A fungus infected environment does not alter the behaviour of foraging ants

Abstract

Eusocial insects are exposed to a wide range of pathogens while foraging outside their nest. We know that opportunistic scavenging ants are able to assess the sanitary state of food and to discriminate a prey which died from infection by the entomopathogenic fungus Metarhizium brunneum. Here, we investigate whether a contamination of the environment can also influence the behaviour of foragers, both at the individual and collective level. In a Y-maze, Myrmica rubra ants had the choice to forage on two prey patches, one of which containing sporulating items. Unexpectedly, the nearby presence of sporulating bodies did not deter foragers nor prevent them from retrieving palatable prey. Ant colonies exploited both prey patches equally, without further mortality resulting from foraging on the contaminated area. Thus, a contamination of the environment did not prompt an active avoidance by foragers of which the activity depended primarily on the food characteristics. Generalist entomopathogenic fungi such as M. brunneum in the area around the nest appear more to be of a nuisance to ant foragers than a major selective force driving them to adopt avoidance strategies. We discuss the cost-benefit balance derived from the fine-tuning of strategies of pathogen avoidance in ants.

Figure 1

Individual experiment: distribution of the number of workers as a function of the total number of trips they made toward the Y-maze (N = 45).

Figure 2

Individual experiment: distribution of the individual preference indices of foragers (n = 45). The dotted orange line represents the average of the individual preference indices and the dotted black line represented the value of 0 expected if contamination has no impact on foragers’ preference. Student's t-test; NS: non-significant.

Figure 3

Individual experiment: distribution of the spatial fidelity indices for the foragers that performed several trips to the Y-maze (N = 39). The dotted orange line represents the median value of the observed individual fidelity indices and the dotted black line represents the value of 0 expected if the ants were as inclined to go toward the same platform as they were to switch between successive trips. One-sample Wilcoxon signed rank test; **: P < 0.01

Figure 4

Individual experiment: proportion of foragers returning to the previously visited area as a function of the rank of the foraging trip. N values represent the number of ants for each trip's rank. A binomial test was carried out for each trip’s rank. Binomial tests; NS: non-significant, *:P ≤  0.05, ***:P ≤  0.001.

Figure 5

Individual experiment: proportion of foragers that first visited the contaminated foraging area (CFA) over the first eight trips. N values represent the numbers of ants for each trip's rank. A binomial test was carried out for each trip’s rank. Binomial tests; NS: non-significant.

Figure 6

Collective experiment: ant flows arriving at the UFA (in blue) and the CFA (in orange) every 5 min during the 3-h experiment. Values were averaged over the number of workers entering the foraging area per 5 min of observation. Circles and shadings represent the mean ± standard deviation, respectively (N = 10 colonies).

Figure 7

Collective experiment: the proportion of dead flies still on the UFA (in blue) and on the CFA (in orange) as a function of the time (min). The coloured shadings surrounding lines represent the 95% confidence interval. Curves were drawn by pooling all the conidia-free prey given on each platform for the ten tested colonies (N = 150 dead prey). The two vertical dashed lines indicate the half (i.e., 1 h 30) and the end of the experiment (i.e., 3 h).