What Can Humans Learn from Plants in Insect Defense Without any Synthetic Pesticides?
A review of biochemicals used by nature to protect crops
According to many analysts, the widespread promotion of synthetic petroleum-based pesticides post World War II was driven by massive commercial lobbying efforts to liquidate large stockpiles of unused warfare chemicals and monetize the capacity of their production lines, which would be otherwise obsolete in peace times. We explored the major chemical types of pesticides and risks associated with each in my previous article “Do Pesticides Impact Fertility, Masculinity and Miscarriages?” But what did humans do for centuries to repel insects and bugs before the 20th century and age of synthetic pesticides?
Well, as always, humans tried to learn from the intelligent design of nature by watching how plants repel bugs. Plants often activate direct morphological defenses (like thorns and prickles) or indirect defenses, which act via the attraction of organisms from an additional trophic level, e.g., of enemies of the attacking herbivores. The indirect mechanism involves the release by herbivore-infested plants of certain volatile organic compounds (VOCs) above and below-ground, consisting mainly of terpenoids, fatty acid derivatives, and a few aromatic compounds (such as extrafloral nectar) which can attract parasitoids (such as ants, or the ladybird beetles, hoverflies/syrphids or green lacewing, all well-known natural enemies of aphids), as predators of the feeding insect (such as aphids or bark/leaf beetles).1
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But there are also chemicals known as “Antifeedants”, which are organic compounds produced by plants to repel herbivores (such as often-hungry deer, rabbits, goats, etc.) and insects (aphids, caterpillars, leaf beetles, spider mites) through distaste or toxicity. This why, for example, Western gray kangaroos and koalas avoid feeding on essential oil–containing Myrtaceae (myrtle, eucalyptus, allspice, clove) family, or giraffe can be toxically poisoned by acacia trees, which produce high concentrations of cyanide upon giraffe browsing2. In almost all cases, upon herbivore attack, an inducible defense is established locally on the site of infestation as well as systemically throughout the whole plant, albeit in some cases with lower intensities.
Here are some examples of natural pest-repellent chemistries used by nature and humans (by imitation, in synthetic, denatured or concentrated forms):
Rosin (known in commerce as crude turpentine) is the resinous constituent of the oleo-resin exuded by various species of pine trees and conifers. The chief component of rosin is Abietic acid (or sylvic acid) is an irritant for the skin and mouth of animals scratching or chewing the tree barks. (The acidic Rosin has been historically used to make soaps, adhesives, calks, adhesives, inks).
Terpenoids (such as the non-volatile components of extracted pine resins) have natural antimicrobial activity and their many mechanisms of action (depicted in the opening figure) include direct (for example by inhibition of acetylcholine esterase, which is what nerve agents and Organophosphorus pesticides do in large scale, as explained in my earlier article) or indirect (by attracting parasitoids which are predators of insects and parasites). Terpenoids also include essential oils like D-limonene3 (in orange peel extract) and Eugenol (derived from clove, nutmeg, cinnamon, basil and bay leaf), which are hydrophobic and hence can easily penetrate and dissolve (strip away) the lipopolysaccharide cell membrane (the protective wax coating) from the exoskeletons of insects.
Glucosinolates (found in Brassicaceae or Cruciferae, family of flowering plants commonly known as the mustards, the crucifers, or the cabbage family) and their reactive metabolites isothiocyanates affect a wide range of insects and herbivores such as mammals, birds, mollusks, and nematodes. The general mechanism of toxicity is believed to be the electrophilic spontaneous reaction with biological nucleophiles (proteins of the body of herbivore) such as -NH2, -SH, and -OH.
Capsaicin and several related compounds are called capsaicinoids and are produced as a secondary metabolite by chili peppers, probably as deterrents against herbivores. It is a chemical irritant and neurotoxin for mammals (may include humans if ingested in “large” quantities) and produces a sensation of burning in any tissue with which it comes into contact. In small doses, purified forms of capsaicin are approved as a topical treatment of neuropathic pain (neuralgia) due to its nerve-numbing effect.
Alkaloids, are mainly found in the Solanaceae, or the nightshades such as potato, tomato and eggplants, and are toxic to animals, vertebrates as well as arthropods. Alkaloids include nicotine (in tobacco) and caffeine (in coffee or Coffea arabica) and act as their natural defense by paralyzing insects feeding on the plants. Other alkaloids include natural neurotoxins such as rotenone (from the roots of certain tropical pea plants and legumes), veratridines (from Veratrum plants, which belong to the lily family) and Pyrethrins (found naturally in some chrysanthemum flowers)4.
