Insect herbivory, plant-host specialization and tissue partitioning on mid-Mesozoic broadleaved conifers of Northeastern China

Abstract

Insect-mediated damage was examined on 756 specimens of three broadleaved conifers Podozamites, Lindleycladus and Liaoningocladus, originating from five mid-Mesozoic localities in Northeastern China. These localities are the Late Triassic Yangcaogou Fm. (T3, ca. 205 Ma), the latest Middle Jurassic Jiulongshan Fm. (J2, 165 Ma), and the mid Early Cretaceous Yixian Fm. (K1, 125 Ma). Plant hosts from these three time intervals harbor five functional feeding groups (FFGs) of herbivores and 23 distinctive damage types (DTs), categorized using the widely applied DT system. The DTs were classified into the five FFGs of external foliage feeding (6 DTs), piercing and sucking (5 DTs), oviposition (3 DTs), galling (8 DTs) and leaf mining (1 DT). Damage-type richness and abundance was established for each FFG, encompassing from 10 to 16 DTs for each of the three time intervals examined. For this 80 million-year-long interval, foliar herbivory on broadleaved conifers was transformed from early predominance of external foliage feeding (T3), later replaced by an emphasis on piercing and sucking (J2), followed by bimodal expansion of endophytic interactions from oviposition and leaf mining (K1). This trajectory of herbivore succession indicates that, from T3 to K1, plant–insect associations were transformed from earlier reliance on a greater number of exophytic modes of herbivory to a later, increased variety in endophytic consumption. The transformation also was demonstrated by finer-grained partitioning of food resources and specialization on particular host-plant tissue types. This subdivision of tissue types likely promoted greater dietary saturation of tissue space by functional feeding groups. Possible explanations for these shifts in herbivory include ecological causes, long-term environmental changes, or both. Ecological factors, such as (i), evolution of a more differentiated plant-host spectrum available for consumption; (ii), long-term changes in plant physiognomy and deployment of antiherbivore defenses; (iii), change in herbivore partitioning of plant-host tissues; and (iv), emergence of the parasitoid guild for efficient regulation of insect herbivores. Long-term environmental variables may be linked to these shifts in insect herbivory style.