Plant volatiles influence the African weaver ant-cashew tree mutualism

Abstract

Plant volatiles influence virtually all forms of ant-plant symbioses. However, little is known about their role in the mutualistic relationship between the African weaver ant and the cashew tree. In this study, we tested the hypothesis that cashew tree volatiles from plant parts most vulnerable to herbivory viz. inflorescence, leaves, and fruits, are attractive to weaver ants. Using behavioral assays, we show that these volatiles attract weaver ants but without significant difference in preference for any of the odors. These same plant parts are associated with extra floral nectaries (EFNs') and therefore we evaluated the possibility that the ants associate the volatiles with food rewards. We found that perception of the odors was followed by a searching response that led the ants to non-volatile sugar rewards. More importantly, we observed that weaver ants spent significantly more time around the odor when it was paired to a reward. Chemical analysis of volatiles showed that the plant parts shared similarities in chemical composition, dominated by monoterpenes and sesquiterpenes. Additionally, we evaluated the attractiveness of a synthetic blend of three ocimene isomers ((E)-β-ocimene, (Z)-β-ocimene and allo-ocimene) identified in cashew leaf odor and shown to constitute a candidate kairomone for the cashew pest Pseudotheraptus wayi. We found that the attractiveness of the blend was dose dependent, and the response of the ants was not significantly different to that established with the crude volatiles from plant tissues. These results present new and interesting possibilities for improving weaver ant performance in cashew pest management.