Overcoming a chemically limited future for weed, pest and disease control in agriculture
Chemicals come and go
Agricultural chemicals and veterinary pharmaceuticals (agrichemicals) can substantially improve animal and plant health and productivity by controlling weeds, pests and disease, or “agripests”. But the range available for use changes over time. Many products contain the same active chemical, so the ‘options’ available concern the number of actives available for use, not the number of products.
The different ways chemicals kill target organisms – their modes of action – are important and chemical groups have been defined to help users understand this. History has seen an evolution in actives with sometimes whole chemical groups being phased out over time. This has happened globally, and an example is the organochlorine group of insecticides to which DDT (Dichlorodiphenyltrichloroethane) belonged.
Although these chemicals were initially effective and thought to have no toxicity to vertebrates, their use contaminated the environment, causing a range of negative effects. For example, it reduced eggshell stability in predacious birds, threatening their survival. Organochlorine insecticides have been phased out across many countries, and the last active in this chemical group for agricultural use, dicofol, was phased out in Australia during 2020.
However, social, economic and environmental factors are producing a chemically limited future, where our current use of agrichemicals will be constrained. Our work at CSIRO is looking at alternatives to help the industry transition into new ways of thinking.
The future will have fewer chemicals
The rate at which new agrichemical actives are brought to market has slowed during the 21st century. This is mostly due to a high failure rate for developing new products and the high cost to make them.
Products intended for agricultural applications need to have very low per unit costs. Although agriculture is very large globally, it is very fragmented by commodity, geography, climate and national boundaries. These factors mean that research for new agrichemicals is a higher risk investment compared to research for similar products sold for medical, household or manufacturing uses.
Meanwhile, social pressure to phase out the use of current agrichemical actives and groups may soon outweigh the rate of introduction of new actives and groups.
Increasing sophistication of trade deals internationally and of consumer preferences has also drawn more attention to production methods, including the use of agrichemicals, and this results in market and regulatory pressures to limit or restrict agrichemical usage, for example restrictions of some antibiotics in meat destined for the Russian market.
Resistance can be slowed down, but not stopped
If you have heard the argument against overuse of antibiotics, as it can lead to bacterial superbugs, the same applies to the usefulness of agrichemicals for food and fibre production.
Genetic selection underpins change in all organisms over time. Over relatively short time frames, selection can lead to changes in the characteristics of organisms, and chemical resistance is a classic example. Chemicals used to kill agripests apply significant selection pressure to these populations; this is intrinsic to what they do.
A few factors are important for determining the rate at which resistance develops to a chemical; the proportion of the population exposed, the amount of genetic variation pre-existing in the population, as well as how often the treatment takes place and any advantages or costs to being resistant in the selection environment.
In addition, migration, dispersal mechanisms and mating systems of the organisms can alter the rate at which resistance develops locally and determine the rate at which resistance genes from other populations can enter the population. Mutation rates are also important with larger populations offering more opportunities for individuals to harbour new mutations. Agripest populations tend to be large, for example barber’s pole worms can lay up to 10,000 eggs a day, and thousands of worms can reside within an individual sheep.
Being objective about when it is necessary to use chemicals can help limit application frequency. Increasing the complexity of the selection process will slow resistance. This can be achieved by using chemicals with differing modes of action in rotation, or in combination products.
Rotating between products, and using products containing multiple actives require the target organisms to alter multiple resistance genes, complicating and slowing the selection process. Alternating chemical with non-chemical control options can produce the same effect.
Vaccines stimulate the immune system of animals to produce a variety of effects on the target organisms, as illustrated by CSIRO’s recent work to develop ways of identifying livestock with resilient immune responses.
Natural predators released in fields or glasshouses also adopt a variety of mechanisms to kill their prey, and even better can undergo population change themselves to evolve to an adapting prey species.
Alternatives to chemicals
Globally, research organisations have produced a large suite of approaches to agripest control that do not depend on chemical products.
Some of these are used in a very similar way to agrichemicals including biological-based sprays, periodic release of biocontrol organisms and protected antigen vaccines. Other approaches have much longer-term effects, such as traditional vaccines, genetic selection of disease resistant crops and livestock and release of stable populations of biocontrol agents.
Re-design of agricultural systems can also produce situations where the impact of agripests is reduced. The timing of operations, the mix of different production enterprises and the manipulation of farm landscapes are examples of re-designing agricultural systems. For example, a current CSIRO project has investigated altering the time of lambing to reduce exposure of lambs to parasitic diseases occurring in summer and so reduce reliance on parasiticides.
A bright future if we keep our eyes open
The experience of developing and using agrichemicals through the 20th century has taught humans some valuable lessons which should not be forgotten. Even in a chemically limited future, the principle of developing agripest control measures which do not have harmful side effects is imperative.
Objective assessment of the consequences of any control measures for farm profitability, the environment and the safety of farm workers, local communities and consumers remains the principal concern. The existing regulatory processes which have developed for agrichemicals are already being applied to some non-chemical products, but we must ensure that similarly rigorous investigations underpin the use of other non-product-based redesign systems.
Equally, the use of control methodologies and the impacts of these practises should be monitored over time to improve agripest control and guard against unforeseen consequences.
Agriculture is arguably the most important undertaking that ensures the continuance of human civilisation. Agripest pressures can have dramatic effects on our ability to produce food and fibre (of which there is a growing need as population increases), and the loss of methods to control or avoid these effects is a significant threat.
A wealth of basic and applied scientific research has led to a situation where multiple possibilities for agripest control exist or are adaptable for new applications. With minds open to change combined with an objective and cautious approach, the future of food and fibre production need not be limited by a decreased reliance on chemicals for agripest control.
CSIRO would like to know your thoughts on this topic. Please let us know:
1. How agrichemicals impact your day-to-day life right now
2. What you see as the biggest issue in a chemically limited future scenario
3. What you think are the highest priority research areas to focus on