Blade-running: An efficient yet simple behavior to potentially combat summit disease in ants

doi:10.21203/rs.3.rs-8202305/v1

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Research Article

Blade-running: An efficient yet simple behavior to potentially combat summit disease in ants

https://doi.org/10.21203/rs.3.rs-8202305/v1

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Background

Social systems are attractive targets for parasites. Once infiltrated they are able to manipulate the host to contribute to their further dispersal. A wide array of parasites causes summit disease: driving their host up on elevated points on plants from where propagules are then dispersed. In ants, previous observations suggested the existence of a simple social prophylactic strategy that could help reduce the pathogen’s dispersal success through early corpse disposal e.g., in the case of summit-causing pathogenic Pandora fungus.

Results

We experimentally tested the efficiency of a simple prophylactic mechanisms in a large nest complex by modelling summit diseased ants with the use of fresh uninfected ant corpses and dummies fixed to grass blades. Indeed, ants discovered and disposed of corpses very efficiently, primarily of those close to the nest margin.

Conclusions

We argue that this behavior is not necessarily specific, but rather part of a general set of behaviors that could also be of use to fight other pathogens as well as those that cause summit disease.

corpse

extended phenotype

Formica exsecta

Pandora formicae

prophylaxis

social immunity

A social lifestyle is frequently associated with an increased level of parasitism [1–3]. Nevertheless, group-living results in novel social strategies that reduce the frequency and virulence of pathogens and parasites, known as social immunity [4]. Social insects like ants are extreme examples: they live in highly aggregated groups, which are composed of genetically highly related individuals and overlapping generations. Ants have developed a wide variety of defense strategies, both physiological and behavioral, individual and social alike: they produce antibacterial and fungicidal secretions, practice auto- and allogrooming, pathogen avoidance, nest hygiene, exclusion or emigration of infected individuals from colonies, and, in extreme cases, they even move the nest [5–13]. Highly complex polydomous systems containing tens or hundreds of interrelated nests [14–17] may face even greater exposure to parasites: since they involve food-source sharing, exchange of individuals and brood among nests [14, 18–24]. Among the diverse parasitic pressures ants face, fungal pathogens that manipulate host behavior are particularly notable [25–35]. In these systems, infected individuals exhibit the characteristic “summit disease” syndrome: moribund hosts climb to elevated structures such as leaves, grass blades, or twigs and anchor themselves with their legs or mandibles, after which the fungus emerges from the cadaver and releases spores, enabling efficient aerial dispersal over the susceptible host population below [25–35]. The summit disease syndrome is widespread across insect–pathogen systems, with similar behavioral manipulation reported in ants, grasshoppers, beetles, and flies infected by various Entomophthoralean fungi [25, 27, 35]. However, summit disease in eusocial insects presents a unique evolutionary context. While this extended phenotype facilitates pathogen dispersal, social insects possess sophisticated collective defences that may suppress or counteract such manipulations [4]. The rarity of summit disease in well-defended eusocial colonies—despite the presence of specialized pathogens—suggests that social immunity is particularly effective against parasites whose transmission depends on infected ants dying in predictable, elevated locations. This leads to the prediction that sanitary behaviors specifically targeting conspicuous, elevated corpses should be more developed in eusocial species than in solitary insects.

