Alkaloid protective role

Quinolizidine alkaloids, pure or in mixtures of plant extract, can be used to protect plants against noxious insects. Scientific data has shown that quinolizidine alkaloids play a role in the resistance of some lupine varieties to the pea... 

Orr A. B., Trigo J. R., Witte L. and Hartmann T. (1996) Sequestration of pyrrolizidine alkaloids by larvae of Tellervo zoilus (Lepidoptera Ithomiinae) and their role in the chemical protection of adult against the spider Nephila maculata (Araneidae). Chemoecol. 7, 68-73. 

In addition, copulating male and female arctiid moths provide PAs that are mainly tranferred to the eggs to provide protection against predators.24 Males provide females with a copulatory bonus in the form of a seminal ejaculate fortified with pyrrolizidine alkaloids. The PAs contributed by the male are allocated to both the female and the eggs, supplementing the female s own PA contribution. Obviously the alkaloids play a pivotal role in ensuring reproductive success. 

As was the case with terpenes, the function of alkaloids in plants is not known. It has been proposed that they are merely nitrogen-containing waste products of plants, like urea in animals. However, most plants reutilize nitrogen, rather than wasting it. Furthermore, it is difficult to imagine why such complex structures would be needed to store waste nitrogen. Like terpenes, alkaloids have been proposed to serve as protection from herbivores and insects. However, only a few examples of such protection can be demonstrated. Whatever the role of alkaloids is, some 70% to 80% of plants manage to do quite nicely without them. 

Host plants play a key role in the production and use of sex pheromones by herbivorous insects through larval or adult sequestration of chemically active compounds and pheromone precursors [210]. One of the best examples of sequestration of plant chemicals by larvae and their subsequent use by adult males in sex attraction or courtship interactions is shown in Utetheisa ornatrix (Arctiidae), whose courtship pheromone derives from pyrrolizidine alkaloids (PAs) ingested at the larval stage from the host plant Crotalaria spectabilis [211]. U. omatrix larvae sequester PAs (e.g. monocrotaline) and retain the alkaloids through metamorphosis into the adult stage to provide egg protection for the next generation. 

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