Our findings demonstrate that F. exsecta workers dispose of corpses appearing on grass blades mostly near the nest mound as suggested by Marikovsky (1962). This simple defense mechanism could specifically lower the chances of P. formicae infection but could also reduce the prevalence of any other parasite causing summit disease in ants as eg. Dicrocoelium fluke worms.
Among ants, corpses are important information carriers. Workers can discriminate among corpses of different species, and their responses are consistent with the status of the ant species (Maák et al. 2014). Usually, the appearance of dead colony members represents a threat of infection, which triggers general prophylactic behavior, such as aggression or corpse removal (Soeprono and Rust 2004; Choe and Rust 2008; Diez et al. 2012; 2013). A widespread prophylactic strategy (Wilson et al. 1958; Diez et al. 2012; Qiu et al. 2015) in social insects is the relocation of dead or diseased individuals to external or internal refuse piles, since corpses can be potential carriers of infection (Howard and Tschinkel 1976; Soeprono and Rust 2004; Choe et al. 2009; Diez et al. 2013; Kesäniemi et al. 2019).
After discovering the corpses, ants removed them quickly. More active nests with more foragers on the grass blades, in general, reacted more promptly. This efficient reaction prevents the development of the conidia, which needs at least 2–4 days in natural conditions (Marikovsky 1962; Boer 2008), but in favorable conditions, with elevated temperatures and levels of moisture, the spores can develop even after one day (pers. obs.), which offers a narrow time window to the ants to hinder the appearance of conidia. After that any corpse handling would just increase the chance of contacting the infection.
It is unclear what happens with corpses that are carried away. There are several different scenarios, but all could serve the same purpose: reducing the chance of an outbreak. (1) Several specimens were seen taking bodies towards the nest, probably to deposit in underground cemeteries (Czechowski 1976), which, due to increased humidity, could facilitate the development of the fungus, but, on the other hand, may restrict its transmission. (2) In addition, carriers may apply formic acid to the corpses, and even contact with the mixture of gland secretions on the carriers’ bodies could at least slow down the development of conidia (Otti et al. 2014). (3) As also noticed by Marikovsky (1962), the practice of taking apart the bodies may also hinder the development of the fungus. (4) In some cases, ants are also known to eat ant corpses (Czechowski et al. 2008; Maák et al. 2020), which, in the case of corpses that have been infected, might enable the acquisition of appropriate immune-competence. All things considered, however, once it has developed, the fungus might escape the ants since as Marikovsky has observed (1962), ants don’t handle corpses with conidia. Our pilot experiments (Erős unpubl.) also confirm that ants do not react to corpses with conidia. It is possible that once the conidia appear, due to sudden changes on the level of Pandora (Małagočka et al. 2015), certain proteins are released by the fungus that repel ants or even make them neutral towards corpses by altering corpse specific clues on the surface of cadavers. On the other hand, contact with corpses covered by conidia would be dangerous to ants, as it would expose them to infection.