The specialized myrmecopathogenic fungus Pandora formicae (Humber & Bałazy) infects wood ants of the genus Formica, which form large supercolonies [25–29]. The fungus produces infective spores that attach to, germinate on, and penetrate the host cuticle, ultimately killing the worker [25, 29–31]. Infected individuals display the characteristic summit disease behavior (described above), after which P. formicae rapidly colonizes the cadaver: within hours rhizoids grow out of the ventral plates and attach the ant to the leaf even more strongly [29–30]. In one or two days, the fur-like fungus appears on the surface of the individual [25–26, 29, 32], and sporulates directly on the nest mound below [7, 25, 27, 31–32]. This efficient transmission mechanism would predict high fungal prevalence. Even though the fungus has been found in the largest known polydomous system in European Formica ants (F. exsecta), prevalence remains extremely low, rarely exceeding 1% of the nests, mostly with a single infected ant found at such nests [26]. The paradox of efficient transmission mechanisms yet extremely low prevalence (≤ 1%) might suggest that F. exsecta has evolved effective counter-strategies to limit fungal spread. Marikovsky [32], based on his field observations, proposed a hypothesis concerning the existence of a simple defense strategy: “Whenever they [the ants] discovered an infested ant, they painstakingly removed it from the grass.” Simple as it is, the efficiency of this sanitary behavior has never been tested experimentally in this host–parasite system.

To test this idea in F. exsecta, we directly assessed workers’ efficiency in discovering and removing visually conspicuous summit-diseased-like corpses and foreign objects. We attached dead F. exsecta foragers and artificial dummies to grass blades around active nests, allowing us to disentangle visual cues from chemical ones. This experimental design enabled us to determine whether visual stimuli alone could trigger removal behavior, thus contributing to the low prevalence of a fungal pathogen.

During the study period, only two Pandora-infected zombie ants were found at two separate nests out of the 512 verified mounds (0.39% population-level prevalence). In addition, we detected a single infected alate gyne (Fig. 1a and Fig. 1b), also fixed to a grass blade in an early necrotic stage. Although this was a rare observation, it highlights the possibility that sexuals can also succumb to P. formicae infection.

To evaluate how efficiently workers detect and remove summit-disease–like corpses and foreign objects, we pinned a standardized ant corpse (“nest corpse”) near the nest at a height typical summit-diseased individuals, and compared its discovery and removal with that of an identical corpse positioned 0.5 m away (“distant corpse”) and an ant-sized dummy at the nest edge. We monitored discovery and removal during a 2-h observation period and checked their status again over the following two days. We first assessed ant activity on grass blades surrounding nests (mean = 2.55, SE ± 2.13), which was significantly predicted by nest size (LMM χ² = 8.48, p = 0.003): larger nests exhibited greater activity on grass blades. Ants discovered corpses rapidly. At the first observation point (15 minutes after placement), ants had already discovered 20 nest-corpses (25%, of which four were removed), 10 distant corpses (12.5%, one removed), and seven dummies (8.75%, one removed). By the end of the 2-hour observation period (eight observations at 15-minute intervals), ants had discovered the majority of nest-corpses (58.75%), but fewer than half of the distant corpses (35%) and dummies (25%) (Fig. 2a). Nest-corpses were discovered significantly faster than both dummies (z = 4.34, p < 0.001) and distant corpses (z = 3.21, p < 0.001, Fig. 2a), with no difference between the latter two (z = 1.34, p = 0.37; Cox regression: χ² = 31.19, p < 0.01). Discovery rate increased with worker activity on grass blades (z = 2.56, p < 0.01) but was independent of nest size (z = -0.13, p = 0.89).

Figure 1. Corpse discovery and removal. (a) Estimated survival (hazard) functions from the Cox regression model describing the number of observations until discovery for the two corpse categories and the dummy within 2 hours. (b) Estimated survival (hazard) functions from the Cox regression model describing the number of observations until removal for the two corpse categories and the dummy within 2 hours. Shaded areas represent 95% confidence intervals around the corresponding functions.

Ant workers not only discovered but also removed a large proportion of nest-corpses. By the end of the 2-hour observation period, nest-corpses were removed at a significantly higher rate than dummies (z = 3.1, p = 0.005, Fig. 2b) but did not differ from distant corpses (z = 1.9, p = 0.13), with no difference between distant corpses and dummies (z = 1.62, p = 0.22; Cox regression: χ² = 15.1, p < 0.001). Removal rate was independent of both nest size (z = -0.80, p = 0.42) and worker activity on grass blades (z = 1.51, p = 0.13). After one day, the vast majority of nest-corpses had been removed (77.5%), followed by distant corpses (60%), while only 12.5% of dummies were gone. By days three and four, removal was nearly complete: 92.5% of nest-corpses, 85% of distant corpses, and 32.5% of dummies had been removed.

