Histological changes induced by the cynipid wasp Dryocosmus kuriphilus (Hymenoptera: Cynipidae) in leaves of the chestnut Castanea sativa (Fagaceae): Mechanisms of galling impact on host vigor

Abstract

Dryocosmus kuriphilus (Hymenoptera: Cynipidae), the Asian chestnut gall wasp (ACGW), is an invasive pest that threatens native stands and orchards of European Sweet Chestnut (Castanea sativa Mill.). ACGW induces galls in stems, petioles, and midribs. These galls cause inhibition of tree growth and fruit production. An understanding of morphogenetic changes in host organs is important to evaluate how plant resources are redirected to galls. Structural divergences in C. sativa petioles, midribs, and respective galls were investigated. Larvae of D. kuriphilus are found in the central region of young petioles and midribs in the spring. They are positioned in the pith region of petioles and midribs, surrounded by vascular tissues. The increase in cell layers and volume is evident in the ground tissues of galls, i.e., parenchyma, collenchyma, and sclerenchyma that originate from ground meristem. Gall formation causes the separation of the original vascular system into several collateral and amphicribral vascular bundles. The vascular web branching likely favors the redirection of resources from developing leaf blades to the galls by compensatory hydraulic mechanisms. The rapid growth rates of galls are likely supported by an increased water supply to gall sites. Cytoplasmically dense and metabolically active nutritive linings of the larval chambers are the sole source of food for larvae. Nutritive cells are maintained by a rich vascular supply. The redifferentiation of mechanical tissue surrounding the nutritive tissue also requires energy and protects the D. kuriphilus larva until pupation. These vascular alterations impact the normal formation of tissues in distal regions of the leaf, which reduces the productivity of chestnut trees.