Host location behaviour of gravid females in the Mediterranean corn borer Sesamia nonagrioides: external morphology of antennae and ovipositor sensilla

The behaviour of gravid females of Sesamia nonagrioides (Lefebvre) during host plant location in a wind tunnel is described using an ethogram and compared with unmated females, males and gravid females deprived of antennae. The sequence of behaviours exhibited by the majority of the intact gravid females began with a resting period followed by the unfolding of the antennae towards the plant plume and wing buzzing. The females then started to walk while fanning their wings, displayed erratic flights striking the walls of the wind tunnel, and finally executed a zigzagging flight oriented towards the maize plant. The behavioural sequence was disrupted in the absence of both antennae. One third of the amputated females took flight; although the flights were not oriented, they attempted to fly towards the plant. The external morphology of the antennae’s sensilla in females and males and of the ovipositor was studied using scanning electron microscopy and staining with silver nitrate to detect the presence of pores. Males and females showed sexual dimorphism in length, size, shape, and the presence and distribution of sensilla along the antennae, which may be related to differences in the odor sources that each sex seeks. The distribution of sensilla chaetica and sensilla trichoidea showed a type of compartmentalization with structures for the detection of volatiles concentrated on the middle flagellomeres and structures for mechanoand gustative-reception concentrated on the distal flagellomeres. The only argyrophilic sensilla on the ovipositor were type I, located primarily on the ventral surface of the ovipositor to detect chemical stimuli on the substrate.