Plants under attack by arthropod herbivores have been shown to employ a strategy of indirect defense by attracting natural enemies of the herbivores. Such interactions have been intensively investigated for aboveground tritrophic systems, whereby leaf-feeding insects induce the emissions of specific volatiles that attract predatory and parasitic arthropods. Recently it was found that similar interactions also occur belowground when insects-damaged roots release compounds that attract entomophatogenic nematodes. We investigated such root signals for maize plants under attack by larvae of Diabrotica virgifera virgifera, a ferocious pest on maize. With the use of a newly developed below ground olfactometer we found that the nematode Heterorhabditis megidis was highly attracted to Diabrotica-damaged maize roots, compared to mechanically damaged roots or healthy roots. Additional experiments showed that D.v.virgifera-damaged roots produce large quantities of the sesquiterpene (E)-beta-caryophyllene. This sesquiterpene was indeed found to be a key attractant for the nematode. Interestingly, most North-American maize varieties do not emit this signal, which results in dramatic differences in the attractiveness between different maize lines and in larval-infection rates under field conditions. Measurements of diffusion of a range of typical plant volatiles show that (E)-beta-caryophyllene is the most effective and cost efficient as a belowground signal, suggesting that this is indeed a function for this compound.