Selection of entomopathogenic nematodes for enhanced responsiveness to a volatile root signal helps to control a major root pest

The efficacy of natural enemies as biological control agents against insect pests can theoretically be enhanced by artificial selection for high responsiveness to foraging cues. The recent discovery that maize roots damaged by the western corn rootworm (WCR) emit a key attractant for insect-killing nematodes has opened the way to explore whether a selection strategy can improve the control of root pests. The compound in question, (E)-beta-caryophyllene, is only weakly attractive to Heterorhabditis bacteriophora, one of the most infectious nematodes against WCR. To overcome this drawback, we used a six-arm below-ground olfactometer to select for a strain of H. bacteriophora that is more readily attracted to (E)-beta-caryophyllene. After six generations of selection, the selected strain responded considerably better and moved twice as rapidly towards a (E)-beta-caryophyllene source than the original strain. There was a minor trade-off between this enhanced responsiveness and nematode infectiveness. Yet, in subsequent field tests, the selected strain was significantly more effective than the original strain in reducing WCR populations in plots with a maize variety that releases (E)-beta-caryophyllene, but not in plots with a maize variety that does not emit this root signal. These results illustrate the great potential of manipulating natural enemies of herbivores to improve biological pest control.