Biologicals: biostimulants and biopesticides
What agricultural biologicals are, how biopesticides, biostimulants and biofertilisers differ, how they are used and regulated, and what the evidence says.
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Biologicals are farm inputs based on living organisms or natural substances: microbes that kill insect pests or suppress root diseases, pheromones that confuse moths, seaweed extracts that help crops cope with drought, bacteria that fix nitrogen. They are one of the fastest-growing segments of agricultural inputs, driven by pesticide restrictions, resistance, residue limits and demand for lower-impact farming.
They are also a field full of loose claims. Knowing what the categories mean, how the products work and what evidence to look for is increasingly expected in crop protection, agronomy and agribusiness courses.
The three families
Biopesticides
Biopesticides control pests, diseases or weeds using organisms or naturally derived substances. Three groups are usually recognised:
- Microbial pesticides: bacteria, fungi, viruses or protozoa.
- Bacillus thuringiensis (Bt), whose crystal proteins are toxic to specific insect larvae once eaten, is the most widely used. Bt sprays have been sold since the late 1930s.
- Entomopathogenic fungi such as Beauveria bassiana and Metarhizium species infect insects through the cuticle; a Metarhizium product was developed for locust and grasshopper control in Africa.
- Baculoviruses (nucleopolyhedroviruses, granuloviruses) are highly specific to caterpillar species; a programme using a baculovirus against the velvetbean caterpillar treated soybean on millions of hectares in Brazil.
- Trichoderma fungi and some Bacillus and Pseudomonas bacteria suppress soil-borne plant pathogens by competition, antibiotics and induced resistance.
- Biochemical pesticides: naturally occurring substances that control pests by non-toxic mechanisms or that are of natural origin, such as insect sex pheromones for mating disruption and monitoring, plant extracts, and some mineral and fermentation products.
- Macrobials: living natural enemies (predatory mites, parasitic wasps, beneficial nematodes) released in biological control. Some regulators classify these separately from biopesticides.
Biostimulants
Plant biostimulants act on the plant's own processes, not on pests, and are not primarily nutrients. The European Union's fertilising products regulation (2019) defines them by function: products that stimulate plant nutrition processes independently of their nutrient content, to improve one or more of nutrient-use efficiency, tolerance to abiotic stress, quality traits, or the availability of nutrients confined in the soil or rhizosphere. Main types:
- Seaweed extracts, mostly from brown seaweeds.
- Humic and fulvic substances.
- Protein hydrolysates and amino acids.
- Silicon compounds and some other beneficial elements.
- Microbial biostimulants: plant-growth-promoting rhizobacteria and mycorrhizal fungi.
Biofertilisers
Biofertilisers are microbial inoculants that supply or make available nutrients: rhizobia for legumes, free-living and associative nitrogen fixers, the Azolla–Anabaena partnership in rice, and phosphate-solubilising and potassium-mobilising microbes. Legume inoculation with the right rhizobial strain is the best-established biological input in agriculture: where a legume is new to a field, inoculation can make the difference between a nodulated, nitrogen-fixing crop and a nitrogen-starved one. In some legal systems, microbial biofertilisers fall under biostimulant rules.
Why interest is growing
- Pesticide restrictions and resistance: many older synthetic actives have been withdrawn, and pests have become resistant to others.
- Residue limits set by importing markets and retailers, often stricter for fresh produce.
- Integrated pest management and organic farming, in which biologicals are core tools.
- Safety: most biopesticides have low toxicity to people and non-target organisms, short or no pre-harvest intervals, and fewer residue problems.
- Resistance management: biologicals with different modes of action can be rotated or mixed with synthetic products.
How to use them well
Biologicals are living products or delicate chemistry, and they behave differently from synthetic inputs.
- Timing and targeting: microbial insecticides usually work best against young larvae and must be eaten or contact the pest; they act more slowly, often over days.
- Environment: ultraviolet light, heat and dryness kill many microbes on leaves; applying in the evening, with adequate coverage and sometimes with UV protectants, helps.
