Latitudinal variation in constitutive chemical defense compounds in two host plants of Lymantria dispar (Lymantriidae): Betula pendula (Betulaceae) and Larix sibirica (Pinaceae)

Dynamics of changes in Earth's climate cause movement of ecotones in the northern direction. Phytophagous insects often expand ahead of populations of their host plants. Due to this range expansion, phytophagous insects will face new populations/species of host plants, which may differ in constitutive chemical defense compounds affecting the insects' growth and development. In this paper, we studied the distribution of potential determinants of toxicity among different populations of host plants of Lymantria dispar: one of the most widespread invasive forest pests in the Holarctic region. We focused on Betula pendula and Larix sibirica populations in Western Siberia, using a 1000-km-long north–south transect covering both L. dispar's historical geographic range and recent natural distribution. We analyzed fatty acids, diterpenoids, β-sitosterol, alcohol nonacosan-10-ol, and nonacosan-10-on in L. sibirica spring needles. We also assayed triterpenoids in B. pendula spring leaves at the same phenological stage, considering these compounds potential bottom-up regulators of pest expansion. We found a significant decrease in concentrations of dehydroabietic acid and nonacosan-10-ol (antifeedant compounds for L. dispar), from south to north in spring needles of L. sibirica. Additionally, we observed a significant increase of fatty-acid levels in these needles along the same south-to-north gradient. There was a nonlinear pattern of the distribution of triterpenoids along the latitudinal gradient in the spring leaves of B. pendula populations. Studies on the variation of plant secondary metabolites — chemical defenses against herbivorous insects — have not been conducted for L. sibirica, and existing research on B. pendula did not include information on the phenological stages of leaf development, despite the known temporal dependence of chemical variability. Our current study fills these gaps. Our findings reveal that dehydroabietic acid and nonacosan-10-ol in L. sibirica may facilitate the northern expansion of the forest defoliator L. dispar and triterpenoids in B. pendula appear unlikely to impede this range shift.