The impact of pests, weeds and diseases
For many urban consumers, the fact that animal pests, diseases and weeds (or ‘agripests’) pose a significant economic burden to farmers and a continuing threat to food security goes largely unnoticed.
However, examples of the impacts abound. In Ireland beginning in 1840, epidemics of the disease potato blight (caused by Phytophthora infestans) led to the ‘Great Hunger’, a famine resulting in mass starvation, the death of roughly a million people and the ensuing migration of at least another million between 1845 and 1852, resulting in a 20 per cent drop in the Irish population.
To this day, locust swarms have similarly destroyed crops and been a contributory cause of famines and human migrations throughout history. Meanwhile, globalisation of trade and motorised transport of people has enabled unprecedented opportunities for agripests to rapidly spread and establish around the globe. For example, fall armyworm, an invasive moth that can damage a wide variety of crops, has spread rapidly around the world (including to Australia) in the space of less than five years. It is an alarming rate of spread, greatly aided by globalisation.
Weeds, while less catastrophic in their effects, have also been a major threat to productivity throughout the history of agriculture. Prior to the adoption of modern approaches to weed control (beginning in the 19th century), one of the main tasks on farms was removing weeds from fields, generating a huge demand for human labour in rural areas.
Another concern is the deliberate introduction of pests for nefarious purposes. Today, most of the world’s cacao is produced in Africa. But as recently as the early 1990s, Brazil was also a major producer, until a fungal pathogen (witches’ broom) was deliberately introduced into cacao plantations to weaken powerful landowners. The industry was devastated, with the desired outcome achieved: production in Brazil fell by 75 per cent.
The history of agricultural pesticides
Farmers have been looking for new ways to control agripests since the advent of agriculture. Many of the earliest pesticides were simply based on dried plant leaves.
One product familiar to many gardeners, Pyrethrum, is based on a plant-derived organic compound sourced from flowering Chrysanthemum plants, which was used by the Persians as early as 400 BC. The 19th century saw interest in the agripest control properties of inorganic chemicals, mostly containing arsenic, sulphur or copper.
One of the earliest inorganic chemical pesticides to be developed was ‘Bordeaux mixture’, a combination of copper sulphate and lime. Originally used as a visual deterrent to stop children from stealing grapes in the 1880s, French viticulturalists quickly realised that the mixture was highly effective in controlling grape powdery mildew. While valuable in some contexts (e.g. Bordeaux mixture is still an effective fungicide by modern standards), inorganic pesticides are often toxic to humans and other mammals, while plant derived organic pesticides are expensive to produce and often unstable (e.g. natural pyrethrins break-down in sunlight).
Beginning in the 1940s, chemists and chemical companies started to more widely utilise organic chemistry to synthesise and commercialise pesticide products. Many of these were broad spectrum (i.e. poisonous to entire groups of organisms) and initially proved to be spectacularly effective compared to previously available pesticides.
However, high levels of residual toxicity and the indiscriminate use of many of these broad-spectrum, first-generation pesticides resulted in significant harm to both the environment and human health. These problems are clearly illustrated by the story of DDT (dichlorodiphenyltrichloroethane).
A broad-spectrum insecticide, DDT was one of the first synthetic organic pesticides to be released for widespread use. Despite initially proving to be of great benefit for pest control, the cautionary tale of DDT is well known, owing in large part to the 1962 publication of “Silent Spring” by Rachel Carson, which documented both the ecological devastation caused by indiscriminate use of DDT and the problems emerging due to the widespread evolution of insect pest resistance. Its legacy continues to be unearthed to this day.
Increasing understanding and awareness of the environmental and health effects associated with pesticides has led to better regulation, use and monitoring of agricultural chemical usage. In Australia, the responsibility for regulation of new products rests with the Australian Pesticides and Veterinary Medicines Authority (APVMA), while environmental monitoring is largely the remit of state government agencies.
Regulation and monitoring have led to the withdrawal of many chemicals from the market, and increased effort to develop pesticides with low residual toxicity and increased specificity to the target pest. However, increased regulation has increased the costs and slowed the development of new chemicals. Chemicals that leave persistent residues have been replaced with alternatives that in many cases are more immediately toxic to humans and other species.
