This is a huge loss, and a significant barrier to feeding an ever-growing human population. The UK’s National Institute of Agricultural Botany (NIAB) is addressing this challenge. Professor Xiangming Xu and Dr Michelle Fountain both work at NIAB East Malling, although they have very different research backgrounds. Xiangming’s career has focused on genetics and how plant diseases spread, while Michelle’s work centres on the ecology of insects that interact with crops. Both these areas of expertise, among many others, are proving vital to tackling crop pests and diseases.
Their work can be divided into these four key areas.
1) UNDERSTANDING MICROBIOMES TO IMPROVE PLANT HEALTH
Every plant or animal has its own microbiomes: communities of microbes interacting with one another, and with the organism that hosts them. Crops are no different. “Scientists have known about the roles of individual microbes on plant health for years. However, it is only recently that we have been able to profile whole microbiomes,” says Xiangming. This has been made possible through advances in DNA sequencing techniques. Scientists can now extract DNA from an environmental sample and work out the genetic codes of microbes present within the sample. These codes can be used as ‘fingerprints’ to identify the communities of microbes that live on or in a particular plant or plant tissue.
These advances have opened a lot of doors. “We need to establish whether plant diseases are associated with particular microbial communities in different parts of a plant – for instance, on the roots, the leaves or within the plant,” says Xiangming. Once researchers have uncovered these relationships, they can investigate the biological processes behind these associations, as Xiangming goes on to explain: “This can help us to design better crop management measures that influence these microbial communities. For instance, we can change growing conditions (such as through irrigation and ‘watering’) to promote a certain microbiome component. We could also add specific beneficial microbes into the soil or onto plant roots or leaves.”
A novel way of fending off fruit flies provides a great example of this. These insects are a pest of many fruits, laying eggs on the plants and feeding off the fruits’ sugars. Female flies that are developing eggs also need a source of protein, which they obtain from certain yeasts that grow on the surface of fruits. “It may be possible to manipulate this microbiome to change the community of yeast species that grow there,” says Xiangming. “By promoting species that are less attractive to fruit flies, we may deter them from laying eggs on the fruit.”
2) USING PESTS’ NATURAL ENEMIES AS BIOCONTROL AGENTS
Biological control agents (biocontrol agents for short) are organisms that naturally keep pest/pathogen species in check. These ‘natural enemies’ include predators, parasites, competitors of pests and pathogens. Ladybirds, a voracious predator of aphids, are a familiar example often welcomed by gardeners. “By adopting crop management practices that encourage natural enemies, we hope that their populations will suppress pests and diseases of crops,” says Michelle. “This reduces the need for growers to use other potentially harmful control methods, such as pesticides.”
Understanding exactly how to encourage these natural enemies is where researchers like Michelle come in. “In order to harness the benefits of biocontrol agents, we must first understand their optimum environmental conditions for survival and reproduction,” says Michelle. Temperature and humidity provide two straightforward examples of this. Optimum conditions are also affected by interactions with other species in the ecosystem, such as pests and diseases, the crop itself, and each other.
Since some pesticides can kill natural enemies just as effectively as the pests themselves, reducing their use is an important strategy for encouraging natural enemies. Other methods involve encouraging plants that support natural enemies (such as by providing nesting sites, resting, hibernation or pollinator-friendly vegetation) or altering management patterns to create climatic conditions that favour natural enemies.
Agriculture does not exist in an isolated bubble, and this is especially true when it comes to natural enemies. Any member of the public with a garden can grow flowering plants or provide nesting sites for insects and other creatures – log piles, ‘bug hotels’ and not being a super tidy gardener are all good examples. “If you do not have a garden, getting involved with local nature reserves, allotments and conservation groups will help to improve habitats for natural enemies,” says Michelle.
