It has been demonstrated that entomopathogenic nematodes are attracted to herbivore-induced volatile organic compounds (VOCs) from plants when fed upon by their insect pests. Thus these attracted nematodes can attack and kill the insects present in the vicinity of plants. Please read following papers for more information on VOCs released by plants and nematode attraction.
Ali, J.G., Alborn, H.T. and Stelinski, L.L. 2011. Constitutive and induced subterranean plant volatiles attract both entomopathogenic and plant parasitic nematodes. Journal of Ecology 99: 26-35.
Rasmann, S., Erwin, A.C., Halitschke, R. and Agrawal, A.A. 2011. Direct and indirect root defenses of milkweed (Asclepias syriaca): trophic cascades, trade-offs and novel methods for studying subterranean herbivory. Journal of Ecology 99: 16-25.