Subterranean herbivore-induced volatiles released by citrus roots upon feeding by Diaprepes abbreviatus recruit entomopathogenic nematodes

It is well-documented that herbivore induced volatile emissions benefit plant hosts by recruiting natural enemies of herbivorous insects and our understanding of the dynamics of such above ground tritrophic interactions continue to expand. Recently, there has been a renewed interest in the fact that similar signals also occur belowground and that this might be an equally important aspect of the indirect defenses of plants. The larvae of the root weevil (Diaprepes abbreviatus) are a serious pest on citrus roots in Florida. Infestations can be controlled by the use of entomopathogenic nematodes, yet the interactions between the plant, insect and nematode are poorly understood and remain unpredictable. In bioassays using a belowground six-arm olfactometer, citrus roots (‘Swingle citrumelo’ rootstock) recruited significantly more entomopathogenic nematodes (Steinernema diaprepesi) when infested with root weevil larvae than non-infested roots, mechanically damaged roots, or larvae alone. By dynamic collection and GC/MS analysis of volatiles from the soil we determined that infested plants produced novel volatile terpenoids not found in samples from non-infested roots or in soil containing only larvae. These findings suggest Swingle citrus roots release induced volatiles as an indirect defense in response to herbivore feeding and some of these induced volatiles most likely function as attractants for entomopathogenic nematodes. Future work will focus on evaluating the use of these nematode recruitment chemicals to enhance control of D. abbreviatus in the field. This study may also be the first to utilize direct sampling of volatile induced compounds from an intact host-herbivore subterranean interaction.