Many insect pests rely on signalling chemicals (known as semiochemicals) to find food and mates or to avoid predators. Farmers can manipulate these chemicals to send out the ‘wrong’ signals to these pests and prevent them affecting crops. For instance, pheromone traps can lure male insects to a certain point where they mistakenly believe they are finding a female; instead they meet a sticky end. In a similar vein, flooding female attractant (sex) pheromones through the whole field can make it impossible for males to pinpoint where the females are, so preventing reproduction. Farmers can also apply ‘insect alarm pheromones’ to crops – chemicals usually used by pests to signal danger, provoking a hasty retreat.
“Because these chemicals are naturally produced at very low concentrations by the insects themselves, and are often species specific, they have fewer negative impacts than pesticides,” says Michelle. Rather than applications of insecticides that may harm non-target insect species in the crop, semiochemicals only target the pest species at key times in their reproductive cycle and are only needed in small amounts.
Michelle has worked on a trap that captures two pest species simultaneously by using two different semiochemicals. The first is an aggregation pheromone, normally produced by male strawberry blossom weevils to gather other weevils for mating. The second is a sex pheromone, normally produced by female European tarnished plant bugs to attract males. By attaching an attractant lure to traps, both strawberry pest species are captured. Unlike the application of some insecticides, the impact on other species is minimal.
Farm to Fork: New rules to reduce the risk and use of pesticides in the EU
4) CREATING THE FUTURE WITH PRECISION TECHNOLOGY
Mathematical models might seem a world away from fields and orchards, but in fact they are revolutionising the way that crops are grown. “By understanding the relationships between pests, pathogens and environmental conditions, we can develop mathematical models to predict how factors such as weather will influence the severity of pests and pathogen in a crop” says Xiangming. Natural enemies can be included in these models, too.
The more factors included in these models, the more sophisticated they become, as Xiangming explains: “We can use remote sensing techniques to measure the spread of pests or diseases in a crop in real time, and use this information to make better decisions, such as precisely adjusting the dose of biocontrol products,” says Xiangming. This ‘precision dosing’ means that no more product is used than is necessary, reducing costs for farmers and minimising any negative outcomes of biocontrol use.
Precision technologies are becoming increasingly important throughout agriculture. One up-and coming spraying system involves real time monitoring of pests and diseases throughout an orchard via remote sensing; only those areas with pests and pathogens detected will be treated when absolutely necessary. This means overall pesticide use is dramatically reduced, which is good news for biodiversity (including natural enemies) and reducing airborne or waterborne pesticide drift into the wider environment.
In some cases, there could be the potential to eventually eliminate pesticide use entirely. “By being able to detect pests and pathogens early, and predict their development more accurately, we can use prevention methods (such as biocontrol) to stop them significantly damaging crops,” says Xiangming. Combining a deep ecological understanding with the technological means to act on findings is becoming ever-more feasible. The future could be bright for the way we grow our food.
Key facts
Some of the older, less costly pesticides can remain for years in soil and water. Many of these chemicals have been banned from agricultural use in developed countries, but they are still used in many developing countries.
Pesticides play a significant role in food production. They protect or increase yields and may increase the number of times each year a crop can be grown on the same land. This is particularly important in countries that face food shortages.
To protect food consumers from the adverse effects of pesticides WHO reviews evidence and develops internationally-accepted maximum residue limits.
Pesticides are used to protect crops against insects, weeds, fungi and other pests.
Pesticides are potentially toxic to humans and can have both acute and chronic health effects, depending on the quantity and ways in which a person is exposed.
People who face the greatest health risks from exposure to pesticides are those who come into contact with them at work, in their home or garden.
Pesticides from farming leach into world’s waterways at rate of 710 tonnes a year, UN research shows
Overview
There are more than 1000 pesticides used around the world to ensure food is not damaged or destroyed by pests. Each pesticide has different properties and toxicological effects.
Many of the older, less costly (off-patent) pesticides, such as dichlorodiphenyltrichloroethane (DDT) and lindane, can remain for years in soil and water. These chemicals have been banned by countries which signed the 2001 Stockholm Convention, an international treaty that aims to eliminate or restrict the production and use of persistent organic pollutants.
The toxicity of a pesticide depends on its function and other factors. For example, insecticides tend to be more toxic to humans than herbicides. The same chemical can have different effects at different doses, that is, the amount of chemical to which a person is exposed. Toxicity can also depend on the route by which the exposure occurs, such as by swallowing, inhaling or direct contact with the skin.
