Science Makes Sense-Week 32: Chemistry and Social Justice-Pesticides

Rachel Carson, wrote her book “Silent Spring” in 1962 and changed the way we looked at pesticides especially DDT. She may have been quite responsible to have initiated the environmental movement of today. But along with this movement, we continue to produce a lot of new pesticides that make your head spin when you do a search on the internet! So in the 21st century, when we talk of pesticides, it covers such an extensive area that we are not only looking at chlorinated organo-compounds like DDT but so many others including, to name a few, herbicides, fungicides, rodenticides as well.
Today we shall look at the chemical composition of many common pesticides, fungicides etc., their classifications and understand how it impacts humans and the environment. We will also touch on what are the alternatives to using synthetic pesticides.
The word “pesticide” is used to describe a substance (or mixture) that kills a pest, or it can prevent or minimize the damage that a pest causes. Examples of pests include insects, mice or other animals, unwanted plants (weeds), fungi, bacteria or viruses. Pesticides can also include any substance that is used to modify a plant. They are usually chemicals, but can also be made from natural materials such as animals, plants, bacteria, etc.(Ref.1)
The chemical families that most pesticides belong to are the following, organo-phosphates (organic compounds containing phosphorus), chlorinated hydrocarbons including DDT, carbamates (salts from carbamic acid which is really formic acid,HCOOH, with its free hydrogen replaced by an amine NH2 group). There are also thiocarbamates (‘thio’ is when an oxygen atom, O, is replaced by a sulfur,S, atom) and pyrethroids. Pyrethroids are benzenoid derivatives of pyrethrum which is obtained from chrysanthemums. Pyrethrum has the characteristic cyclopropane structure along with hydrocarbon chains with oxygen.(Ref.2)
How do all these different kinds of products broadly called insecticides affect humans and the environment?
Let us first look at its effects on humans. Starting with mild results like headaches and nausea, pesticides can also cause more serious and lasting health effects such as cancer, disruption of the endocrine system and reproductive harm. Disruption of the endocrine system means that the complex production of hormones is disturbed, resulting in infertility and babies born with birth defects. Children are especially susceptible to pesticides, because till the age of twelve, their brains are not fully developed and pesticides can impact the central nervous system.(Ref.4)
Pesticides affect the environment as well. Depending on whether the pesticide is water soluble or fat soluble, it can enter water bodies or get into fish and other aquatic life. Then when we as humans ingest them, we get health problems as well.(Ref.4)
In short, these are the harmful effects of all kinds of pesticides:
1. Pesticides damage ecosystems
2. They may damage/harm un-targeted animals(see Nuggets under ‘bioamplfication’)
3. Pesticides decrease biodiversity
4. Pesticides may cause a decline in populations and/or extinction of species
5. Pesticides “mess up” food chains/webs
6. Pesticides disrupt the ecosystems’ natural balance.
Knowing all this, organic farmers try going in one of two directions. The word ‘organic’ here has nothing to do with the definition in chemistry, it simply means the use of no pesticides or the absence of synthetic fertilizers. When they use no pesticides they allow crops to be spotted or eaten by a few pests and not look ‘perfect’ and allow weeds to coexist with crops/or do a lot of hand weeding. The alternative is to use naturally grown pesticides like pyrethrum that comes from chrysanthemums instead of synthetically applied pesticides and pheromone traps. Too much application of these pesticides can also lead to toxicity, so organic farming is a tricky business. But crop rotation, sprays and pheromone traps combined with minimal use of even naturally obtained pesticides by organic farmers will lead to less toxicity in humans and the environment.(Refs.5,6)
Activities for Middle School Teachers
Study the time-line of the different kinds of pesticides from the early 60’s to right now out there. How ‘natural’ are some of these pesticides? Take students on a field trip to organic farms and orchards. What alternative types of pest control is used here and how effective are they?
Look at the chemical structures of the different pesticides and note if there is a connection between type of chemical classification and specific use of the pesticide.
Nuggets of Information
Pesticides kill or repel insects and include bug sprays/repellants, baits, commercial and garden sprays, shampoos and moth balls. Herbicides kill weeds and unwanted plants. To achieve this we use weed killers, weed and feed fertilizers and tree stump treatments. Meanwhile, fungicides kills mould, mildew and other fungi. here we use commercial/ garden flower sprays,treated seeds and paint additives. Rodenticides kill all kinds of rodents using mouse and rat baits. Then we have disinfectants to kill bacteria, mould and mildew and use bleaches, ammonia, kitchen and bathroom cleaners as well as pool and spa cleaners. Finally, wood preservatives protect wood from insects and fungi and we use pressure-treated wood.
Broad Spectrum: Insecticides vary in what insects they kill. Some kill only a few types of insects. Then you could choose these insecticides when you wish to kill only one insect pest and not other beneficial insects in the area. Many insecticides are wide range killers or “broad spectrum”. Such pesticides are used when several different kinds of insects are a problem. One chemical can kill them all. No broad spectrum insecticide kills all insects; each varies as to the kinds of insects it can control.
Narrow Spectrum: While many insecticides are broad spectrum, a new group of insecticides attack the central nervous system, and are much more selective. The chitin inhibitors only affect animals with chitin in their exoskeleton (i.e. insects). (Chitin is the growth on an insect and inhibiting that growth can also kill the insect.) Growth regulators are even more specific. They affect certain groups of species that have a particular hormone. Finally, when we talk about pheromones, they are the most restrictive because they react with only one species or one sex of a single species.(Ref.3)
When grasshoppers, that are lower on the food chain eat pesticide-laden foods, they get the toxins in their systems. Then shrews, who are their predators, eat several grasshoppers and get more of the toxins in them. Owls in turn, eat several shrews and get even more toxins in them. This process is called Bioamplification. Remember, there could be a decrease in the owl population which may die because of so many toxins in their systems, leading to an increase in shrew population and a consequent decrease in grasshoppers. All this leads to a complete change in the balance of species.
Cesar Chavez was a fighter for better working conditions for the California grape and lettuce pickers. He gave the famous ‘wrath of grapes’ speech in the 80s after twenty years of organizing and forming a union called the United Farm Workers(UFW) in California. He urged the government to ban the use of pesticides in the growing of grapes, demanded better working conditions, better pay for the workers not only in California, but all over the country..(Ref.7)


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