Introduction

The materials in living plants and animals are divided by scientists into two groups: primary and secondary metabolites. Primary metabolites are the substances fundamental to all living matter: simple sugars, amino-acids, nucleotides and fatty acids, etc. Secondary metabolites are substances made by one or a group of species which are not generally vital to the life of the organism. Secondary metabolites may be structural materials, such as bone, chitin or hair, antibacterial or antifungal compounds, they may give protection from predators or foragers, they may be signalling substances (hormones or pheromones) or they may have, as yet, no known function in that organism. The range of secondary metabolites is enormous and presents a never-ending source of research and exploration. What is equally surprising is that this great array of substances are made from relatively few basic building blocks. Figure 1.1 attempts to summarize, very briefly, the way in which all these types of compounds found in nature are made. Notice that the carbon atoms of all substances, from plant or animal, are ultimately derived from carbon dioxide via photosynthesis. The figure shows that many groups of compounds are formed via relatively few biosynthetic paths. Biosynthesis is the building up of chemical compounds through the physiological processes that take place in living animals, plants and micro-organisms.

There are by some estimates about one million insect species. They have colonized almost the entire terrestrial world, and are very varied in habitat and behaviour. They share some biochemical characteristics with all living organisms, others with all animals, but others are peculiar to insects alone, or to a few species or even a single caste of a single species. In the words of Jerrold Meinwald and Thomas Eisner, pioneers in insect chemical ecology, "The ability to synthesize or acquire an extremely diverse array of compounds for defence, offence and communication appears to have contributed significantly to the dominant position that insects and other arthropods have attained." The kind of compounds the insects produce are therefore a challenge to our ability to understand their structures and functions. The groups of compounds that are of special interest to us in the study of insects are indicated in boxes in Figure 1.1.

The great diversity of secondary metabolites indicated in Figure 1.1 are often spoken of by chemists as natural products. They are varied in their chemical structure, but they are all made by one of these few biosynthetic pathways (in some cases, a combination of more than one of them). By understanding their biosynthetic origins one can make some sense of this great diversity of natural products and group them according to their origin. Moreover, as we come to understand better these biosynthetic mechanisms, we gain greater insight into how we might regulate such reactions in pest species, as well as understanding how these pathways evolved.

The general principles are considered first in each case and then their application to insects is discussed. In some cases the principles are discussed first in relation to micro-organisms or plants, because that is where they were first studied or where more is known of them. It should also impress upon the reader the unity and diversity of biosynthetic products.

1.1 THE STRUCTURES OF NATURAL PRODUCTS

Knowing the probable biosynthetic origin of a new compound can help to decide what is its likely structure, and what is an improbable structure, and help us to arrive at its structural formula. It can be difficult to rule out a possible structure completely, because nature is full of surprises. This book should help the reader to decide which among some alternative structural possibilities is the more likely. In Figure 1.2, the compounds on the left are insect pheromones where the likely biosynthetic origin can be easily deduced from the structures, while it is very difficult to see how those on the right can be made by the routes we know, but both the structures on the right have been found in at least one insect. When structures like those on the right are proposed, it is particularly important to show that they are correct by synthesizing the structure proposed and comparing it with the natural compound.

1.2 COMPOUNDS AND FUNCTION

Many of the compounds from insects considered here are pheromones (Greek, phero = carry or convey), defensive or offensive substances (allomones, Greek, allos = other), or hormones (Greek, hormao = excite or impel). Pheromones can be considered as chemical communication between individuals of the species, while hormones are chemical communication within the individual. In evolutionary terms, it has been suggested that pheromones were among the first communication chemicals affecting animal behaviour, and the pheromones of primitive single-cell organisms may have evolved into the hormones of multi-cellular animals. On the other hand, the types of compounds used as pheromones and allomones are so varied, they appear to have evolved many times, while the hormones are relatively conserved, and the same hormones serve many or all insects and can be common to many invertebrate classes. Chemicals for communication (semiochemicals, Greek semeion = sign or signal) between species and between plants and animals are called collectively allelochemicals, and are further sub-divided in a system depending upon whether they benefit the sender (allomones, as above), receiver (kairomones), or both (synomones), and other categories.

Pheromones are the group of insect compounds that have found greatest application in agriculture and forestry. For example, a large number of lepidopteran species are important agricultural pests. They use sexual pheromones to attract males for mating. The pheromones can be used to aid control of pests in one of several ways. Traps baited with synthetic pheromone can be used to detect the arrival of a pest, or to assess the build-up of the species in a crop, so that insecticides can be used more sparingly and at the correct time. In a few cases, trapping alone can be effective in removing enough of the males to control the pest. Sometimes the pheromone is scattered throughout the crop so that males are unable to locate females (mating disruption). Sometimes a wrong isomer can completely inhibit the response to a pheromone, so a lure containing some of the inhibitor can disrupt mating. Both Coleoptera and Lepidoptera can be pests in forestry, and there too, pheromone traps have been found effective. Pests in stored products are particularly suitable for pheromone trapping, where use of insecticides is undesirable. Sales of pheromones world-wide...