Natural plant compounds, Insect control

Natural Plant Compounds Useful in Insect Control... 

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. 

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. 

Plant phytoalexins [natural plant antibiotics] [1] have the potential of becoming a new class of useful compounds in the control of insect pests. Some phytoalexins have been demonstrated as deterrents to insect feeding. Considerable progress has been made to characterize them chemically and to extend the study of their function in plant disease resistance, but exploration of their role in the control of insect pests is just beginning. 

Small quantities of compounds in natural extracts are often a problem when these need to be evaluated in bioassays. Sometimes there is just not enough of the compound isolated to carry out the usual bioassay.20 Microassays have been developed10 1 to overcome this problem. Typically, a microassay is carried out on a thin-layer chromatography plate with a cellulose layer. A small droplet (1.5 pi) of the tested compound in a solvent (1-102 nmol cm-2) is then added on the plate. After the solvent has evaporated a small amount (5 pi) of sucrose solution lmoll-1 is added to the place where the compound was added. In the control the same procedure was followed on a different plate, but with the solvent alone, with no compound added. These two plates are then placed in a petri dish with the test insect species. In the past, when paper chromatography was widely used, a crude plant extract was placed on the origin of the paper and then eluted into bands. The paper was freed of solvent, sprayed with sugar solution, and used directly in a bioassay to see which parts of the paper were not eaten, and therefore of interest for further examination. 

They may be a source of structures for screening. They may possess activity that is applicable to pest control directly or after structural modification of the original structure. Finally, the recognition of their function in nature may suggest new approaches to pest control. However, their practical application may be limited by economics. Resistant plants are important in managing insect pests and their resistance may arise from many factors. Some plants contain insecticidal principles that may be exploited. Compounds that modify Insect behaviour are not directly lethal, but may be valuable in pest control. However, their efficacy may be difficult and costly to determine. 

The chemical control of insect pests dates back thousands of years. The earliest insecticides were inorganic compounds of copper, lead, and arsenic, as well as some naturally occurring organic compounds such as nicotine (Fig. 7.30a). Few of these first-generation insecticides are in use today because of their adverse side effects on plants, animals, and humans. 

---