Lack of self-medication by fungus infected Lasius platythorax ants in a multitrophic experiment

Ants live in dense colonies with low genetic diversity between nestmates, creating favourable conditions for the spread of diseases. Due to the constant pressure by infectious disease, ants have evolved different mechanisms to cope with it. One way to combat pathogens is via consumption of biologically active compounds, which can inhibit the growth of the microbes or eliminate them. Although ants are known to respond to pathogen infections by altering their foraging to incorporate reactive oxygen species (ROS) into their diet to successfully combat disease under laboratory conditions, it is still unknown how ants would self-medicate in the wild. By designing a multitrophic environment, we try for the first time to identify potential sources of medicinal compounds for ants to use against fungal infections under natural conditions to better understand how the behaviour is expressed in nature. We investigated whether a bean plant suffering from an infestation of Megoura viciae aphids would be a source of ROS which the ant could use for self-medication, and whether the ant Lasius platythorax changes its foraging behaviour on bean plants if exposed to infectious spores of the entomopathogenic fungus Beauveria bassiana. We found no clear evidence that the ants significantly change their foraging behaviour on extrafloral nectaries in response to the pathogen or that the nectar produced by aphid stressed plants contains ROS. The aphids contained ROS in levels which ants could potentially use, however Megoura viciae is considered unpalatable to ants and we saw no evidence of predation on the aphids. Our results mean that the combination of species of the ant - plant - aphid interactions which we studied does result in a detectable self-medication response in ants. Therefore, we suggest that alternative species of aphids as well as other natural sources of ROS should be studied for self-medication.