Direct and indirect plant defenses reduce damage by a generalist but not by a specialist pod borer

Abstract

The efficiency of different plant defenses is expected to vary depending on the combination and abundance of herbivores. Effectiveness of ant-mediated defenses can also depend on ant activity and local ant assemblage, which can vary due to human activities such as intensive grazing. We used the plant Crotalaria pallida (Fabaceae), its two pod-boring herbivores and the ants that visit the extrafloral nectaries (EFNs) to understand how chemical defense and ant–plant associations affect plant damage. We hypothesized causal relationships between plant chemical defense, ant presence on plants and herbivore damage in scenarios of constitutive and induced plant defenses and tested the different possible relationships using path analysis. The constitutive model explained the data more adequately, and each plant defense had different effects on each herbivore species. First, chemical defense significantly reduced damage by the generalist pod borer Etiella zinckenella but not by the specialist Utetheisa ornatrix. Second, ant presence reduced damage by the generalist, but had no effect on damage by the specialist. Third, a larger number of pods on the plant increased ant presence, as expected since more pods results in more EFNs. Finally, we show that these density-dependent responses by ants mediate indirect cascades on herbivory and generate context-specific responses reducing damage by E. zinckenella in sites with cattle grazing. In synthesis, we highlight that plant defenses have distinct effects on co-occurring herbivores and that indirect defenses can mediate ecological cascades that generate context dependency in the efficiency of different defenses.