Inducible systems of defense

In the coevolutionary interactions of plants and animals, lipids play a major role. They function as ecomones (pheromones, allomones and kairomones) and have been classified by their function. Host plant resistance is partially dependent on these chemical constituents. Lipids may be subdivided into two types. Volatile lipids are generally involved in long distance interactions whereas non-volatile lipids are generally involved after the insect has contacted the host plant. Several examples of each are reviewed. Utilization of these compounds to promote increased host plant resistance could be accomplished by selection of plants rich in allomones, lacking kairomones for a particular pest or those with inducible systems of defense. Another approach is to isolate the defensive compounds of one plant and apply them to crop plants. Trap crops could also be used to lure insects away from other crops. 

In the plant, part of the metabolic cost of producing and maintaining pools of secondary compounds may be reduced by using compounds which contain only carbon, hydrogen and oxygen (which are rarely limiting), by recycling the compounds or by use of inducible systems of defense. 

Inducible systems of defense (phytoalexins) are widespread in plants and are effective against many types of fungi and bacteria (37,38). Similar systems have been demonstrated in a few cases with insects and are probably common in nature (see for example reference 39). Although Inducible systems of insect resistance would seem to be efficient and effective, no system is foolproof. The larvae of Epilachna tredecimnotata cut a circular trench in Cucurbita leaves and prevent mobilization of the deterrent substances to the area which is then consumed (40). 

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