Soap-like organic compounds called saponins have an amphiphilic character (affinity to both oily and watery substances) and can therefore disrupt cellular membranes of insects. Saponins have shown insecticidal effects against aphids, beetles, weevils, leafhoppers, worms and moths. Oat also use saponins such as avenacosides as allelochemical (A chemical that is hostile to other nearby organisms) against fungi. For humans, Saponins have shown immense therapeutic potential as hypolipidemic (lowering lipids in the blood), hypoglycaemic (lowering blood sugar), anti-asthmatic, antioxidant, anti-hypertensive (lowering blood pressure), and anti-microbial activity along with few adverse effects such as cytotoxicity. Saponins are present in the form of triterpenoid glycosides or steroids in plant-based foods. Triterpenoid saponins can be found in legumes such as alfalfa, chickpeas, broad beans, soybean, lentils, kidney beans, peanuts, sunflower seeds, ginseng roots, horse chestnut, tea leaves, spinach leaves, quinoa seeds, sugar beets and other alliums species, whereas steroidal saponins are found in Yucca, tomato seeds, ginseng roots, yam, eggplants, fenugreek seeds, asparagus, capsicum peppers.
Fruits use protein enzymes against injury and to defend against feeders. These proteins include peroxidases (found in almost all vegetables, especially “fresh” cabbage and green beans to protect the cells against the effects of oxidative stress and cell damage/injury due to hydrogen peroxide) and polyphenol oxidases (PPO in the diagram, found in fruits like apples, potatoes, pears and bananas, that turn brown upon being injured by forming the antibacterial oxidant/antioxidant quinones or melanin).
The Incas in the South American Andes grew mashua (Tropaeolum tuberosum) plants (tubers) as intercrop to protect their potato plants.
Salicylic acid (a precursor to aspirin), is a common, nearly ubiquitous, phenolic secondary metabolite of many plants (Such as broccoli, cauliflower, cucumber, mushrooms, radishes, spinach, zucchini, tomatoes and the nightshade family). The acid not only affects plant thermogenesis, stomatal dynamics, seed germination, cell growth, vegetative growth, flowering, photosynthesis, but also its responses to abiotic stresses (such as UV exposure) and defensive responses against pests and pathogens. Resorcinol (found in certain plants such as barley and rye) is another phenolic compounds with antifungal properties.
Simple aliphatic organic acids like Oxalic acids (found in spinach, kale, Swiss chard, sweet potatoes, almond, rhubarb, beets, potato skins) also provide self-defense against insect pests and grazing animals. Oxalic acid is perhaps the strongest organic acid in plants and used against varroa mites in honeybee hives. Its conjugate base strongly chelates cations like calcium, potassium and magnesium, and the resulting oxalates are deterrent to planthoppers and toxic to grazers (calcium oxalate is the main form of kidney stones in humans too5).
So it appears that before the age of synthetic pesticides (many of which use plant-based insecticide chemistries as precursors anyway) humans had a large number of plants and herbs in their arsenal against insects and pathogenic microbes. Example of these include roots or leaves of legumes, brassicas, nightshades, and herbs like lavender, cinnamon, nutmeg, thyme, lemon balm, rosemary, basil, mint, peppermint, bay leaves, tea tree, and marigolds.
Quercetin and Tricosane are higher molecular weight non-VOC molecules effective sometimes as antifeedants in close proximity.
The mechanism often includes release of neurotoxic hydrogen cyanide (HCN), when inactive cyanogenic glycosides (in vacuoles) are mixed with activating hydrolases (in adjacent cytoplasm) upon cell destruction by a feeding herbivore. HCN affects cellular respiration in general by inhibiting the binding of oxygen to the cytochrome-c-oxidase within mitochondria.
Being a solvent of cholesterol, d-limonene has been used clinically to dissolve cholesterol-containing gallstones. In farming and gardening, however, certain monoterpenes, such as D-limonene serve an allelopathic role by inhibiting respiration, blocking the nitrogen cycle, or inhibiting growth and seed germination of neighboring plants.
Natural pyrethrin is relatively safe and biodegradable as it will break down within a few days in direct sunlight. Synthetic variations of pyrethrin are marketed as insecticides in the “pyrethroids” family.
Spinach, chard and Beet greens are among the main culprits in formation of calcium oxalate.