Biological plant protection products and biostimulants – formulation challenges

Plant protection products play a significant role in protecting crops against pests, weeds, diseases and moulds, while biostimulants support plants against adverse weather conditions and improve their overall condition. Their usefulness is beyond doubt. In recent years, the search for substitutes for chemical pesticides has intensified, with biological alternatives gaining in importance. This category includes naturally occurring substances that control pests through natural biochemical mechanisms. These are substances of plant or animal origin (e.g. oils, saponins, organic acids) or bacterial metabolites (e.g. spinosad, abamectin, Bt toxins). In the case of plant growth regulators, preparations containing natural substances such as auxins, cytokinins and gibberellins have also been developed [2].

Biological plant protection products also include macroorganisms and microorganisms. Microorganisms that are active ingredients in biopreparations include viruses, bacteria (mainly Bacillus and Pseudomonas) and fungi (Trichoderma, Beauveria, Coniothyrium, Matharhizium, Pythium) ^[2].

Significant progress has also been made in the development of biostimulants that support biological processes in plants. There are many products available on the market based on: seaweed extracts, plant extracts, amino acids and proteins, humic acids and microorganisms. The latter group is unique because microorganisms can support plants in many ways: by promoting root system development, converting nutrients into more readily available forms, and additionally combating insects or fungi. Therefore, biostimulants based on microorganisms can often also be registered as plant protection products ^{[3, 4, 5]}.

Both groups of agrochemicals described above act in a very specific manner, are easily biodegradable and safe for the environment and consumers, and, most importantly, very effectively increase crop yields ^[3].

Biological preparations based on microorganisms

Microbiological plant protection products are the most widely used and researched class of biopesticides. These are preparations containing microorganisms such as bacteria, fungi and viruses.

Microbiological pesticides kill insect larvae or inhibit weed growth in cultivated fields. They act on plant pathogens and pests through various mechanisms, such as toxin production, enzyme secretion, volatile compounds, direct colonisation or host consumption. They can control many different types of pests, although each individual active ingredient is relatively specific. They are characterised by their ability to potentially multiply on crops after application. They also promote plant growth and improve crop quality ^[6].

Biostimulants and plant protection products based on Bacillus

Various species of bacteria can be used against soil-borne plant pathogens, insect pests or plant parasitic nematodes. These are spore-forming bacteria, including Bacillus (e.g. B. amyloliquefaciens, B. subtilis and B. thuringiensis) and Streptomyces, as well as non-spore-forming species from genera such as Pasteuria and Pseudomonas ^[2].

Biostimulants based on Bacillus bacteria occupy a special place among the microbiological agents used in agriculture. They consist of lyophilised spores mixed with additives and adjuvants (supporting substances) and bring a number of benefits to plants ^[7].

The metabolic and genetic diversity of Bacillus spp. allows them to adapt to different environmental conditions. It has been shown that bacteria associated with plants increase their resistance to stress caused, for example, by soil salinity or drought. They are also capable of producing phytohormones (plant hormones) that have a positive effect on plant growth and development ^[7].

The key to success – a well-prepared formulation

The process of formulating biopesticide preparations leads to the creation of the final product by mixing the microbiological component with various carriers and adjuvants for better protection against environmental conditions, greater survival of biological agents, and improved bioactivity and storage stability ^[6].

Biopesticide formulations can be divided into liquid and dry types. Liquid preparations can be based on water, vegetable oils, esters based on these oils, fatty alcohols, lactides or combinations thereof. Water-based preparations (suspension concentrates (SC), concentrated emulsions (EW), capsule suspensions (CS), etc.) require the addition of inert ingredients such as dispersants, emulsifiers, wetting agents, thickeners, rheology modifiers, cross-linking monomers, dyes, antifreeze compounds and additional nutrients. Depending on the type of formulation, the formulation process involves simple or high-shear mixing, wet grinding or in situ polymerisation. Dry formulations (dispersible powders (WP) and dispersible granules (WG), granules (GR) or tablets (TB) for direct application, etc.) can be produced using various technologies such as spray drying, freeze drying, air drying, with or without the use of a fluidised bed. There are also several granulation techniques: wet granulation by coating, dry granulation by powder compaction and crushing, as well as by extrusion and spheronisation. Here, too, binders, carriers, dispersants, wetting agents, etc. are added.

The most popular formulations of biopesticides and biostimulants are: SL, OD, CS and WP or WG. As already mentioned, microorganisms are particularly sensitive to external factors such as solar radiation, moisture or excessive storage temperature fluctuations, which is why OD and CS formulations are particularly suitable for them. Oil protects microorganisms from UV radiation, insulates them from moisture and protects them from temperature changes. In CS formulations, microorganisms are enclosed in capsules, which also makes them less vulnerable to external factors. Endospores or spores of bacteria and fungi can be dried, which is why they are also found in WP and WG formulations. Most extracts used as biostimulants are highly soluble in water and can be preserved with preservatives, which is why they are often found in SL formulations ^{[7, 8]}.

The organisms used in the formulations are suspended in a suitable carrier, which is supplemented with additives to maximise survival, optimise application to the target and protect the organisms after application ^{[7, 8]}.

The formulations contain one or more active substances and a number of additional ingredients to improve their effectiveness. These include safener (herbicide protectants), synergists, carriers, thickening and adhesive substances, wetting agents and others.

