Are sawflies adapted to individual host trees? A test of the adaptive deme formation hypothesis

Question: Are populations of two sawfly species adapted to individual host trees? Hypothesis: The adaptive deme formation hypothesis suggests that selection may lead to populations (demes) adapted to host individuals. Organisms: Larvae of the black sawfly Tomostethus nigritus and the privet sawfly Macrophya punctumalbum (Hymenoptera: Tenthredinidae) living on ash (Fraxinus excelsior). Methods: We reciprocally transferred larvae of the two sawfly species to leaves of different ash individuals of a floodplain forest in Central Germany. After 24 h, we assessed the relative growth rate of the larvae. Conclusions: There was no effect of the transfer between host individuals on the relative growth rates of sawfly larvae. Based on our results, together with published literature, we conclude that under certain conditions local adaptations may fine-tune herbivore populations to individual hosts. However, the formation of adaptive demes does not appear to be a general phenomenon in herbivorous insects.