Introduction: The Chemistry of Plants and Insects Free

This book is an introduction to the natural chemical compounds that are part of the many different interactions between plants and insects. Fragrant scents from flowers attract pollinators. Bright or drab pigments in flower petals tempt different kinds of pollinating insects to visit. Flowers with stamens full of pollen provide food to honey bees, and sweet sugary nectar offerings in flowers invite butterflies, all as an enticement to encourage pollination and, with this, to promote reproduction of the plants (Figure 1.1). Organic, i.e. carbon-based, chemical compounds compose the floral scents, pigments, and nectar components that perform key roles in the communications between plants and insects.^1,2 

Most insects, on the other hand, use and need plants as vital sources of food, in particular to obtain the basic nutrients of carbohydrates, amino acids, and fatty acids. Insects and other animals (including humans) are heterotrophs and as such cannot synthesize all the primary metabolites in their own systems but must obtain them from plant-related sources. (Refer to the Glossary at the end of this book for definitions and brief explanations if needed.) Huge numbers of beetles, caterpillars, grasshoppers, and aphids find their basic nutrients in tender plant parts (Figure 1.2(a)). Some insects obtain them from plant roots, tree barks, or fruits while bees, butterflies, and many beetles find nourishing proteins and sugars in pollen or nectar from flowers. There are, of course, insects that feed on other animals, like mosquitoes sucking blood, or on animal products, like dung beetles using animal feces as food (Figure 1.2(b)). But these insects get their basic nutrients from animals that had earlier ingested plant foods and with them the required primary metabolites.³ 

Plants, as autotrophs, are able to undergo photosynthesis. They can convert carbon dioxide from air and water into oxygen gas and simple sugars like glucose, in the presence of chlorophyll, light, enzymes, and mineral ions. From the simple sugars, plants can synthesize larger carbohydrates, amino acids, and fatty acids, as well as vitamins. All are basic nutrients that need to be ingested by insects and other animals. In further biosynthetic steps, plants can produce attractive scents and colorful pigments that call out to pollinators.

Plants not only produce a wealth of organic compounds that serve to attract pollinators, but also synthesize highly diverse defensive compounds. Strong smells in leaves, bitter substances in leaves or roots, and toxic compounds in fruits, leaves, or roots, fend off insects that would otherwise eat the plants and destroy them. Aside from many physical defenses like thorns, prickles, and tough skins, these defensive substances protect plants from being devoured by insects keen on feeding on leaves, roots, or heartwood. Tender plant shoots with low concentrations of natural defensive chemicals are most susceptible to insect attacks. So are plants that are stressed by environmental factors and so are less capable of synthesizing compounds that are distasteful or toxic to insects.

Unlike most plants, insects are short-lived, from a few days to a couple of months.⁴  Beetles, caterpillars, aphids, and other insects can produce generation after generation in large numbers and are able to rapidly evolve to adapt to the chemistry of the plants they visit. Many insects have developed a tolerance and even a preference for previously unpalatable plant sources. An example is the Colorado potato beetle (Leptinotarsa decemlineata), infamous for its damaging infestations on potato plants (Figure 1.3(a)).⁵  The beetles most likely had their original ranges in Mexico and Southwestern North America, feeding on some native plants there. With the development of cultivated potatoes (Solanum tuberosum) in North America, the beetles adapted to feeding on the cultivated crops as their host plants. Through the export of potato plants, the voracious insects were soon further introduced to temperate climates all over the world. Rapid cycles of generations enabled the potato beetles to adapt and infest the crops in spite of toxic chemical defenses in the plants.⁶ 

Numerous species of insects have not only learned to tolerate plant defensive compounds without harm, but use the toxins for their own protection from predators. For example, the caterpillars of monarch butterflies (Danaus plexippus), as well as specialized aphids and beetles like the red milkweed beetle shown in Figure 1.3(b), have adapted to feed on milkweed plants (Asclepias spp.) and can ingest defensive plant compounds from the plants’ milky latex without harm. The insects then use the plant toxins for their own defense, becoming ill-tasting and unpalatable to their predators.⁷  Insects have even evolved mechanisms to slightly alter the chemical structures of defensive plant compounds, transforming them to pheromones that attract more insects. The coevolution of insects and plants continues, and crucial organic compounds, produced by the respective plants and ingested and occasionally tolerated by feeding insects, play key roles.⁸ 

The examples of insect–plant interactions chosen in this book and the chemical compounds that participate in them are necessarily a very limited selection from the huge number of such relationships. The case studies here are chosen from around the world and selected for their fairly well-known roles, be they desirable or undesirable to mankind, or because of the particular interest of their organic compounds. The great diversity of chemical compounds that are part of these interactions will serve as a presentation of the major families of organic compounds occurring in living things. As the book progresses, the selected sequence of topics and the related examples of chemical compounds will gradually proceed from the relatively simple organic structures of plant volatiles, like geraniol 1.1 (Figure 1.4(a)) found in scents of flowers, to increasingly more complex molecules, like the toxic alkaloid solanidine 1.2 (Figure 1.4(b) found in potatoes.

