Biological fitness of an oligophagous herbivore Chrysomela populi on two different poplars.

Elucidating the mechanism by which oligophagous herbivores adapt to host plants is crucial in determining the co-evolution and interactions between insects and plants. Throughout a co-evolutionary process, the oligophagous herbivore Chrysomela populi L. relied on Populus trees to develop and form specialized larval glandular chemical defenses. However, the current understanding of the growth, development, and chemical defense of C. populi on different Populus trees remains insufficient. Here, the effects of Populus canadensis and Populus tomentosa on the development and chemical defense of C. populi were examined. The larvae develop well on P. canadensis achieving higher pupation and improved survival. In contrast, they only survive up to 2nd instar after feeding on P. tomentosa. Meanwhile, the synthetic salicylaldehyde content in C. populi larvae after feeding on P. canadensis leaves was higher than after feeding on P. tomentosa. Finally, transcriptome sequencing found insufficient synthesis but excessive metabolism of trehalose in C. populi larvae after feeding on P. tomentosa. After analyzing the genes involved in salicylaldehyde synthesis in the larvae, the CpMRP gene was upregulated when C. populi fed on P. tomentosa, while the β-glucosidase and salicyl alcohol oxidase (SAO) were not upregulated. The results preliminarily revealed the divergent response and possible molecular basis underlying the biological fitness and chemical defense of C. populi to different poplars varieties and also provided a multidimensional perspective to understand the interaction between Chrysomela leaf beetles and salicin-containing plants.