We are now in mid-summer, high season for bugs and bug sprays. We spray ourselves with DEET, we spray our plants with neonicotinamides, and we directly spray bugs that take up residence anywhere in our sphere of activity with pyrethrum and prallethrin. In this article, I will trace the history of the use of chemicals that kill the bugs that torment us or eat our food, and their effects on the larger ecosystem. Insects are food for other animals. A lack of insects for food means fewer frogs, means fewer snakes, means more rats…and we are back to the arms race against not only insects but also other animals that we compete with for food. Additionally, loss of biodiversity and potential exposure to more diseases follow. With so much at stake, why is there such a big market for pesticides? What are the benefits, and is there such as thing as a cost-benefit metric to guide their use?
The suffix “cide” means to kill, and the prefix identifies the target: bactericide (bacteria), fungicide (mold), herbicides (plants), nematicides (roundworms), molluscicides (clams and snails), pediculicides (lice), acaricides (ticks and mites), insecticides (insects), piscicides (fish), rodenticides (mice and rats) and avicides (birds); but most don’t selectively kill only pest insects. Since insects are animals, and the term “animals” includes everything from aquatic insect larvae, mollusks, fish, and amphibians, to terrestrial worms, bugs, reptiles, birds, and mammals, they have similar genes, metabolic cycles, and chemical messengers and receptors. If insecticides leach into the water, they may kill insect larvae, amphibians, fish, and even accumulate up the food chain to detrimental effect. For example, the insecticide type called neonicotinoids are considered moderately toxic and teratogenic in humans [1]. The herbicide RoundUp, a glyphosate formulation, has been shown to have quite a wide range of detrimental outcomes on animals, including cytotoxicity and hormonal disruption [2]. Their use in the fields may leach into our diet. Do we stop to consider how well cereals are cleaned of pesticide residues before they show up in our breakfast Cheerios? How many of us test our fish for imidacloprid levels? Still, concern about pesticide residues is probably one of the key reasons why organically grown foods are increasing in popularity.
Pest control measures probably began in earnest when humans started farming, which led to the stockpiling of grains after harvest. This was naturally an attraction to other animals – either ones that ate any part of the plant, or ate the stored harvest. Thus scarecrows, slingshots, guns, traps, and other animals were employed to deter them in the fields. But there were sneakier thieves – insects with their chewing mouthparts and their voracious larvae, too small to scare away, shoot, trap or hunt. Biological control was possible to some extent with predatory insects such as ladybugs whose rapacious nymphs eat aphids; ichneumonid wasps that lay their eggs in other insects’ larvae; hornets that eat the flesh of other insects; dragonflies that catch other insects on the wing; and the incomparable praying/preying mantises. Even throwing in fish, frogs, lizards, snakes, birds, voles and other insectivorous mammals that perform biological control, predators were still unequal to the task of obliterating insects in farmers’ fields or storehouses. Early farmers used natural control methods like low density planting, co-planting with plants that naturally repelled insects, seasonal plant cycling, and the physical removal of insects. Some small effort towards chemical control involved the use of copper compounds, smoke, sulfur dust, sticky substances, salt water, and powdered chrysanthemum flowers, which were later discovered to have the insecticidal chemical, pyrethrum. Toxic chemicals such as arsenic and mercury were also in the toolkit, but their side-effects on humans were so debilitating that they were used only in the direst circumstances. On the ecologically positive side, many farming societies ate the insects gathered from fields as food, and occasional devastations from locusts and boll-weevils were accepted as part of the cycle of life.