The book begins with the plant perspective, with an introduction to the structures of organic compounds that plants synthesize in order to communicate with insects. Many components of fragrances in flowers are relatively small organic molecules (on a molecular scale), with few carbon atoms assembling them. Their structures will serve as an introduction to typical organic molecules. The composition of flower nectars and their sugars will introduce additional chemistry concepts. We will then progress to the more complex molecules of colorful pigments, with continued reflections on how the plant compounds influence insect responses. Pollen in flowers is not only transferred by insects onto stigmas of other flowers, inducing fertilization of the plants; it is also offered to attract insects as a nutritious source of starch and proteins, examples of natural polymers. Some plants, namely the insectivorous plants, lure insects to intricately-shaped modified leaves, trap the insects, and then use special enzymes to digest them and use the digestion products as nutritional supplements. The subsequent chapter describes the great diversity of plant compounds that act as insect repellents. It further illustrates the wealth of organic plant compounds that are important in interactions with insects.

In the chapters of Part 2, previously introduced families of organic compounds will reappear, but as part of the insect perspective. The focus will be first on the special chemistry of insects themselves, and then on the nutrients that insects obtain from eating plants. Some insects, like many types of aphids (Aphis spp.), are generalists that feed on lots of different types of plants (Figure 1.5(a)). Others, like the elderberry beetle (Desmocerus californicus, Figure 1.5(b)), are selective about the plants they feed on. These choices are based on the chemistry of the respective plants. Numerous insects use plant defenses for their own defense. Insects that can ingest plant toxins without harm often advertise their acquired toxicity with bright warning colors and distinct patterns, as illustrated in Figures 1.3(a) and (b), and 1.5(b).

Comparing plants and insects with regards to the composition of their structural materials, their pigments, or their defensive substances is fascinating. The respective chapter sections will include comparisons of the organic compounds that compose them.

Interactions between plants and insects and their ecology are of key interest to mankind, and the chapters in Part 3 concentrate on the human dependence on insect–plant communications. People would go hungry without the work of pollinating insects in orchards and on other food crops. Some insect products, like honey, beeswax, and silk, are of great interest to humans. They are products of specific insects feeding on plant sources. On the other hand, insects can appear as destructive pests on crop plants, threatening the supply of food for humans. Therefore, studies on insect attractants and repellents, as well as genetic studies on how to breed plants that are less susceptible to insect infestations, are active topics of research. Various approaches on how to manage insect pests, including thoughts on environmentally sensible methods, will be discussed.

Plant–insect interactions comprise a field of ongoing and vigorous research. Previously unknown insects and plants keep being discovered, especially in environments like tropical rain forests, and with them new mechanisms of communications between plants and insects. Often the amounts of essential chemical compounds that are involved in specific interactions are extremely small and therefore difficult to detect or to define. Thanks to increasingly sensitive methods of instrumentation, previously unknown mechanisms and their participating chemical compounds have been elucidated, and great recent progress has been made in explaining them.^9,10  Examples of investigations and their findings will be shown.

The subtitle of this book is, somewhat tongue-in-cheek, “Plants, Bugs, and Molecules”, the word ‘bugs’ used here for insects in general, just as in everyday language. In entomology, i.e. the scientific studies of insects, true bugs represent a subcategory, called an order, of the class of insects. True bugs are scientifically known as the order of the Hemiptera. We shall use the scientific definition in this book. Many ‘bugs’ from everyday language are actually beetles (order Coleoptera), like ladybugs which are more correctly called lady beetles or ladybird beetles. True bugs have very different mouthparts from beetles. As another example of a striking difference between bugs and beetles, the larval stages of beetles, like the lady beetles shown in Figure 1.6(a) and (b), look significantly different from the adult insects. Beetles go through a complete metamorphosis during their development, whereas true bugs, like the milkweed bugs shown in Figure 1.6(c), look quite similar in their immature stages and as adults. More details on insect and plant biology will be shown in the respective chapters if they are relevant to insect–plant interactions and their chemistry.

There is a wealth of common names for individual plants and insects, and many local variations exist aside from language differences. On the other hand, the systematic names for individual plants or insects are unique and internationally used. Therefore, we'll include systematic names after common names of plants or insects throughout the book. Thus, a ladybug would be referred to as ladybug or lady beetle (Coccinella californica) (Figure 1.6(a)).

Refer also to the Glossary at the end of this book for definitions and brief explanations, as well as for a short guide to reading structures of organic compounds.

The end-chapter references include books as well as review articles and journal articles that relate to the topics of the respective chapter. While most referenced books and articles are in-depth texts, others are for popular reading.

Many excellent texts can provide detailed background on organic chemistry, biochemistry, plant biology, and entomology if needed.

Some examples are:

• J. E. McMurry, Fundamentals of Organic Chemistry, Brooks/Cole, Belmont, CA, 8th edn, 2012.

• D. R. Klein, Organic Chemistry, J. Wiley and Sons, 2nd edn, 2013.

• R. F. Evert and S. E. Eichhorn, Raven Biology of Plants, W. H. Freeman, New York, NY, 8th edn, 2012.

• D. L. Nelson and M. M. Cox, Lehninger Principles of Biochemistry, W. H. Freeman, New York, NY, 6th edn, 2012.

• H.-W. Heldt and B. Piechulla, Plant Biochemistry, Academic Press, London, 4th edn, 2011.

• P. J. Gullan and P. S. Cranston, The Insects: An Outline of Entomology, Wiley-Blackwell, Chichester, West Sussex, UK, 4th edn, 2010.

• R. J. Elzinga, Fundamentals of Entomology, Pearson, Upper Saddle River, NJ, 6th edn, 2004.

• J. B. Harborne, Introduction to Ecological Biochemistry, Academic Press, London, 4th edn, 1993.

• For reference on organic structures:

• The Merck Index: An Encyclopedia of Chemicals, Drugs, and Biologicals, ed. M. J. O'Neil, The Royal Society of Chemistry, Cambridge, UK, 15th edn, 2013.