It has been demonstrated that the plants when attacked by herbivorous insects can emit volatile compounds that can attract natural enemies of the insects. For example, the roots of maize plants when attacked by western corn root-worms (a noxiuos insect pest of corn) can synthesize and emit a volatile compound called (E)-beta-caryophyllene that attracts insect-parasitic nematodes that infect and kill many soil dwelling insect pests (Rasmann et al., 2005; Degenhardt et al., 2009). Read following scientific papers for more information on insect induced plant volatiles that attract natural enemies of insect pests.
Degenhardt, J., Hiltpold, I., Kollner, T.G., Frey, M., Gierl, A., Gershenzon, J., Hibbard, B.E., Ellersieck, M.R. and Turlings, T.C.J. 2009. Restoring a maize root signal that attracts insect-killing nematodes to control a major pest. Proceedings of the National Academy of Sciences of the United States of America. 106: 13213-13218.
Rasmann, S., Kollner, T.G., Degenhardt, J., Hiltpold, I., Toepfer, S., Kuhlmann, U., Gershenzon, J., Turlings T.C.J. 2005. Recruitment of entomopathogenic nematodes by insect-damaged maize roots. Nature 434: 732–737.