Then chemistry was deployed. In the 1930s, organophosphates were produced as nerve agents for use in war, and later formulated as parathion and malathion for use as sheep dip and in fields. Organophosphate poisoning is one of the most common causes of poisoning and deaths in the farming world, with estimates of 1 million poisonings and 100,000 deaths each year [3]. Organochlorides are less toxic, but more environmentally persistent pesticides exemplified by dichloro-diphenyl-trichloroethane, abbreviated DDT. When I was a 5-year old, this was the first chemical’s name I learned, tutored by my organic chemist father. I would sing it aloud, little understanding how this musically-named chemical devastating the natural world. Rachel Carson’s seminal book, Silent Spring, tells of the bioaccumulation of this chemical up the food chain from prey to predator, which leads to thin-shelled bird eggs which break before they can hatch. The numbers of birds plummeted as a consequence, hence the silent Spring. The petrochemical industry spurred by the synthetic chemistry renaissance used coal tar and petroleum by-products to produce naphthalene, nitrophenols and chlorophenols. Herbicides were added to the mix with the dioxins 2,4,5-T and 2,4-D which entered the Vietnam-era lexicon as “Agent Orange”.
The 1970s and 1980s were periods of more targeted development, but also when scientists tried to better understand non-target and off-target activities of chemicals. The list is shocking, with every class of aquatic and terrestrial animal and plant suffering deleterious outcomes [4]. Managed and wild bees are particularly hard hit by insecticide use in agricultural fields. Having said that, bear in mind that, while it is easy to criticize rampant use of pesticides, few would question their undeniable benefits for controlling insect vectors of disease or crop destruction. After the Second World War, the world pivoted to food production with the application of fertilizers, herbicides, and pesticides and the demand grew and grew, as did the global population. Of course, the realization had already dawned that some target insects were developing resistance to previously effective pesticides and biological control agents, and that dioxins were having detrimental effects on soldiers and civilians exposed to them. Scientists studied environmental persistence, and the effects of contamination due to spray drift, and the chemicals’ inadvertent entry into processed foods and meats.
The total pesticide consumption of the world nearly doubled between 1990 and 2017 from 2.3 million to 4.1 million tons [5]. The 1990s were also the period of the mainstreaming of genetic engineering and efforts to harness specificity of toxin to target. For example, Bacillus crystal toxins were discovered to be a targeted biological control agent for insect larvae. Various relatives of this toxin were engineered into corn and sold as Bt-corn, or formulated to be spread as Bt-spray. While it was an improvement over large-scale spraying of fields with broad-spectrum pesticides, there were concerns over humans ingesting recombinant molecules that were incorporated into food products derived from genetically modified plants. I am aware there is no simple answer to food production without the use of chemicals. I am not starving, nor hopefully are you, and we can make value judgements about the use of pesticides that may be immoral in other societies on the verge of food insecurity. Consider that, right now, the greatest locust plague of the last twenty years is laying bare fields in Kenya, Ethiopia, Yemen, Pakistan and Northern India. Huge quantities of insecticides are being sprayed to control them and to save the cropfields.
But in the absence of really pressing need to use pesticides, let’s endeavor to make small changes to our own lives by spraying less inside and outside, by safely relocating to the great outdoors centipedes and spiders, by knocking down wasp nests to persuade them to move instead of spraying them with wasp killer, by using more common sense and less chemicals inside the home and outside in the garden, and by encouraging a healthy and active food web that maintains dynamic control of pest populations. We can also give over one portion of the garden to a little wild space with no sprays and plenty of diversity of flowers and plants – these will be both a reservoir and an oasis for wildlife. Food webs will thrive again, unmolested. And then bumblebees and sweat bees, hummingbird moths, and frogs will reestablish themselves in our gardens, and bees will drink again from dandelions. Is that not worth something?
References:
1. Wang, X., et al., Mechanism of neonicotinoid toxicity: impact on oxidative stress and metabolism. Annual review of Pharmacology and Toxicology, 2018. 58: p. 471-507.
2. Gill, J.P.K., et al., Glyphosate toxicity for animals. Environmental Chemistry Letters, 2018. 16(2): p. 401-426.
3. Eddleston, M., Novel clinical toxicology and pharmacology of organophosphorus insecticide self-poisoning. Annual review of pharmacology and toxicology, 2019. 59: p. 341-360.
4. Wikipedia-Environmental Impact of Pesticides. Available from: https://en.wikipedia.org/wiki/Environmental_impact_of_pesticides.
5. FAO. Pesticides Use. Available from: http://www.fao.org/faostat/en/?#data/RP/visualize.
