Protect crops by increasing plant diversity in agricultural areas

Without a pest control strategy in place, plant pathogens, weeds and insect pests can all cause damage to crops, diminishing not only yields but also the quality of the harvest. Farmers mainly rely on chemical solutions, but these products have a significant impact on the environment and human health. Society wants farmers to stop using pesticides, and while national and European public policies are increasingly taking this expectation into account, they often have little effect on the ground. This reality led the French Ministries of Agriculture, Research and the Ecological Transition to ask INRAE, in the framework of the Ecophyto plan, to produce a collective scientific assessment to evaluate the effectiveness of crop protection strategies based on plant diversity and to analyse the barriers and levers involved in their deployment.

This joint, multidisciplinary undertaking, which was based on an analysis of international scientific literature, reveals findings that are well established by research:

• Increasing plant diversity in fields and landscapes to protect crops does indeed work.
• Plant diversity also promotes biodiversity and ecosystem services for farmers and society, such as water regulation and carbon storage.
• These agroecological solutions go hand in hand with yields that are often higher and more stable from one year to the next than in conventional systems.
• Adopting such solutions will require lifting various barriers as well as a major shift away from today’s specialised cropping systems.
• However, there are ways to overcome these barriers, and public policies have an essential role to play in promoting plant diversity.

Video of the final symposium (with English interpretation)

How much diversity is enough?

Introducing greater plant diversity can make agricultural areas less hospitable to insect pests, weeds and plant diseases by reducing their host plants and promoting a favourable environment for their predators and competitors. But how much diversity is enough? International scientific literature can offer some insights. For example, varietal mixtures should consist of four to five varieties to effectively control diseases. Today, such mixtures include just two to three varieties and account for 17% of France’s agricultural area planted with wheat crops. For intercropping, selected species should have different sensitivities to diseases and insect pests and be complementary in terms of their nutrient needs. Today, intercropping is implemented on only 0.1% to 3% of crop areas and combine a cereal and a legume. Different crop sequences in the same field (crop rotations) should be planned for more than three years. They should alternate winter- and spring-sown crops, legumes (which fix atmospheric nitrogen in the soil), and crops such as rape to better regulate weeds. To support biodiversity, the optimal arable field size is around 2.8 ha, whereas more than half of the agricultural land is occupied by fields larger than 6.8 ha. Finally, cultivated areas need to include 20% semi-natural landscape elements (hedges, woods, etc.) to regulate the insects, mites and other pests that attack crops and to conserve biodiversity and the ecosystem services it provides. With 300 metres of hedgerows per hectare, both yield and biodiversity can be maintained.

The practice of agroforestry, which combines trees and crops (and sometimes livestock) in a single field appears to be a particularly interesting approach because of the variety of associated ecosystem services it provides: biodiversity maintenance, water regulation, water and soil quality, carbon storage. And yet, it is practised on less than 1% of agricultural area in France.

Aside from the use of cereal varietal mixtures, these diversification practices are currently not widespread among farmers. In today’s economic context, farmers often feel that adopting diversification practices to protect crops is a riskier choice than relying on pesticides. These products are seen more as inputs that reduce risk at a low cost.

What are the major barriers and levers?

Strong incentives must be provided to change practices

The industrialisation of agriculture was strongly encouraged by public policies in the second half of the 20th century. These policies led to a narrow specialisation of farms and regions, made possible by the widespread use of synthetic pesticides, and resulted in a loss of plant diversity. The current system is thus plagued by systemic lock-in that is resistant to change.

Protecting crops is no longer about aiming for zero pests, weeds or diseases, but rather about regulating their populations by marshalling the biodiversity present in the agricultural ecosystem. Insect pests, weeds and diseases cause damage to crops (e.g., holes in fruit) when present above a certain threshold, but that damage may not necessarily lead to yield losses (e.g., if the damage is limited to non-harvested parts of the plant). A farm’s overall profitability also depends on the selling price (i.e., consumers’ willingness to pay for products that do not meet conventional standards) and costs related to inputs, labour and equipment. Stable long-term profits are also an important factor for farmers.

Using varietal mixtures requires the availability of suitable varieties, while intercropping requires equipment adapted to sowing and harvesting (grouped purchases, subcontracting certain tasks, or making one’s own equipment can help farmers better cope with associated costs). Integrating semi-natural elements into the landscape requires an organised collective effort and coherent public policies. Sharing knowledge and experiences is crucial in all cases, as is the development of markets for products from diversified systems. Cooperatives, which liaise between upstream and downstream stakeholders, could be key players. For example, researchers in Italy observed increased plant diversity and better yields when more cooperatives played an active and incentivising role.

How can we support plant diversity to protect crops?

Regulations – and specifically the requirements farms must meet for organic certification – are the main tool used today to curb synthetic pesticide use. However, this type of certification does not impose plant diversification techniques. But combining these two approaches seems to offer a promising way to optimise and sustain biodiversity conservation, ecosystem services and yields while also improving the economic value of diversified production systems. Another inspiring model can be found outside France. Diversified systems, and even ultra-diversified systems such as the “Creole garden” approach, are used in 70% of farms in France’s overseas territories and are resilient to global change, whether environmental (climate change) or economic (market fluctuations).

The conclusions of this collective scientific assessment were presented and debated during a conference attended by nearly a thousand people, both online and in person. Many discussions revolved around ways to encourage the dissemination of this knowledge, starting at the vocational high school level, and support its implementation. As we shift towards agricultural systems with greater plant diversity, each and every one of us (farmers, industrial players, consumers, public authorities, research stakeholders) can play an active role.

INRAE, which is committed to research to support the agroecological and food transitions, is mobilising its research facilities, in particular the Ca-Sys platform for field crops and the Gotheron orchard, which implement diversity with zero pesticides. Workshop zones and living laboratories that bring together actors in the field are also essential to innovation in this area. The collective scientific assessment notes the benefits of developing remote sensing and modelling to better track diversification practices and understanding of their effects. Research should also aim to better understand the combined effects of different diversification methods to simultaneously regulate various crop pests. Finally, public policies that are both better designed and well evaluated are also necessary.