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Insect Repellent Plants

New Zealand's unique native flora is proving a useful hunting ground for naturally occurring substances which repel insects, opening up possibilities for commercial development.

The 80 million years of isolated development for New Zealand plants and insects has resulted in a high number of insects which are dependent on only one species or genus of plant for feeding.

"That is, the majority of plants are unsuitable hosts to most insects except the species that is adapted to it," says Canterbury University chemist Graeme Russell. "This is a higher proportion than any other flora in the world and emphasises the long association and co-adaptation that has occurred."

The substances he is studying aren't necessarily toxic to insects, but are antifeedants which lead the insects to reject the plant as a suitable food source. In some cases, they interact more directly with the insects, interfering with feeding techniques or growth.

"New Zealand flora is likely to be a rich source of insect-active chemicals and, in earlier searches, insect moulting hormones and other compounds which disrupt insect growth and development were isolated."

Of 18 native plant extracts tested to date, 14 showed activity against at least one of the five insect species used as test insects (grass grub, brownheaded and greenheaded leaf roller, tropical armyworm and ant). Kowhai, kawakawa and kohekohe were particularly effective, with antifeeding activity against at least three of these insect species, and several of the active compounds have now been identified.

Russell sees two main ways in which such compounds could be used in insect control: as applied control agents or as factors in genetically modified crops. By applying an antifeedant chemical to a particular part of a crop -- for example, the crown or top of the plant -- the insects could be encouraged to move to the lower leaves or to a nearby trap crop where they could then be controlled through insecticides.

This technique depends on clear identification of the compounds involved and cost-effective extraction or production of them.

"Research into biotechnological and synthetic routes for these materials offers hope in this respect," Russell says.

Genetically engineering crops to produce antifeedants would involve inserting the genetic instructions for production of the bioactive compound from the original plant to an unprotected crop. Inducing a plant to produce a suite of compounds may offer a chance to stay ahead of the selection pressure which can produce resistant insects if only one factor were involved.

"Plants counteract insect resistance evolution by employing a large number of defensive chemicals, usually antifeedants and/or repellents and usually in multicomponent mixtures and at variable concentrations," Russell notes.

"If transgenic plants are to be developed, chemical diversity needs to be introduced and this can only be accomplished through the knowledge of the chemical, biochemical and ecological interactions between plants and insects. In this respect, the study of insect antifeedants from plants is likely to present us with exciting opportunities for future development."