The positive impacts of the development of a large, profitable and global commercial agrichemical industry can be observed in the form of abundant, affordable food in markets and grocery stores around the world. Pesticides were an enabling tool of the “green revolution”, an extraordinary period of food crop productivity growth. It led to countries like Bangladesh rapidly becoming self-sufficient for staple foods.
There have also been environmental benefits. In Australia, herbicides have allowed the wide-spread adoption of reduced tillage systems, improved profitability in broad-acre agriculture, and the subsequent reductions in soil erosion and improvements to soil health.
However, pesticide use has also come with unwanted consequences, including toxicity and off-target effects. The intensive use of registered products and a lack of alternatives has resulted in another negative side effect: the widespread emergence of chemical resistance. Resistance emerges when genetic changes in target agripest populations results in decreased susceptibility to a previously effective pesticide.
New pesticides have been developed via increased knowledge of botanical insecticides, (e.g. the pyrethroid insecticides from pyrethrins). However, these botanical extracts are still very toxic insecticides, and several neonicotinoids (a class of insecticides) have attracted concerns over their impact on non-target species such as bees.
Meanwhile, despite most farmers using herbicides within regulatory limits, no-till farming has resulted in Australia having one of the biggest problems worldwide with weed species resistant to herbicides, while farmers are required to keep livestock away from sites once used for dipping sheep due to potential arsenic poisoning having leeched into the soil.
Scientists continue to raise concerns regarding wide-spread pesticide use on farm worker health, the environment and resistance evolution. These concerns often reflect the fact that regulatory systems were designed to mitigate acute risks, but do not address the impacts that wide-spread and cumulative use of multiple chemicals have in our farming systems today.
Increased understanding of unintended consequences of pesticide use has led to development of ‘an integrated approach to manage weed, diseases and pests’. While integrated approaches have not been universally adopted, there are some successful examples.
The Australian cotton industry today has moved from a catastrophic situation in the 1970s of 20-30 insecticide applications per season to control moth pests. Today cotton farmers see the benefits of a well-used integrated pest management system combined with an effective resistance management strategy to support the long-term use of genetically modified cotton plants that express a naturally occurring soil bacterium, Bacillus thuringiensis (Bt). Bt controls cotton’s major pest Helicoverpa and as a result, the amount of insecticide used per hectare by cotton farmers has dramatically declined to only one or two insecticide applications per season for other pests.
The future of agripests and treatments against them
The reality today is that alternatives to pesticides are often more costly, harder to implement, and riskier for farmers. There is also little incentive for farmers to manage resistance evolution and adopt stewardship practices, for which they are almost solely responsible. These challenges are compounded by a general unwillingness from industry and consumers to absorb costs associated with growers adopting more sustainable practices.
The R&D investment needed to make alternatives a viable option for many farmers is unlikely to come exclusively from agri-businesses. But public funds devoted to agricultural R&D has decreased in many developed countries in recent years. Who will pay for alternatives that cannot be packaged and sold from a shop front?
Food shortages caused by agripest outbreaks are rare in the modern world, but with the ongoing evolution of resistance and withdrawal of chemicals from the market, the need for new tools for crop protection will continue. The devastating health and societal impacts of the ongoing novel coronavirus pandemic illustrate the importance of preparing for the potential emergence of new and even more destructive pathogens, as increasing global connectivity helps facilitate their evolution (e.g. via hybridisation) and spread.
Many countries around the world are conducting research to make pesticide alternatives more user-friendly, economically profitable and target specific. Furthermore, we have a greater understanding of the motivations and drivers for change of practice (or absence of change) at the farm, community, and country-level.
Agri-chemicals will continue to be used in some way to address pest problems for the foreseeable future, but we must find ways to make their negative consequences less far-reaching and long-term. Changes are also needed to mitigate resistance evolution and extend the practical effective lifetime of pesticides.
We do not advocate abandoning the pest-control innovations that are so crucial to present-day food-security and farmers livelihoods. But rather we are looking ahead and focusing on a new wave of pest control approaches that are truly sustainable and integrated into farming systems.