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Introduction

In the first part of this article series, we explored opportunities for producers to increase efficiencies of inputs via strategies such as seed treatments, foliar inputs and carbon based inputs. Achieving ‘more with less’ is a common catch phrase often heard within many industries, particularly so in agriculture. Optimising any input is in every farmers interests for their own financial benefit but this also typically comes with a win-win for environmental sustainability as well. There has been growing concern regarding potential negative effects and unintended impacts of agricultural inputs particularly so with pesticides¹. Consequently, a range of biological and botanical alternatives are emerging – typically of microbial or plant origin – and currently becoming more common in the marketplace. There is significant investment in research and development of novel compounds for commercialisation and in 2019 it was estimated the value of the biostimulant market worldwide was over US$2 billion². It has also been estimated that approximate 3000 tonnes of biopesticides are manufactured annually³. Many of these natural compounds can have herbicidal, fungicidal or insecticidal properties and undoubtedly more and more will be commercialised in coming decades. These inputs can play an important role in substituting synthetic inputs and can assist farmers when transitioning towards soil health.

Bioinsecticides

Bt-based biopesticides are one of the more common synthetic alternatives and constitute over 50% of the market share, followed by other microorganisms and active compounds of plant origin³. Microorganisms particularly are an abundant source of insecticidal compounds. Actinomycetes are a highly diverse bacterial phylum who are known to produce an extraordinary array of metabolic compounds that have been exploited as antibiotics for human use and also for a host of compounds that can attract and kill insect pests⁴. Entomopathogenic fungi are also a well studied group of fungi who are known to cause disease and death in their insect hosts. There are 750 species known and they are a ubiquitous fungi in many ecosystems around the world⁵. Although there are many commercialised products available based on these microorganisms, they are generally present in most soils and hence strategies to improve soil health in a general sense will help to optimise the presence and activity of these organisms. During high insect pressure situations however, targeted application and use of these task specific organisms can be warrented^6,7

Among the plant world, there are many active ingredients that have insecticidal, repellent or antifeedant properties and have been identified and extracted from plant material. Ground or whole plant material and oils or aqueous extracts of garlic, neem, chili pepper, clove, marigold, rosemary, nettle, orange, tobacco and thyme have all proven to be effective against many insect pests.^8–10 Many of these types of botanicals have yet to break through to mainstream adoption often due to the laborious process of manufacture or cost of production. However, this will likely change in future with stricter pesticide regulations and many pesticides being removed from the marketplace.

Biofungicides

Plants are known to possess a myriad of biocidal compounds that can compromise bacteria, fungi, nematodes and virus’ and generally these compounds possess lower human and environmental adversities. I must stress of course, just because something is naturally occurring does not mean it is somehow harmless, but on balance, naturally derived compounds do typically possess lower toxicity and lower persistence in the environment^6,7. Comprising a diverse mix of active ingredients rather than a singular active, plant extracts also induce much slower development of resistance in pest populations⁶. There are a host of compounds that plants synthesise internally for their own protection such as essential oils, phenolic compounds and flavonoids for example, and these compounds can be extracted from plant material for in-field applications⁶.

Additionally, there are a host of beneficial microbes which can antagonise disease causing organisms via competitive exclusion or via the production of antagonistic antibiotic metabolites¹¹. Microbially diverse inoculants such as manures, composts, compost extracts and other organic amendments have all demonstrated suppressive properties against many pathogens^7,12. An ever increasing range of commercial inoculants are also becoming available with task specific species – noteworthy microbes known to antagonise plant pathogens include Trichoderma spp¹³, mycorrhizal fungi¹⁴, Bacillus spp, and Pseudomonas spp^7,11. One other noteworthy mention is the role of plant nutrition to combat diseases. We might automatically consider certain nutrients such as copper or sulphur which have direct biocidal effects of pathogens, however, there is a significant body of evidence highlighting that optimising plant nutrient status can help fuel the plants own immunity against invading pathogens^15–17. Standout nutrients that can specifically be managed against pathogens include silicon, calcium, boron, manganese, copper, sulphur and potassium, but that said, a limitation of any one of the essential plant nutrients in one way or another can undermine plant physiology and development and hence plant immunity.

Bioherbicides

There has been a body of interesting work done on alternative herbicides but of all the bioalternatives, bioherbicides seem to have made the least progress. Much like it has held back innovation with synthetic herbicides, the widespread use of glyphosate has perhaps arrested some progress in the natural herbicide arena also. That said, with resistant weeds becoming a more prominent problem, the need for alternatives is becoming more pressing. A range of bioactive compounds have been demonstrated to possess herbicidal properties – common ingredients such as acetic acid, mineral oils, pine oil, seed meals or allelopathic compounds from plant extracts or stubble residues¹⁸. Biological agents such as fungi or bacteria have also been deployed with to induce disease and death in select weed species¹⁹. Like many biological products, this style of biological warfare often has rather mixed success when graduating from glasshouse to field trials. Once again, plant extracts and essential oils are emerging as an interesting candidate in this space. Ingredients that would appear more at home in your kitchen pantry rather than the chemical store – essential oils from clove, cinnamon, thyme, rosemary, eucalyptus, mint and citrus have all been shown to possesses varying degrees of herbicidal properties^18,20,21. Although the results of these studies are mixed and complete kill is not always achieved, these types of bioherbicides may have a place as a temporary knockdown or in companion and intercropping systems where a temporary suppression of one of the plant partners may be advantageous.

Integrating into a Systems Approach

If there is one take home from the discussion on bioalternatives, it is that there is significant potential for a vast array of naturally occurring compounds and microbials that can play a major role in mitigating losses to weeds, disease and insect pests. That said however, being of an organic origin, active ingredients in natural products like these can have variable concentrations and hence have variable effects on target pests. This variability is undoubtedly a key hurdle that limits their adoption, but in time however, can and will be improved and overcome. Like is often the case, more research and development is still needed to optimise the practical application of such technologies. However, this hurdle becomes much less of an issue when these bioalternatives are used as part of an integrated pest management approach and not as the sole mode of control – like we have traditionally used synthetic pesticides. They may not provide 100% control, 100% of the time, but by leveraging a multifunctional approach, integrating multiple tools into the management strategy, means that individual actions work together to strengthen the overall success of an ecological pest management program. Bioalternatives generally are more effective when combined with other tools such as intercropping, cover crops, crop rotations and cultural controls. All of these tools when integrated into the production system help to reduce the use of synthetic chemicals and obtain economically acceptable levels of pest control.

Conclusion

After optimising input efficiencies and achieving more with less, substituting synthetic pesticides for biological based alternatives can be an important next step in a transition toward soil health. There are many blind spots and knowledge gaps to fill in terms of achieving reliable pest management with bioalternatives, but it is clear the potential is there and innovation in this space is moving in the right direction. With pesticide resistance and product withdrawals more and more immanent, the widespread adoption of these alternatives may happen sooner than we think. This transitional, substitution phase can help pave the way towards redesigning production systems within regenerative and agroecological frameworks. This will be the focus of the next print article in terraHORSCH toward the end of the year and before that, we will explore emerging concepts into the nature of soil organic matter and the microbial pathway to SOM accumulation [once again online].