Formulation challenges

The effectiveness of biological plant protection products largely depends on their form. These preparations, based on natural substances, are much more sensitive to environmental conditions than their chemical counterparts. This is particularly true for biostimulants based on Bacillus bacteria. Therefore, the industry faces a number of formulation challenges to ensure that live microorganisms remain effective and stable over a long period of time ^{[7, 8]}.

Sensitivity of microorganisms to environmental factors

Some microorganisms are unstable and degrade rapidly when exposed to light, air or high temperatures. A short duration of action and variable efficacy limit the number of natural substances that can be marketed in the plant protection products sector. One solution in this area is the development of more stable and effective formulations that maximise the viability and efficacy of derivatives, particularly those of Bacillus. Microencapsulation is a significant advance. In this process, spores are enclosed in a protective matrix that shields them from environmental stressors. The coating gradually degrades, releasing the bacteria at the right place and time ^[8].

Controlled release of the active substance

Controlled release of active substances increases their effectiveness, thereby reducing the need for frequent applications and lowering overall costs. One of the frequently used solutions is the aforementioned use of microcapsules. Dedicated matrices are also used, which release the ingredient in response to specific stimuli, e.g. changes in pH or humidity. An interesting solution is the use of polymer carriers. The use of biodegradable polymers as carriers for targeted and controlled delivery of substances to plants through leaves or roots is very promising ( ). Release occurs slowly as the polymer is broken down (depending on the location) by sunlight or soil microflora. The release rate of biopesticides can be regulated by using different monomers and cross-linking additives and selecting their appropriate ratio. An equally interesting example is the use of nanoporous structures ^{[8, 9]}.

Introduction of nanoformulations to the market

Nanotechnology has the potential to offer new solutions to increase the effectiveness of biopesticides. One example is the creation of nanometric coatings that protect the active ingredient from external factors. Advances in the creation of nanoemulsions offer interesting possibilities. In this case, the active substance is dispersed in a liquid carrier. Such preparations are characterised by greater dispersibility, allowing for even coverage of plants. In addition, their small size allows for better penetration and more targeted action.

The introduction of nanoformulations to the market is a great opportunity, but also presents a number of challenges and doubts. One of the most significant is the relatively high cost of production, as well as the still unknown long-term effects of introducing nanomaterials into the environment ^[8].

Multifunctional formulations for comprehensive plant protection

An interesting direction in the development of biopesticide formulations is the synergistic combination of Bacillus strains with other biocontrol agents or biostimulants to create multifunctional preparations that provide comprehensive plant protection and support plant growth. Different strains have different effects and properties, and combining them provides comprehensive support for the plant. In addition, the addition of other supporting ingredients, such as humic acids or algae extracts, can further enhance the action of the bacteria. ^{[3, 6, 7]}

PCC Exol as a supplier of solutions for the formulation of biopesticides and biostimulants

Surfactant manufacturer PCC Exol has been supporting the agricultural industry with its products and technical advice for many years. We are also constantly looking for optimal solutions for the biopesticide and biostimulant formulations described above, which is why we can recommend several of our products for these categories.

For SL formulations, we offer a wide selection of wetting agents, spray formation and retention enhancers, and leaf cuticle penetrants that increase absorption. All of these products are easily biodegradable and harmless to the environment. As wetting agents, we particularly recommend products developed specifically for this purpose, such as EXOwet D15, EXOwet L5, EXOwet T7 and EXOwet OS. Many products from the ROKAnol group also have good wetting properties, with NL8P4, L5P5, the DB series, the GA series and the ID series. As agents improving spray formation and retention, we recommend the ROKAmer series, including ROKAmer G4300 and G3800, and in particular ROKAmer 6500 or its water-based and more convenient version, ROKAmer 6500W. As leaf cuticle penetrants, we recommend products based on unsaturated fatty acids or fatty alcohols with medium HLB: ROKAcet O7 or ROKAnol O10 ^[10].

In OD formulations, it is necessary to ensure adequate suspension stability and emulsification of the oil phase after addition to water, which is why surfactants that combine both these functions work well. In addition, these surfactants must not harm microorganisms, therefore we recommend the following for this formulation: ROKAcets from the R series and the ROKwin and ROKwinol product groups, which also have a positive effect on the properties of the spray and increase the effectiveness of spraying ^[10].

In the CS formulation, adequate emulsification of the oil phase during interfacial or in situ polymerisation must be ensured, which requires the precise selection of emulsifiers and protective colloids, otherwise the micelles will clump together and the capsules will be of an unsuitable size. In addition, many of the solvents and emulsifiers used for encapsulating conventional substances kill microorganisms, which is why vegetable oils and appropriately selected emulsifiers are most commonly used. After the encapsulation process, appropriate wetting agents and water dispersants should be added. The same ROKAnols or ROKAmers used in SL formulations can be used as wetting agents. ROKAmers additionally stabilise the suspension by acting as co-dispersants. We recommend EXOfos PT­ K25 or PT­ K60, SULFOROKAnol TSP95, EXOdis PC40, Rodys OP and Rodys KP ^[10] as dispersants.

PCC Exol also provides technical advice on solving various formulation-related problems and develops new products or variants thereof at the request of customers. We encourage you to visit our product catalogue and contact us.