Record of an Ectoparasitic Fungus on Eggs of the Neotropical termite Nasutitermes corniger (Blattaria, Isoptera

Insects and fungi are abundant in many environments and often interact. However, little information exists on the fungal infestation of insect eggs. Here, we report an entomogenous fungus similar to Hormiscioideus filamentosus (Blackweel & Kimbrough, 1978) infesting eggs of the termite Nasutitermes corniger (Motschulsky, 1855). The fungus arises from egg chorion and has long and simple filaments. The small haustorial cells infiltrate the egg chorion and the fungal thalli number varies from two to ten. Future studies will elucidate the infestation levels of termite eggs by this entomogenous fungus in the Neotropical region and its effects on termite colonies.

According to Schmidt-Hempel (1998), the lack of information on the incidence of fungal infestations of eggs can reflect a lack of studies on this subject.
Different genera of ectoparasite fungi infesting termites have been reported from the following families: Mastotermitidae, Stolotermitidae, Kalotermitidae, Rhinotermitidae and Termitidae (Wilson et al., 2021), indicating that fungal infestations reach five families among the nine included in Isoptera (Engel et al., 2009).
A variety of microbes colonize termite colonies, including diverse pathogenic fungi (Lee & Wood, 1971;Blackwell & Rossi, 1986).Recent research showed that termite embryos have microbial protection owing to embryonic microbiomes and grooming by nestmates (Cole et al., 2020).Thus, according to this study, embryos are immature but not defenseless since they have extra-chorionic protection (grooming) combined with intra-chorionic defense (embryonic microbiome), which influences the fitness of the whole colony.However, some fungi breach this defense and infest the termite eggs, although their impact on egg survival has yet to be evaluated.The current scientific note describes an entomogenous fungus similar to Hormiscioideus filamentosus (Blackweel & Kimbrough, 1978) infesting eggs of the termite Nasutitermes corniger (Motschulsky, 1855).Nine genera of fungi are specialized for infesting termites, with the genus Antennopsis being the most common and the only one recorded as a termite egg parasite (Wilson et al., 2021).H. filamentosus has been described as infesting the termite Armitermes neotenicus (now named Embiratermes neotenicus) (Holmgren, 1906) from Brazil and is distinguished from other termite-infesting fungi by the combination of its basalcell arrangement, filament arrangement, and reproduction by filament fragmentation with internal proliferation (Blackwell & Kimbrough, 1978).
In the present research, eggs were obtained from an arboreal nest of N. corniger collected in a small area of natural vegetation.The nest was elliptical and had 11 imaginal physogastric queens and six imaginal kings.The eggs were fixed in FAA fluid (formol, alcohol, and acetic acid; 1: 3:1), cleaned with detergent in an ultrasound apparatus, dehydrated, and critical point dried in a CPD 030 Balzers desiccator.Later, they were sputtered with gold and examined under a Philips (SEM 505) scanning electron microscope.The results show a fungus with long and simple filaments arising from the egg chorion (Figs 1 and 2).The highest number of observed filaments was ten.For comparison, Myles et al. (1998)  Some eggs had two fungal thalli attached to a cellular mass with small haustorial cells infiltrating the egg chorion.Moreover, the basal cellular area presented a peculiar structure with a cushioned aspect.This morphological aspect is quite different from the basal cells of Antennopsis but very similar to the crustose mass or pulvinate structure described for the fungus H. filamentosus.Other characteristics of Antennopsis, such as the conidial heads, which are the terminal ends, are also absent.Moreover, fungi of the genus Termitaria and Mattirolella present a far more elaborate thallus.
H. filamentosus was recorded as infesting different termite castes of E. neotenicus, including soldiers, workers, and alate reproductives, but not eggs.Other fungi of the genus Hormiscioideus have been found in various insects, including beetles, ants, and flies (Blackwell & Kimbrough, 1978).2021), most ectoparasitic fungal infestations of termites cause low-level damage to their host.Therefore, light infestations are not detrimental to the termite colony because the molting rate is fast enough to ensure the low abundance of the fungus (Myles et al., 1998).Guswenrivo et al. (2018) reported that the infestation rate and thallus number per termite can measure the strength of a fungal infestation.However, it needs to be clarified what effect a given level of infestation has on the colony.N. corniger and N. ephratae infested by Mattirolella crustosa died rapidly compared with non-infested individuals (Thorne & Kimbrough, 1982).Similarly, termites infested by the ectoparasitic fungus Termitaria sp.tended to survive less than non-infested termites (Lenz & Kimbrough, 1982).The mortality of termites infested by ectoparasitic fungi might be due to a combination of fungal toxins, physical obstruction of hemolymph circulation, nutrient depletion, and invasion of organs (Goettel & Inglis, 1997).
It is known that grooming can reduce fungi infestation because it affects fungal growth (Cole et al., 2020).However, there is a lack of modern studies showing the pathogenic relationships between fungal ectoparasites and termite hosts (Wilson et al., 2021).Moreover, factors that affect grooming activity may allow the infestation of eggs by parasitic fungi.The ectoparasite fungus Antennopsis is known to interfere with swarming success (infested imaginal swarming termites perish) and infests eggs.Consequently, Wilson et al. (2021) comment that it can potentially be used as a microbial control agent for termites from small confined colonies.Future studies will elucidate the infestation levels of termite eggs by entomogenous fungi in the Neotropical region and their effects on the colonies.

Fig 1 .
Fig 1. Egg of Nasutitermes corniger infested by a fungus similar to Hormiscioideus filamentosus.a. Thalli (arrow) of the fungus attached to the micropilar region of the egg.b. and c.Detail of the six fungal thalli arising from micropilar process (arrowheads).D. Detail of the crustose mass (cm), which is a basal feature of this fungus similar to H. filamentosus.t = thallus.