Our follow-up experiment addressed whether corpse removal from grass blades represents a specific adaptation to summit disease or general necrophoric behavior. Comparing discovery and removal of corpses placed on grass blades versus directly on the ground over a 90-minute observation period (six 1-minute scans per nest), we found that discovery probabilities differed significantly among treatments (χ² = 37.08, p < 0.001). Corpses on the ground were most likely to be discovered (90%), dummies on the ground had intermediate discovery rates (50%), while corpses and dummies on grass were relatively rarely discovered (15% and 5%, respectively). Similarly, removal probabilities also differed significantly across treatments (χ² = 36.8, p < 0.001). Pairwise comparisons showed that corpses placed on the ground were removed more frequently than all other treatments (p ≤ 0.001), while removal of dummies on grass was lowest and did not differ significantly from dummies on the ground or corpses on grass. Overall, removal was most frequent for corpses on the ground (75%), intermediate for corpses on grass (10%) and for dummies on the ground (15%), while no dummies placed on grass were removed.

Our findings provide the first experimental evidence that F. exsecta workers dispose of corpses appearing on grass blades mostly near the nest mound supporting Marikovsky’s [32] field observations. The efficiency of this sanitary behavior becomes evident when considering the timing of fungal development. P. formicae requires 2–4 days to produce infectious conidia under natural conditions [25, 32], creating a critical window for intervention. Our results show that 92.5% of nest-corpses were removed within 3–4 days, predominantly within the first 24 hours (77.5%). By removing infected corpses before sporulation, workers could eliminate the primary source of transmission — the release of millions of conidia directly onto nest mounds below [7, 25, 27, 31–32]. This rapid response, combined with the spatial pattern of more efficient removal near nest margins, where infected ants typically attach themselves [25–26], provides a mechanistic explanation for how F. exsecta maintains infection prevalence below 1% despite the fungus's presence in the system. Among ants, corpses are important information carriers that trigger general prophylactic behaviors such as aggression or corpse removal [36–38]. A widespread strategy [37, 39–40] in social insects is the relocation of dead or diseased individuals to refuse piles, since corpses can be potential carriers of infection [38, 41–43]. However, the specific sanitary response to summit disease syndrome — where corpses are attached to elevated vegetation near nests—had never been experimentally tested, despite its potential importance for understanding low parasite prevalence in natural populations.

Our follow up experiment addresses whether corpse removal from grass blades represents a specific adaptation to summit disease or general necrophoric behavior. As expected, ground corpses were discovered (90%) and removed (75%) more frequently than grass corpses (15% and 10%, respectively), reflecting higher encounter probability during routine foraging. Nevertheless, several patterns point to a possible functional relevance of corpse removal on vegetation: (1) despite low discovery rates, workers still removed a substantial proportion of nest-corpses from grass blades (77.5%) within 24 hours, which falls within the critical pre-sporulation period observed in the first experiment. This suggests that elevated corpses can still be intercepted in time. (2) In the follow up experiment, grass-placed corpses elicited some removal (10%), whereas dummies did not (0%), indicating that ants distinguished actual corpses from non-biological objects even when both were elevated. (3) Removal was always more efficient close to nest margins, a pattern consistent across both experiments. Because naturally infected ants typically attach in these peripheral zones [25–26], this may reflect greater patrol effort in areas of higher ecological relevance. Taken together, these observations suggest that efficient ground-level necrophoresis, combined with targeted patrol activity near nest edges, may suffice to intercept both early-stage infected individuals and those reaching elevated positions. Although this does not necessarily indicate a behavior specialized for summit-disease suppression, such a combination of responses could contribute to keeping P. formicae prevalence very low (< 1% in our study system) and may help mitigate parasites that transmit via externally attached corpses.

