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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.

What is biological control of insect pests? Biological control is a method in which natural enemies are introduced in the fields or greenhouses to suppress the populations of economically important insect pests of many plant species.

Natural enemies may include predators, parasities and pathogens.

Predators: Predators are the group of vertebrate animals and invertebrates that generally kill and feed externally on their host and also complete their entire life cycle outside the host body as opposed to parasites. Vertebrate predators of insect include birds, amphibians, reptiles, fish, mammals and invertebrate predators include dragon flies, damselflies, soldier beetles, ladybird beetles, ground beetles, rove beetles, lacewings and hover flies. Several species of mites and Spiders are predators of many species of pest insects and mites. Insect predators are generally used in biological control programs against many small insects like midgeflies, gnats, mosquitoes and larvae of many soft bodied insects.

Parasites (Parasitoids) : Larvae of parasitic insects also called parasitods sometime during their life cycle generally enter the body of insect host where they feed, develop, eventually kills their host and emerge in the environment as adult. These adults are typically free-living but they can feed on other insects as predators and on honeydew, plant nectar or pollen when their prey is not around. Since parasites complete part of their life cycle inside their hosts, they have to adapt with the life cycle, physiology and defense mechanisms of their hosts. Many kinds of parasitoids have been included biological control programs and successfully used against many insect pests especially in European greenhouses. For example, parasitic wasp, Pediobius foveolatus is commercially available and widely used against Mexican bean beetles in the fields.

Pathogens: Pathogens are group of microorganisms including bacteria, fungi, protozoans and viruses can infect and cause diseases in insects.  These diseases caused by pathogens may inhibit the rate of feeding, reproduction, development of insect pests or kill their populations entirely.  One of the most popular insect pathogen is a bacterium, Bacillus thuringiensis (Bt), which has been widely used against many economically insect pests of both field and greenhouse crops.

Beneficial nematodes: Beneficial nematodes include Steinernema spp. and Heterorhabditis spp. These nematodes are considered as parasites or pathogens of insects. These nematodes are commercially available and have been used as biological agents in controlling many soil dwelling insect pests of many economically important crops. You can find more information on these nematodes somewhere ealse in this blog.