An experimental test of ant effects on herbivory and pathogen infection on wild cotton (Gossypium hirsutum L.)

Abstract

Ant-plant interactions are widespread and have profound effects on plant fitness, food webs, and species diversity. Whereas most experimental work has tested for ant effects on herbivores, few studies have investigated ant effects on plant pathogens, and even less dual effects on herbivores and pathogens. In this work, we tested whether ants protect wild cotton (Gossypium hirsutum L.) plants against insect herbivores and fungal pathogens by means of an ant exclusion field experiment. In addition, we measured extrafloral nectar (EFN) traits (volume and concentration) to assess their role in ant attraction, and controlled for cotton direct chemical defenses (phenolics) in testing for ant-mediated defense. Ant exclusion did not have a detectable effect on either insect herbivory or pathogen infection levels on wild cotton leaves, and, accordingly, did not impact plant growth (height) over the growing season. We found that EFN traits were associated with ant recruitment to wild cotton plants, with both nectar amount and concentration being positively correlated with ant abundance on control plants. The above tests accounted for leaf phenolics, a type of direct defense, indicating no independent effect of indirect defense mediated by ants on wild cotton during the study. These results provide limited evidence of biotic defense by ants against insect and pathogens on wild cotton at the study site, but call for further assessments of spatiotemporal variation in wild cotton-ant interactions.