Some of the Laboulbeniales species associated with bat flies penetrate their hosts with haustoria, rhizoidal structures that make contact with the body cavity for nutrition and as a holdfast. Haustorial Laboulbeniales are those that have recently been referred to as the true biotrophic members of the order (Reboleira et al., 2021). Bat flies with haustorial Laboulbeniales are often deformed and their integument is severely blackened (due to melanization) at the site of infection (Jensen et al., 2019). The extent of damage to the hosts is largely unknown and probably varies among Laboulbeniales (Kaishian et al., 2023). However, Szentiványi et al. (2020) showed that Arthrorhynchus spp. reduced bat fly survival in Penicillidia conspicua bat flies.
Different studies point at very low parasite prevalences of bat flies with Laboulbeniales, ranging from 2.2% to 9.0% (Blackwell, 1980; Haelewaters et al., 2017a, 2018b; Szentiványi et al., 2018; Walker et al., 2018). Except some regional studies focusing on prevalence of parasites and one study reviewing tritrophic associations globally and analyzing host specificity patterns (de Groot et al., 2020), other aspects remain unstudied. Efforts are being made towards a global tritrophic traits database to study some of these aspects based on records resulting from standardized fieldwork (Haelewaters et al., 2021a). One question of interest is how environmental pressures such as changing landscapes and warming climate affect parasitism at these multiple levels.
Other examples of Laboulbeniales that have parasites as hosts are found in two other genera: Rickia on mites of ants, Salganea cockroaches, Nasutitermes termites, and beetles in different families; and Trenomyces on lice of birds, cows, foxes, and rats as well as on louse flies of primates (Lepilemur sp.). In addition, species of Dimeromyces are not only associated with bat flies, they are also found on mites of beetles in different families. It should be mentioned that it is not always clear whether these mite hosts are truly parasites or rather commensals in relation to the primary host. Finally, written notes by Jean Balazuc at the Muséum National d’Histoire Naturelle, Paris reveal an unpublished genus of Laboulbeniales from a human ectoparasite (the sucking louse Pediculus humanus, order Psocodea).