From volatile to visual cues: host location and oviposition in tachinid parasitoids, with insights from Exorista larvarum (L.)

Tachinid flies (Diptera: Tachinidae) are important biological control agents of herbivorous insect pests, yet their oviposition strategies and host selection mechanisms remain far less understood than those of hymenopteran parasitoids. Tachinids use both indirect and direct strategies, with the indirect ones (such as oviposition close to hosts or on host food plants) being notably more common than in hymenopterans. Indirect strategies often rely on chemical and physical cues to locate host habitats or microhabitats and are associated with high fecundity due to increased egg mortality compared with direct strategies. When host location and acceptance are carried out by parasitoid first instar larvae, the signals involved in locating a suitable host are largely unknown. In contrast, direct strategies, which are prevalent among tachinids, involve precise oviposition on or inside the host and rely on a complex interaction of olfactory, visual, and tactile cues for host location and acceptance. Herbivore-induced plant volatiles (HIPVs), host frass, host pheromones and visual cues such as host movement play key roles in host location for direct-type species like Trichopoda pennipes (F.), a parasitoid of stink bugs, or Exorista larvarum (L.), a larval parasitoid of Lepidoptera. Exorista larvarum can be also reared in vitro, even eggs laid outside the host, but inducing oviposition on artificial substrates remains challenging. A better understanding of the sensory and behavioral mechanisms underlying oviposition could enhance the possibility of eliciting egg laying by E. larvarum on selected hosts and even artificial substrates, thus facilitating its rearing and ultimately making its exploitation as regulator of target insect pests more feasible and efficient.