After discovering corpses, ants removed them quickly, with more active nests responding more promptly. The ultimate fate of removed corpses likely influences defense effectiveness. Several scenarios could reduce outbreak risk: (1) deposition in underground cemeteries [44], which may restrict transmission despite facilitating fungal development; (2) application of formic acid or contact with gland secretions that slow parasite development [45]; (3) dismemberment of corpses, which could stop or slow parasite development [32]; (4) consumption of cadavers [46–47], potentially enabling immune competence acquisition. However, once conidia develop, ants avoid handling infected corpses entirely [13], highlighting the importance of early removal.

An unexpected but noteworthy finding in our survey was the discovery of a single infected alate. Generally, infections of sexual forms are rare in social insects [48], yet they can have disproportionately large fitness consequences because they directly affect colony reproduction. While our data are insufficient to quantify the frequency of such infections, this observation raises intriguing questions about whether P. formicae can exploit dispersing individuals to spread between colonies or populations. Since alates leave the nest for mating flights, infection of these individuals could represent a highly effective transmission route for the parasite. Future studies should therefore consider targeted monitoring of sexuals during swarming periods, as this may reveal an overlooked aspect of summit disease dynamics in social insects.

A limitation is the low natural prevalence (only two infected workers among 512 nests). While this supports our hypothesis that F. exsecta effectively controls P. formicae, it prevented direct observation of responses to naturally infected, sporulating corpses. Our experimental approach using fresh uninfected corpses positioned to mimic summit disease therefore provides an indirect but controlled test of the sanitary behavior hypothesis. Future studies incorporating naturally infected corpses in high-prevalence populations would complement our findings. Taken together, our results demonstrate that simple behavioral adaptations can effectively counter sophisticated parasite strategies, contributing to the evolutionary arms race between social hosts and their pathogens.

The summit disease syndrome represents a remarkable example of parasite manipulation, with convergent evolution evident across diverse pathogen taxa including Pandora formicae, Ophiocordyceps species infecting tropical Camponotus ants [49–51], and Dicrocoelium fluke worms [52]. These similar phenologies probably represent convergent evolutionary responses to evade social handling of infected hosts by ant workers, which would considerably decrease the pathogen’s reproductive success unless the pathogen evolved mechanisms to drive newly infected hosts out of the reach of their nestmates [4, 53]. In F. exsecta, simple behavioral acts, such as cutting grass for nest cover, general practices of nest protection involving patrolling on grasses, or foraging on grass blades, could also serve as means of defense against pathogens. While the chances of escaping the ants’ defenses are higher for a parasite such as P. formicae in smaller nests and at bigger distances from the mound (as revealed by our study), the combination of these two factors might critically lower the transmission success of the fungus. We expect that other features of parasites, e.g., their seasonality, or the timing of propagule development within a day when ants could be less active [27], might serve to ensure its stability within a supercolony.

Our study system (3,347 nests across 22 ha) is the largest known European polydomous system of F. exsecta Nylander, 1846, located in a semi-wet meadow in Central Romania in the Eastern Carpathians [54]. This system also hosts the myrmecopathogenic fungus P. formicae (Entomophthoromycota, Fig. 1a and Fig. 1b) formerly reported by us as P. myrmecophaga [see 26], but renamed following the description of Małagočka et al. [27].

In a first experiment, we randomly selected 80 F. exsecta nests from the central part of the supercolony, maintaining a minimum distance of 3 m between neighboring nests. All nests were checked for infected ants before and during the experiments by carefully inspecting vegetation within 0.5 m of the nest margin. As nest mound size is a fair indicator of the number of F. exsecta ants residing within [55] we calculated the above-ground volume of each experimental nest mound [56]: V = 1/2 × π × r₁ × r₂ × h, where r₁ is the largest radius at the bottom, r₂ is perpendicular to r₁, and h is the height aboveground.

