Natural plant compounds, Insect

Natural Plant Compounds Useful in Insect Control... 

The natural plant compounds play a very important role in the natural environment. A particularly important role is played with reference to insects. 

Klocke, a.. Natural plant compounds useful in insect control, in Allelochemicals Role in Agriculture and Forestry (G. R. Waller, ed.), ACS Symposium Series 330, 396-415, American Chemical Society, Washington, DC, 1987. 

There are numerous other examples of plants, insects, large herbivores, and other organisms playing synergistic roles in nature s balance. Multiple functions are typical of plant compounds and do not contradict in any way their main role as chemical-defense and signal-induction compounds. A trait that serves multiple functions in a given plant or animal is more likely to survive the rigors of natural selection. 

Throughout history, mankind has always been interested in naturally occurring compounds from prebiotic, microbial, plants and animals sources. Various extracts of flowers, plants and insects have been used for isolating compounds whose task, color and odor could be used for various purposes. Many natural products, such as plant hormones, have a regulatory role, while others function as chemical defense against pests. The role of certain compounds is to act as chemical messengers, such as sex-attractants (pheromones) in insects, terrestrial and marine animals and humans. What is the origin of natural products ... 

Extracts of plants have been used as insecticides by humans since before the time of the Romans. Some of these extracts have yielded compounds useful as sources (e.g., pyrethrins, rotenoids, alkaloids), others as models (e.g., pyrethrins, physostigmine) of commercial insecticides. Recent technological advances which facilitate the isolation and identification of the bioactive constituents of plants should ensure the continued usefulness of plant compounds in commercial insect control, both as sources and models of new insect control agents and also as components in host plant resistance mechanisms. The focus in this paper will be on several classes of compounds, including limonoids, chromenes, ellagitannins, and methyl ketones, which were found to be components of the natural defenses of both wild and cultivated plants and which may be useful in commercial insect control. 

Many flowering plants attract insect pollinators by releasing odorant molecules that mimic an insect s natural food sources or potential egg-laying sites. Plants pollinated by flies or beetles that normally feed on or lay their eggs in dung or carrion sometimes use foulsmelling compounds to attract these insects. 

The simplest quinones are o- and p-benzoquinone [(3) and (4) respectively]. This quinonoid structural feature is widespread in naturally occurring compounds isolated from moulds, fungi, lichens, plants and insects,52 which include not only substituted benzoquinones but also substituted polycyclic quinones [i.e. the substituted analogues of, for example, 1,2-naphthoquinone (5), 9,10-anthraquinone (6), and 9,10-phenanthraquinone (7)]. 

A unique role is played by chemical communication in the interactions between plants and insects. About half a million insect species feed on plants. The process of reproduction in many plant species is critically dependent upon pollination by insects. It is not surprising, then, to find among the numerous natural products of plants both attractants for useful insects and repellents or even insecticides for plant-eating insects. The remarkable diversity of the these compounds (the list includes acyclic and polycyclic compounds, isoprenoids, aromatic derivatives, heterocyclic compounds, etc.) illustrates the non-selectivity in the structure of the chemical mediators for biological applications. The intimate mechanism of their action is, unfortunately, still insufficiently understood. 

One of the most instructive examples related to the mystery of chemical-mediated relations between plants and animals was described by Meinwald. Alkaloids (natural compounds traditionally referred to as secondary , i.e. non-important, metabolites) are amply produced by various plants. In many cases, alkaloids have been shown to be part of a defense system against various herbivores. However, some plant-eating insects learned to break through this... 

The term terpene refers to one of the largest families of naturally-occurring compounds bearing enormous structural diversity, which are secondary metabolites synthesized mainly by plants, but also by a limited number of insects, marine micro-organisms and fungi [4, 5]. 

Although there are many examples of plant secondary compounds which interfere with the physiology of insects [6, 7], there is debate as to the nature of the selective forces responsible for the evolution of these compounds [9, 10]. The argument has been made that although it appears that these secondary metabolites serve to protect the plant against insect herbivory today, it is likely that they were initially selected as a response to vertebrate herbivory [11]. However, phytochemicals which specifically interfere with the insect endocrine system possibly represent compounds which have evolved as protection against insect attack and therefore represent plant adaptations to insect herbivory. 

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