Mycorrhizal symbiosis effects on herbivore competition and natural enemy effectiveness

Mutualistic plant micro-organisms such as arbuscular mycorrhizal fungi usually enhance plant access to nutrients, and alter plant defensive state. Plant defences can be direct if they take the form of anti-herbivore traits, or indirect if they attract herbivore natural enemies. These defences are often specific, and their impact depends on the species of targetted herbivore or natural enemy. We study here the effect of the mycorrhizal fungi Rhizophagus irregularis on the two closely-related spider-mites, Tetranichus urticae and T. evansi. We found that the mycorrhizal fungi promoted growth of the poorer competitor T. urticae, but did not affect the stronger competitor T. evansi. This result suggests that the plant symbiont can allow the poorer competitor to persist under scenarios of strong competition. A competition experiment confirmed this hypothesis, and suggested that microbial symbionts of plants need to be considered to better understand the dynamics of complex herbivore communities. In a second set of studies we explored the effect of R. irregularis on direct and indirect tomato defences against the tomato pinworm moth Tuta absoluta.The plant symbiont did not have any effect on T. absoluta development, but it reduced the attraction of its natural enemy M. pygmaeus. These results reveal a potential cost that tomato plants may have when they associate with R. irregularis symbionts.