May. 20, 2020
• Naturally occurring substances that control pests by non-toxic mechanisms
• Biochemical pesticides include substances that interfere with mating, such as insect sex pheromones, as well as various scented plant extracts that attract insect pests to traps
• Consist of a microorganism (e.g. a bacterium, fungus, virus or protozoan) as the active ingredient
• Microbial pesticides can control many different kinds of pests, although each separate active ingredient is relatively specific for its target pest(s)
• Pesticidal substances that plants produce from genetic material that has been added to the plant
The application of biopesticides is still fairly limited to only a few percent of all pesticides used for crop protection. However, biological control holds an important position in integrated crop solutions programs to deliver performance advantages alongside traditional plant protection products. There are many factors contributing to the low uptake in use of biological controls, such as: expensive production methods, poor storage stability, susceptibility to environmental conditions and efficacy problems. Some of these problems can be overcome by formulation improvements. With many pressures on product performance, formulation is acknowledged as an area to enhance and extend the activity of biopesticides.
Focusing specifically on microbial pesticides, these formulations can present a range of challenges from end product stability to maintaining viability of a microorganism. Formulation knowledge and expertise is often required to help overcome these challenges and achieve end product success. This is why, at Croda, we are concentrating on formulation developments that address specific needs for microbial biopesticides. The process used to prepare the microorganism for formulation will provide important considerations on the choice of formulation type and ingredients needed to support delivery.
The first key consideration in any formulation development is to know your active ingredient. Microbes are complex and often show variability; within the s¬ame species different strains can report different performance, the same strain can report different performance when it is under external stress, depending on the target, one strain may perform better than another and the morphological structure of the microorganism is extremely important - each strain has its own peculiarities. It is essential to select the right formulation type according to the microbe. Choosing the optimum ingredients to stabilise the formulation is important and is guided by fundamental chemistry. Thermodynamics, phase balance, solution behaviour, surface and colloid science all drive informed surfactant selection. It is key to build an understanding of surfactant interactions with the microbe for compatibility assurance. The most commonly used formulations are oil based (oil dispersions (OD) / emulsifiable concentrates (EC)) or solid based (wettable powders (WP) / water dispersible granules (WDG)) to minimise water exposure, but these are not the only options.
Alongside formulation development, it is necessary to consider the method of application to be used with the product: foliar application, in-furrow or seed treatment, amongst others. This will also drive the selection of the formulation type and components. Lastly, knowledge of the target disease or pest and the mode of action of the microorganism has a driving role in the choice of formulation technology, as well as component selection. Besides providing stability, formulation delivers step changes in efficacy.
To help with formulation troubleshooting and design, it is useful to understand what can impact shelf life in the formulation. The two main factors are stability of the formulation and loss of microbial viability. To help provide stability and performance features, most formulations contain a surfactant system. To deliver the optimum performance, generally the surfactant system is a blend of several components. The total surfactant concentration is usually fixed and the ratio of each component must be carefully balanced. To help determine these ratios Croda has developed the Experimental Design Tool which enables you to change the surfactant ratios methodically so that an optimum composition can be identified. A more detailed explanation of this can be found on Croda’s Crop Care website (www.crodacropcare.com). An OD formulation is one of the best options to maintain the viability for a longer period, by avoiding the presence of free water in the formulation. Croda has extensive knowledge in OD formulations and can help you with new developments.
Besides shelf life stability, microbial formulations must be suitable for dilution in water in the spray tank. Formulation development needs to ensure the use of stabilisers for the dilution phase. Knowledge of the dilution conditions, how the formulation behaves in water or oil and how it interacts with the other materials present in the tank mix is key for the selection of formulation ingredients that will provide the optimum performance.
To successfully develop microbial formulations that do not experience loss of viability, superior formulation additives are essential. Croda’s innovative products simplify formulation development by providing excellent and reliable performance. This is demonstrated in Figure 1, showcasing results from a viability study with Trichoderma sp. Specialised surfactants, a modified styrene acrylic polymer (Atlox Metasperse™ 550S) and/or a polyoxyethylene sorbitol hexaoleate (Atlas™ G-1086) are shown to have no detrimental impact on microbial viability and can be successfully utilised in stabilisation mechanisms during formulation development.
Croda also has the ability to produce improved grades of many of our products. Higher levels of purification reduce the impurities that can impact the viability of the microbe. Options to support developments extend to our manufacturing sites; here we are able to refine our processes to produce high purity variants of our standard materials. This reduces the level of impurities or residual water which can impact the viability of the microbe.
Numerous parameters can impact the viability of microbials, including drying methods, susceptibility to formulation aids and additives, prolonged periods of storage and environmental factors. All of these can ultimately limit the efficacy in the field. Methods for assessing viability, such as colony counting, are an important part of ensuring a successful formulation development. Choosing appropriate technologies for maintaining viability and evaluation allows for a more efficient formulation development process.
When considering in house developments of microbial formulations, available facilities and equipment for continuous testing are vital to success. This can often mean outsourcing of certain aspects of the development work, however, at Croda we have access to all of the necessary equipment and expertise to support your developments. Amongst others, some of the critical testing parameters essential to success include high definition microscope imagery, in-vivo microbial testing, biological viability testing, recovery screening and seed treatment application feasibility. Necessary testing also extends to formulation stability testing more commonly seen in standard synthetic based crop protection products: particle size distribution analysis, emulsion performance and rheometry being some of the most regularly utilised. Croda’s capabilities in formulating with microbials can be found on our website.
As has been detailed in this article, microbial pesticides have great potential to deliver performance advantages but can present a range of challenges from end product stability to maintaining viability of a microorganism. Therefore, formulation design is essential to the success and acceptance of these products. In general, the formulation should: increase shelf life, facilitate handling and application, enhance bioactivity, protect against external factors and maintain a good distribution of the microorganism. At Croda we recognise that biologicals have an important role to play in sustainably improving agricultural yields and contributing to global food security. That is why we actively encourage collaborations for developments in this area. Croda’s capabilities in formulating with microbials can be found on our website.
This article was initially published in AgroPages '2020 Formulation & Adjuvant Technology' magazine. Download it to read more articles.
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