

3. We tested whether microbiota enhance the performance of Melitaea cinxia larvae on their host plant, Plantago lanceolata and increase their ability to cope the defensive compounds, iridoid glycosides (IGs).

3. The gut microbiota was significantly affected by both larval population origin and host plant IG level. Contrary to our prediction, impoverishing the microbiota with antibiotic treatment did not reduce larval performance.

5. As expected for this specialized insect herbivore, sequestration of one of IGs was higher in larvae fed with plants producing higher concentration of IGs. These larvae also showed metabolic signature of intoxication (i.e. decrease in Lysine levels). However, intoxication on highly defended plants was only observed when larvae with history of poorly defended plants were simultaneously treated with antibiotics.

6. Our results suggest that both adaptation and microbiota contribute to the metabolic response of herbivores to plant defense though complex interactions.