Pesticide spray

There is no doubt that the biggest challenges of any farmer are weeds competing with crops for space, light and nutrients, and insects engulfing harvests like wildfire. Effective pesticides (insecticides, herbicides and fungicides) are therefore miraculous inventions in the eyes of many farmers. They dramatically lower costs by saving time and energy, and most importantly, enable farmers to avoid numerous problems. Humans are known to be visual animals i.e. we have faith in what we see. However, many problems resulting from the use of chemical pesticides remain hidden from sight.

A “Nerve-Wrecking” History

What makes a pesticide “visibly” effective? It must be highly toxic and lethal to many different types of pest. Ideally, we need a shotgun that gives pests little chance to survive. First, let’s focus on insect pests, the eternal enemy of every farmer. There are countless ways to kill insects, however not all produce the same visual results. In order to see visible results, scientists figured out that the best way is to attack an insect’s nervous system instantly incapacitate them. This is how organophosphate pesticides were born.

These chemicals work by inhibiting the neuromuscular enzyme acetylcholinesterase which causes the overstimulation of nerves and their subsequent exhaustion. They are also very potent in the sense that they can be absorbed through all surfaces on an insect body – there is little chance to escape unharmed. However, little did farmers know that these “magical” chemicals were evolved (or co-evolved) from some of the most deadly chemical weapons i.e. nerve agents used in wars and genocides. Interestingly, mammals including humans use the same acetylcholinesterase enzyme in our nervous system. Therefore, organophosphates can also kill humans in a similar manner.

For instance, the infamous and highly toxic VX agent  used in Angolan Civil War and several other murders (including the assassination of Kim Jong Nam, Kim Jong Un’s half brother) is analogous to the pesticide Malathion structurally and chemically (figure 1). At low dosages and repeated exposure, organophosphates can cause a variety of cognitive deficits such as impaired short-term memory, information processing capability and shortened attention span and other symptoms such as blurred and dim vision, headache, numbness or tingling, insomnia, difficulty walking, and even abnormal heartbeats.  At high dosage, it will result in a painful convulsion and paralysis followed by death.

On the plus side, organophosphates are generally biodegradable and do not persist long enough to affect a wider spectrum of organisms. However, organophosphate are just a tip of an iceberg. Many “careless” agrochemists have developed less biodegradable nerve-agents pesticides such as organochlorides (including the infamous DDT). This has caused a series of misfortunes because they are relatively longer lasting and their lipophilic nature even allows significant bioaccumulation which affects a wide arrays of non-target organisms including humans. Despite widespread bans in most developed countries, the use of organochlorine pesticides has continued to rise especially in the developing countries.

Zero Chance to Live

Being visually effective and “actually” effective in killing a target pest populations are in fact two distinctly different concepts which most people, including farmers have a hard time realizing. In the case of tiny insect pest, farmers generally feel satisfied when they witness little insect activity. Therefore, a lot of farmers will over-spray insecticides – the majority (> 99%) of the pesticides will be taken up by non-target organisms. This temporarily leaves little chance of survival for the target pest population, but the pest’s natural competitors and predators face the exact same fate. In addition, the surviving pest population will eventually evolve resistance to the chemicals. This ultimately results in a more serious pest infestation during the subsequent growth cycles of the target pest. More and more toxic pesticides need to be applied at greater frequency to curb the increasingly uncontrollable infestations.

Weeds  

Weeds cause arguably more damage than all other pests, such as insects, combined. Curbing weeds is no easy task however. Since weeds in an actual field usually comprise of a mixture of different plant species, you often need a broad-spectrum (or a mixture of) herbicides to really get things under control. What this means is that the timing and location of herbicide application needs to be extremely careful in order to avoid destroying your own crop. In the case of using a selective weed killer, it often ends up that another more resistant weed will just take over.

An ingenious solution to these issues completely overturned the past practices. With the advent of gene-editing technology, scientists have introduced genes into crops (e.g. soybeans, corns, cottons) that confers resistance to broad-spectrum herbicides like glyphosate (the most widely used herbicide ever). In this way, all the hassles are eliminated since the timing, location, application method become irrelevant. All you need is to spray whenever something other than your engineered monocrop rises above the ground. Again, the scenario is the same as with insects, everything is wiped clean.

As good as it seems to be, a farm managed in this way leaves a serious impact on the ecosystem. Host plants of beneficial insect and barriers to plant disease are destroyed. It has also been reported that glyphosate and its metabolic product are found in most environmental samples in the US (soil, water, sediment) as well as in food in both the UK and US, with concentrations that are shown in animal models to cause significant hepatorenal damage and potentially other health defects including cancer and reproductive development impairment. Glyphosate is also widely used in Hong Kong and China. There is a dichotomy between a short-term visual result, and the actual long-term costs. Only one party that is clearly winning all the time, you can probably guess who that is.

A “Schrödinger’s”Future

Okay, you probably get how scary pesticides can be, but they still serve the function of killing pest which is pivotal to feeding the planet, right?  While making a truly justified value judgement is not in the realm of science, I am here to present you facts to reduce the information asymmetry.

Over the 40 years span from the early 1960s to the 2000s, global food production has doubled and land use in agriculture has increased by around 10%. However, pesticide use in the same span has increased by more than 15-fold. This means on roughly the same size of land, we applied at least 15 times more pesticide to secure a 2-fold increase in food production made possible also by other factors such as the much higher use of nitrogen fertilizer (almost 7-fold increase), increased irrigation (1.7-fold) and improved crop genetics. Another shocking fact is that crop damage attributable to pest have slightly increased compared to 50 years ago despite pesticide use skyrocketing. This suggests that our crops are increasingly susceptible to pest attack and requires more pesticide input per unit due to our poor agro-ecosystem resilience management.

It is indeed not easy to measure and quantify the actual damage that pesticides do to our environment and ecosystems, let alone making a fair comparison to the benefits (crop protection) they give us. While one can argue that saving even just one starving life could worth all the damage, it is important to realize that people at present and people in the future are in conflict to some extent, mediated by slow ecosystem processes. Not many would disagree that a progressive and healthy society should look to benefit not just present, but future generations. Therefore, it is important not to allow the act of saving life today to become nothing more than an excuse to maintain the status quo.

Yes, pesticides can be very useful. But this is only true if you have full knowledge and control of it. That is, no unnecessary over-spraying and non-target killing. However, attaining this complete understanding and control could be even more costly than other labor-intensive but target-focused organic methods of pest management. This is why most farmers have no choice but to spray indiscriminately. Again, you know it, who get the last laugh?

 


 

Figure 1. On the left is insecticide Malathion while on the right is the highly toxic VX agent (figure adapted from PubChem). Note that organophosphates act by phosphorylating acetylcholinesterase (i.e. the phosphate group is the main reactive site). Also, insecticides in general are bulkier and more complex structurally in order to reduce the toxicity (by slowing down reaction and making reaction intermediate less stable) and increase its specificity to target organism while nerve agents are designed to kill as quick as it can so there is no consideration of making them less lethal. (check this article for more detailed chemistry of organophosphates and acetylcholinesterase reaction)


 

**Article written by Mr. Chun Chung Yeung. Chun Chung is a former intern at Wildroots Organic. He is currently pursuing a masters degree at McGill University with research focus on greenhouse gas emission from organic farmland. He earned his BSc of Environmental Science with first honours at HKUST. He believes that in order for us to love and respect Mother Nature, we need to first try our best to understand her. Therefore, he is deeply passionate about public education**