Nicotine and Other Insecticidal Alkaloids

Abstract

Alkaloids, with more than 10,000 described, are one of the most diverse and prominent groups of natural products with pharmacological and toxicological importance (Harborne 1993). Plant extracts containing insecticidal alkaloids as bioactive constituents have played an important role in the abatement of insects of agricultural and public health importance for centuries. While the direct use of these substances has recently diminished, they continue to serve as leads for synthetic analogs and are also indispensable biochemical tools in mode of action studies. In recent years, the function of these alkaloids in the host plant has begun to unfold: it is now generally believed that the ecological role of these compounds, often acting in concert with other nonalkaloidal substances, is to provide a chemical defense against predators and pathogens in a sustained manner through multiple biological mechanisms (Wink 1993a; Brown and Trigo 1995). The history and progress of the research on natural insecticides, botanicals in particular, have been surveyed from time to time (Jacobson and Crosby 1971; Klocke 1989; Addor 1995). This paper focuses on alkaloids with insecticidal properties. This structural restriction obviously excludes some other natural antiinsect agents of practical significance, but it is hoped that the importance of this segment of natural product research will be demonstrated. The compounds are grouped according to their first reported isolation: those from plants, arthropods and other animals, and marine organisms as well as microbial sources. The following definition for an alkaloid is adopted: "an alkaloid is a cyclic organic compound containing nitrogen in a negative oxidation state which is of limited distribution among living organisms" (Pelletier 1983). This permits the inclusion of not only alkali-like compounds (e.g., nicotine, steroid alkaloids) but amides (e.g., lipophilic isobutylamides) as well. Compounds with a nitrogen atom in a positive oxidation state (e.g., nitro compounds) are excluded.