To investigate the host-finding mechanisms of Spathius agrili Yang, a parasitoid of the emerald ash borer, Agrilus planipennis Fairmaire, a highly concealed insect pest of Fraxinus spp. The morphology and ultrastructure of sensilla on the antennae and legs of both sexes and on the ovipositors and ovipositor sheaths of females were observed using SEM. Results showed that abundant sensilla trichodea (ST) are distributed on the antennae of both females and males, with the sharp trichoid with a socket at the base being the most numerous sensillum. There are 6-7 sensilla placodea on each segment of the flagellum, arranged lengthways in parallel rows. There are very few sensilla styloconica on the female antennae. The tibiae have mostly sensilla chaetica, whereas three types of ST and the relatively plentiful sensilla chaetica are present on the tarsi. Besides ST, a mass of sensilla squamiformia and small short spines are distributed on the pulvilli. Basically there are no sensilla on the female ovipositor, only steering mechanisms for drilling bark and a variety of v-shaped grooves. However, a large numbers of short sensilla chaetica and microtrichia are found on the internal surface of ovipositor sheath. The outer surface of the ovipositor sheath also has ST. In addition, a clear spring-like extension structure can be observed on the fore parts of ovipositor sheath. These findings suggest that the main function of sensilla on the antennae is olfaction, while the sensilla on legs and ovipositor sheaths are mechanoreceptors and tactile sensors.
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