Ant corpses were obtained by collecting live, uninjured individuals from the nest surface two days before the experiments. Ants were placed in small vials, where they died within hours. To ensure that no individuals were initially infected by the fungus, fresh corpses were placed on moist cotton in vials and kept in a cool, dark room for two days until the experiment. This method promotes fungal growth [25], allowing us to exclude ants that were already infected. None of the corpses showed infection. To avoid potential nest-specific effects of chemical cues, experimental corpses were returned to their nest of origin. The experiments were conducted on the 14th and 17th of August 2012, with 40 nests tested per day. Additionally, 432 nests were checked for fungal prevalence during the study period. All dead ants attached to grass blades were checked for fungal infection as described above.

To mimic the appearance of fungus-killed ants, we fixed a single experimental corpse (hereafter ‘nest corpse’) to a Festuca pratensis grass blade using a minutia pin through the thorax, at ~ 8 cm height and ~ 1 cm from the nest mound edge—consistent with the typical position of infected individuals [25–26]. The exact location of the grass blade was chosen randomly. Formica exsecta ants are active across the nest surface and nearby vegetation. To test the effect of distance, another corpse (‘distant corpse’) was pinned in the same way 0.5 m away from the nest mound along the same axis. In addition, to test the ants’ reaction to a non-biological object, a white ant-sized dummy (constructed from three small polystyrene balls) was pinned to a grass blade ~ 1 cm from the nest mound edge, on the opposite side of the nest. We used only a ‘nest dummy’ (and not a ‘distant dummy’), as our aim was to assess whether ants would treat a non-corpse object as foreign material to be removed from the immediate nest surroundings, rather than to compare it with corpses across distances. Corpses and dummies were placed out 10 min before the first observation in the afternoon, when F. exsecta activity is higher (between 4 and 6 PM). At each nest, we performed eight 1-min scan observations, separated by 15-min intervals. Four observers simultaneously monitored 10 nests per day. We recorded the number of ants active on the vegetation surrounding each nest. Discovery was defined as the first physical contact recorded during these scans. Because observations were conducted every 15 minutes, some first contacts may have occurred between scans. However, this approach provides a conservative and standardized measure of detection probability across treatments, ensuring comparability between nest-corpses, distant corpses, and dummies. Disposal was defined as the complete removal of the corpse from the pin, and the time to disposal was recorded. After the observation session, any remaining corpse or dummy was left in place. Their status was checked the following day at 4 PM, and again on day 3 (for the first group of nests) and day 4 (for the second group).

In a follow-up experiment conducted in August 2025, we compared discovery and removal rates between corpses/dummies placed on grass blades versus directly on the ground to test whether corpse removal from elevated positions (grass blades) reflects a specific adaptive response to summit disease rather than general necrophoric behavior. We randomly selected 20 F. exsecta nests from the same population as in the 1st experiment and assigned them to four treatments (n = 5 nests per treatment): (1) corpse on a grass blade ~ 1 cm from the nest edge, (2) corpse on the ground ~ 1 cm from the nest edge, (3) dummy on a grass blade ~ 1 cm from the nest edge, and (4) dummy on the ground ~ 1 cm from the nest edge. All corpses were prepared as described above. Observation protocols followed those of the first experiment, except that in this case, we performed six 1-minute scans over 90 minutes for each nest. Two observers monitored 10 nests simultaneously.