None of the pesticides currently authorized for use on food in international trade are genotoxic (damaging to DNA, which can cause mutations or cancer). Adverse effects from these pesticides occur only above a certain safe level of exposure. When people come into contact with large quantities of pesticide, the result may be acute poisoning or long-term health effects that may include cancer and adverse effects on reproduction.
Scope of the problem
Pesticides are among the leading causes of death by self-poisoning, particularly in low- and middle-income countries.
Since pesticides are intrinsically toxic and deliberately spread in the environment, their production, distribution and use call for strict regulation and control. Regular monitoring of residues in food and the environment is also required.
WHO has two objectives in relation to pesticides:
to ban the pesticides that are most toxic to humans, as well as pesticides that remain for the longest time in the environment;
to protect public health by setting maximum limits for pesticide residues in food and water.
Who is at risk?
The population most at risk are those who are directly exposed to pesticides. This includes agricultural workers who apply pesticides and anyone else in the immediate area during, and shortly after, pesticides are spread.
The general population – those not in the area where pesticides are used – is exposed to significantly lower levels of pesticide residues through food and water.
Prevention and control
Nobody should be exposed to unsafe amounts of pesticide.
People spreading pesticide on crops, in homes or in gardens should be adequately protected. People not directly involved in the spreading of pesticides should stay away from the area while spreading takes place, and for some time afterwards.
Food that is sold or donated (such as food aid) should equally comply with pesticide regulations, in particular with maximum residue limits. People who use pesticides when growing their own food should follow instructions for use and protect themselves by wearing gloves and face masks as necessary.
Consumers can further limit their intake of pesticide residues by peeling or washing fruit and vegetables, which also reduces other foodborne hazards such as harmful bacteria.
Global impact
The United Nations Population Division estimates that by the year 2050 there will be 9.7 billion people on Earth – around 30% more people than in 2017. Nearly all of this population growth will occur in developing countries.
The Food and Agriculture Organization of the United Nations (FAO) estimates that in developing countries, 80% of the increase in food production needed to keep pace with population growth, is projected to come from either increases in yields and/or the number of times each year crops can be grown on the same land. Only 20% of extra food production is expected to result from an expansion of farming land.
Pesticides can prevent large crop losses and will therefore continue to play a role in agriculture. However, the effects of exposure to pesticides on humans and the environment are a continuing concern.
The use of pesticides to produce food, both to feed local populations and for export, should comply with good agricultural practices regardless of the economic status of a country. Farmers should limit the amount of pesticide used to the minimum necessary to protect their crops.
It is also possible, under certain circumstances, to produce food without the use of pesticides.
WHO response
WHO, in collaboration with FAO, is responsible for assessing the risks to humans from pesticides, whether through direct exposure or residues in food, and for recommending adequate protection measures.
Risk assessments for pesticide residues in food are conducted by an independent, international expert scientific group, the Joint FAO/WHO Meeting on Pesticide Residues (JMPR). These assessments are based on all data submitted for national registrations of pesticides worldwide, as well as all scientific studies published in peer-reviewed journals. After assessing the level of risk, JMPR establishes limits for safe intake to ensure that the amount of pesticide residue to which people are exposed through eating food over their lifetime will not result in adverse health effects.
These acceptable daily intakes are used by governments and international risk managers, such as the Codex Alimentarius Commission (the intergovernmental body that sets food standards), to establish maximum residue limits (MRLs) for pesticides in food. Codex standards are the reference for international trade in food, meaning that consumers everywhere can be confident that the food they buy meets the agreed standards for safety and quality, no matter where it has been produced. Currently there are Codex standards for more than 100 different pesticides.
WHO and FAO have jointly developed the International Code of Conduct on Pesticide Management. The most recent edition of this voluntary framework was published in 2014. It guides government regulators, the private sector, civil society and other stakeholders on best practices in managing pesticides throughout their lifecycle, from production to disposal.
