Fine Morphology of Antennal and Ovipositor Sensory Structures of the Gall Chestnut Wasp, Dryocosmus kuriphilus

SIMPLE SUMMARY: Hymenoptera encompass a large group of insects with different habits, ranging from phytophagy to parasitic/predatory lifestyles. This is also true in the superfamily Cynipoidea, where phytophagy becomes highly specialized towards the exploitation of specific plant tissues (i.e., buds), leading to the induction of galls. In this paper, we investigated the organization of antennal and ovipositor sensory structures in the chestnut gall wasp, Dryocosmus kuriphilus. This insect became a major threat to chestnut production in Italy in the last 15 years. We investigated only females (this is a parthenogenetic species with thelytoky), and on the antennae we found several sensilla with the clear functional specialization to different groups of stimuli, with almost no overlapping among each sensilla. Similarly, specialization was also found on the ovipositor where groups of gustatory and mechanoreceptive sensilla were observed. This information represents an advancement in the knowledge of this pest, which may be useful to understand the biological role of plant derived chemical cues or to implement new control methods. ABSTRACT: Dryocosmus kuriphilus is a gall-inducing insect, which can cause significant damage on plants of the genus Castanea Mill., 1754. Antennae and ovipositor are the main sensory organs involved in the location of suitable oviposition sites. Antennal sensilla are involved in the host plant location, while ovipositor sensilla assess the suitability of the ovipositional bud. On both organs, diverse sensillar organs are present. Here, the distribution and ultrastructural organization of the sensilla were investigated by scanning and transmission electron microscopy. The antennae of D. kuriphilus are filiform and composed of 14 antennomeres, with the distal flagellomere bearing the highest number of sensilla. On the antennae, 6 sensilla types were found; sensilla chaetica, campaniformia, coeloconica-I, coeloconica-II, trichoidea and placoidea. The sensilla placoidea and trichoidea were the most abundant types. On the external walls of the ovipositor, gustatory and mechanoreceptive sensilla were observed. Internally, the egg channel hosted two additional sensory structures. The putative functional role of each sensilla in the context of insect’s ecology is discussed as well as the ovipositional mechanism used by this insect.