Statistical analyses

The relationship between worker activity on grass blades and nest size was assessed using a Linear Mixed Model (LMM, N = 640 obs.) approach. Nest size was introduced as an independent variable, while observation series and nest code were applied as random factors. Differences in corpse/dummy discovery rates were analyzed with the help of a Cox regression model (proportional hazard approach, N = 240 corpses). The type of corpse, the mound size and the average number of ants active on grass blades were applied as independent variables. Nest code was included as a random factor in order to handle dependencies. A similar approach was applied for the analysis of corpse removal rates. In the follow-up experiment, differences in the probability of corpse discovery and removal among treatments (corpse on the ground, corpse on the grass, dummy on the ground and dunny on the grass, N = 80 corpses/dummies) were assessed using Fisher’s exact tests due to small expected counts in some categories. Then, pairwise comparisons of proportions were performed using pairwise.prop.test with Benjamini–Hochberg correction to account for multiple testing [57]. Proportions of discovery and removal were calculated for each treatment and are reported as percentages. All statistical analyses were carried out using R 4.1.0 (R Development Core Team 2021 [58]). LMM was performed using the lmer function in the lme4 package [59]. Cox regression analysis was performed with the coxme package [60]. Tukey’s HSD test was used to calculate post-hoc comparisons for each factor using the glht function in the multcomp package [61]. ggplot2 package [62] was used for graphs.

Ethics Approval and Consent to Participate

This work was conducted in accordance with the ASAB/ABS Guidelines for the use of animals in research. Ants are invertebrates, and more specifically the study subject Formica exsecta is not protected by any law, so there is not any requirement for special permissions for experimentation with. However, an ethical visa was obtained in accordance with the existing institutional regulations of the Babeș-Bolyai University. Ants were handled with extreme care during the experiments. When checking colonies for infected ants, or when mimicking the appearance of infected ant corpses by fixing a single experimental carcass on a grass blade we tried to minimize disturbance. Furthermore, the study was non-invasive and involved observation in the field where ant colonies were free to do their daily activity for example foraging without any disturbance.

Competing interest

The authors declare no competing interests.

Acknowledgments

We are greatly indebted to Zsolt Czekes, Réka Erös and Norbert Fákó for their assistance during our fieldwork, and also to the Apáthy István Society for providing housing. We are grateful for the help of Joanna Małagočka, Gyöngyi Szigeti, and János Varga with the identification of P. formicae, and for the assistance of László Gál with processing the photo.

Funding

E.C. was supported by the CNRS. During the 1^st experiment B.M. was supported by a grant from the Romanian National Authority for Scientific Research and Innovation, CNCS–UEFISCDI, project number PN-II-RU-TE-2014-4-1930, and by the Bolyai János scholarship of the Hungarian Academy of Sciences. For B.M. and Á.Sz. the follow-up experiment was supported by a grant of the Romanian Ministry of Research, Innovation and Digitization, CNCS/CCCDI - UEFISCDI, project number 25/2024 COFUND-BIODIVMON-MonitAnt, within PNCDI IV.

Authors contribution

E.Cs., K.E., L.R., and B.M. designed the experiments. K.E., E.C., Á.Sz., A.B., and B.M. conducted behavioral experiments. K.E., E.C., and M.B. performed data analyses of behavioral assays. The manuscript was written by B.M. and E.C. Further on, all authors contributed to the revisions.

Ethical Note

This work was conducted in accordance with the ASAB/ABS Guidelines for the use of animals in research. Ants are invertebrates, and more specifically the study subject Formica exsecta is not protected by any law, so there is not any requirement for special permissions for experimentation. However, an ethical visa was obtained in accordance with the existing institutional regulations of Babeș-Bolyai University. Ants were handled with extreme care during the experiments. When checking colonies for infected ants, or when mimicking the appearance of infected ant corpses by fixing a single experimental carcass on a grass blade we tried to minimize disturbance. Furthermore, the study was non-invasive and involved observation in the field where ant colonies were free to do their daily activity for example foraging without any disturbance.

Consent for publication

Not applicable.

Data Availability Statement

All data generated or analyzed during this study are included in this article as supplementary information files.

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No competing interests reported.

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Blade-running: An efficient yet simple behavior to potentially combat summit disease in ants

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