Insect parasitoids are shown to be important in maintaining biological diversity and preserving the ecological balance, and also have a major economic impact as the result of infection of beneficial insects (e.g., silkworms, honey bees) and their application in agricultural pest control (Clarke et al., 2019). The insect parasitoids include a vast number of species in Hymenoptera (60,000 described), Diptera (15,600 described), Coleoptera (1,600 described), Neuroptera (50 described) and few species in Lepidoptera, Trichoptera, Strepsiptera. The Hymenoptera and dipteran Tachinidae have the greatest species diversity, and are by far the two most important groups in agroecosystem pest management (Heraty et al., 2017). These parasitoids possess a great diversity in larval respiration; mobility, habit, and adult ovipositor, flight, host range and stages of attack between groups, thereby allowing them to adapt efficiently to different environments (Eggleton and Belshaw, 1993, Kathirithamby, 2009). Extensive literatures on the Hymenoptera have drawn a comprehensive picture of parasitoid research, however, parasitoids in different taxonomic status have a number of striking differences in biology and host relations, the associations between hosts and non-hymenopterans remain poorly defined.

The parasitoids usually rely on their host resources for survival and reproduction at a particular stage in their life cycle. They encounter markedly complex nutritional and immunological environments during the parasitic stage. Successful parasitoids have adapted strategies to evade or even inhibit key elements of host immune responses (da Cos et al., 2019). The pioneer studies on immunological interactions between hosts and parasitoids were focused on the hymenopterans. Generally, on the host’s side, once parasitoid larvae or eggs gain entry into the hemocoel of the host, they encounter innate defense mechanisms involving cellular and humoral responses. The cellular immune response involves hemocyte-mediated encapsulation that participates in parasitoids clearance (Clark, 2020). The humoral immune responses including melanization of hemolymph and Toll, IMD and Jak/Stat immune pathways associated with antimicrobial peptides (AMPs) production are activated to defend against infection of parasitoids (Yang et al., 2021). On the parasitoid’s side, to counteract host defense, the hymenopteran parasitoids produce venoms, virulence factors and saliva secretions which are inoculated into the host hemocoel during deposition of their eggs and larval development (Yang et al., 2021, Pang et al., 2023, Shi et al., 2022). Referring to the non-hymenopterans, for example, the larval parasitoid Drino inconspicuoides (Diptera: Tachinidae) can subvert the host cell coverage used in the encapsulation to build a respiratory funnel where the postabdominal spiracles of larval parasitoids connect in communication with the external air (Zhang et al., 2023). The larval Stichotrema dallatorreanum (Strepsiptera: Myrmecolacidae) can avoid host cellular immunity via establishment of a host-derived epidermal bag where it completes transition from 1st to 2nd instar (Kathirithamby, 2009). Therefore, the immunomodulatory mechanisms used by different parasitoid groups might appear quite different. Nevertheless, all parasitoid species have to overcome the same host immune mechanisms, it is therefore illustrative to see how they have developed parallel strategies to neutralize host immunity.

The Toll and IMD pathways are the regulators of immune responses in insects. The inducible expression of AMPs that fight against bacteria, fungi and viruses depends on the Toll and IMD pathways (Tanji and Ip, 2005). The Toll pathway is activated following infection by fungi and bacteria containing Lysine-type peptidoglycan (PGN) in their cell wall through the transcription factor Dif and Dorsal (Valanne et al., 2011). In contrast, the IMD pathway is triggered by bacteria containing diaminopimelic acid PGN through the transcription factor Relish and NF-κB (Myllymäki et al., 2014). In addition, the Imd and Toll pathways mediate an antiviral response to replication of multiple viruses such as sindbis virus, Drosophila X virus. Loss of function of the two pathways results in decreased viability of infected insects (Chow and Kagan, 2018). Although substantial knowledge has accumulated the Toll and IMD pathway mediated antibacterial; antifungal and antiviral function, far less is known regarding their activities against insect parasitoids. These two pathways act in host resistance to parasitism of the hymenopteran parasitoids, while the parasitoids can produce polydnavirus and venoms encoded IκB-like proteins that interfere with Toll/IMD pathways by blocking NF-κB-dependent transcription of AMPs production in the host (Yang et al., 2021, Bitra et al., 2012, Dostálová et al., 2017, Fang et al., 2016). We have previously determined that several AMPs were persistently induced in B. mori after parasitism of the dipteran parasitoid Exorista japonica (Dai et al., 2022). Given the profound differences between the hymenopteran and dipteran parasitoids (Dindo et al., 2014), whether Toll/IMD pathways are stimulated by the dipteran parasitoids remains unclear.

The dipteran parasitoids, despite their lower number of parasitoid families, attack a taxonomically broader range of hosts than Hymenoptera. In comparison with the Hymenoptera, the strong larval mobility and adult flying ability enable the Diptera to attack the more mobile hosts (Eggleton and Belshaw, 1993). Although hymenopterans are widely applied in biological control programs, the dipteran parasitoids are gaining importance globally as promising biological control agents. The two most studied dipteran parasitoids E. japonica and E. sorbillans are also natural enemies of economically important lepidopteran insects such as the silkworm Bombyx mori, tasar silkworm Antheraea mylitta, eri silkworm Samia cynthia ricini, and muga silkworm Antheraea assamensis, and cause economic loss, such incidence, in turn, invites ecofriendly strategies to control them (Khanikor et al., 2022). In this study, we compared the performance of the two parasitoid species on the host B. mori, and found that Toll and IMD pathways are involved in host defense against the parasitoids. Our study will result in a substantial broadening of the basic theory of parasitology, and illuminate potential points for enhanced biological control with dipteran parasitoids or development of control measures against them in sericulture.