Due to the lack of restraint in how pesticides (for plants, insects, fungi etc.) are applied, it is unsurprising that pesticide and pesticide-related residues, are present in our environment. As pesticides are quite literally designed to kill, the issue comes from its impacts on non-target organisms. Ideally, an ‘ecologically safe’ pesticide should affect only its target organism, be non-persistent and have no unforeseen environmental effects. However, most pesticides lack these traits and hence are directly or indirectly toxic to non-target organisms, accumulate in either the environment or organisms (bioaccumulation) and have the capability to disrupt ecosystems.

As discussed on previous parts, the environmental impact of different pesticides is highly varied and linked to the method, form, and timing of their application.

But how did we as a species let it get this bad?

Merrington et al. (2005), had suggested the following reasons for the exponential increase of pesticide use in agricultural systems during and after the green revolution:

• Relatively low cost of pesticides followed by high returns in improved or higher quality yields, commonly combined with external financial incentives in the form of subsidies.
• Intensification of agriculture, including the introduction of high yielding varieties and large monocultures requiring greater pesticide application.
• Bad practices stemming from a lack of understanding of the consequences associated with excessive pesticide use.
• A tendency to try and achieve total protection from pests especially in high value crops, such as fruit and vegetables. Which is highly impractical in an open system.

After application, the impact of pesticides do not stop at the boundary of the farm, it has the potential to cause point source or diffuse pollution via the following pathways:

• by direct contamination of ground and surface waters.
• by soil contamination and subsequent leaching into ground and surface waters, surface run-off and/or soil erosion.
• by direct contamination of non-target organisms.
• by contamination of non-target organisms by residues or persistent compounds.

Point source pollution is often caused by poor agricultural practices and includes improper or leaking storage facilities, spillages whilst filling equipment, run-off from wash-down areas during cleaning, and the inappropriate disposal of unused pesticides.

Whereas diffuse pollution is a much more concerning problem as it may occur even when good agricultural practices are adhered to because a the vast majority of applied pesticides does not reach its target organism, with Pimentel (1995) reporting that that less than 0.1% of pesticides applied reach their target organism, resulting in more than 99.9% of pesticides moving into the environment where it causes adverse impacts. Which will be elaborated below.

• Emergence of Pesticide Resistance

Decades of extensive pesticide use has led to the rise of pesticide resistance. Through natural selection, pest organisms more adapted to frequent and widespread exposure to a specific pesticide would survive, making each subsequent generation more resistant. The emergence of resistance is not restricted to a single phylum of pest organisms, being seen in plants, animals, and fungi. This has led to farmers either increasing the dose of the current pesticide, further increasing the likelihood of resistance and exacerbating the issue and/or rotating to different types of pesticides, which while effective will also eventually lead to build-up of resistance, such as in the case of the Colorado potato beetle (Leptinotarsa decemlineata), which has developed resistance to 52 different compounds belonging to multiple insecticide classes.

• Persistence in Environment and Contamination

The persistence of a given pesticide is a function of its ability be degraded (e.g. when exposed to sunlight, decomposed by soil microbiota etc.). Some synthetic pesticides are highly persistent and do not break down easily, lingering in the environment for long periods of time. In the environment the fate of pesticides can be quite varied, some may be adsorbed, binding tightly to the surface soil particles and organic matter, which makes it less available for microbes to degrade. Leaching is another issue as the movement of water down through the soil may potentially transport pesticide residues to further soil, surface waters or to ground water. A study by Silva et al. (2019) reported that that the presence of pesticide residues mixtures in agricultural soils are the rule rather than the exception in Europe, with over 80% tested soil samples containing pesticide residues (Fig 1).

Some pesticides can be readily degraded but lead to the production of highly toxic intermediate metabolites that are more hazardous than the original compound. One example is the chemical or microbial breakdown of DDT producing highly toxic DDE, which may potentially cause male sterility. In some cases these breakdown products may be volatile and may readily enter the atmosphere.

• Bioaccumulation

Although it’s moderately safe for humans to handle in low doses, While DDT is relatively safe to handle in low doses (such as the exposure to DDT experienced by farmers during application), it may build up, in the fat tissues of exposed animals, leading to effects such as bioaccumulation and biomagnification, as the effects of the pesticide intensifies down the food chain.

 

Despite all these negative effects, pesticides are an unfortunately necessary evil in order to ensure a robust food supply. But while we cannot simply stop using pesticides, we can improve the way in which they are use. In our next segment we will briefly discuss how to minimize the damage caused by pesticides through better practices and technology.