- Compatibility: some fungicides kill beneficial fungi; some products cannot be tank-mixed.
- Storage and shelf life: viability declines with time and temperature; check expiry dates and store as directed.
- Dose by viable count: microbial products are dosed by colony-forming units (CFU) or international units, not just by weight.
Working out a microbial dose
A Trichoderma product guarantees 1 × 108 colony-forming units (CFU) per gram. A field recommendation calls for 1 × 1012 CFU per hectare, applied to the soil with organic manure.
Product needed = 1012 ⁄ 108 = 104 g = 10 kg per hectare.
If the product has been stored badly and its viable count has fallen to 2 × 107 CFU per gram, the same 10 kg delivers only 2 × 1011 CFU, a fifth of the target: the main reason biological products "fail" in the field. A product's guaranteed count at the end of its shelf life is more meaningful than its count at manufacture.
Regulation
Rules differ greatly between countries:
- Biopesticides are generally regulated as pesticides, often with a faster or reduced-data registration route for low-risk products. Registration still requires evidence of identity, purity, safety and efficacy.
- Biostimulants fall between pesticide and fertiliser law. Some jurisdictions now have specific definitions and rules: the EU since its 2019 fertilising products regulation; others regulate them as fertilisers or soil amendments, or not specifically at all. Claims to control pests normally bring a product under pesticide law.
- Quality control matters as much as registration. Surveys in several countries have found microbial products with lower counts than labelled, contaminants, or the wrong organism.
Always use products registered for the crop and use in your country, and follow the label.
What the evidence says
- Well established: legume inoculation where rhizobia are lacking; Bt and baculoviruses against susceptible caterpillars; pheromone mating disruption in orchards and vineyards; Trichoderma against some soil-borne diseases; augmentative biological control in greenhouses.
- Variable: many biostimulant products show benefits in some trials and none in others. Meta-analyses of seaweed extracts, humic substances and microbial biostimulants report average yield gains that are modest and depend strongly on crop, stress and soil. The benefit is often largest under stress (drought, salinity, low fertility) and smallest in well-managed, high-input fields.
- Questions to ask of any product: independent, replicated field trials in conditions like yours; an untreated control; a stated mechanism; and a guaranteed content or count.
Key points
- Biologicals are inputs based on living organisms or natural substances: biopesticides, biostimulants and biofertilisers.
- Biopesticides: microbial (Bt, entomopathogenic fungi, baculoviruses, Trichoderma), biochemical (pheromones, extracts) and macrobials (natural enemies).
- Biostimulants act on plant processes to improve nutrient-use efficiency, stress tolerance or quality; they are not mainly nutrients and do not kill pests.
- Biofertilisers supply or mobilise nutrients; legume inoculation is the best-proven example.
- Use them as living products: timing, coverage, UV, compatibility, storage and dose by viable count all matter.
- Regulation and quality control vary by country; evidence is strong for some products and variable for many biostimulants.
What the exams ask
- Entomology, plant pathology and agronomy courses: definitions and examples of biopesticides and biofertilisers, the mode of action of Bt, entomopathogenic fungi and baculoviruses, and their role in IPM.
- Research-fellowship and postgraduate entrances: biostimulant categories, mechanisms of plant-growth-promoting microbes, formulation and shelf life, and dose calculations.
- Agribusiness and policy examinations: market growth, regulation, quality control and the place of biologicals in reducing synthetic pesticide use.
Described in general terms: the kind of examination, not a past paper. Laisha never reproduces exam questions.
Questions people ask
Are biologicals always safe?
Most have low toxicity to people and wildlife, but not all natural substances are harmless, and microbial products must be free of contaminants. They are regulated and should be handled according to the label.
Can biostimulants replace fertiliser?
No. They are not mainly sources of nutrients. At best they help plants use nutrients more efficiently or cope with stress; the crop still needs adequate nutrition.
Why do some biological products work in one field and not another?
Living organisms depend on conditions: temperature, moisture, soil, existing microbes and the pest's stage. Product quality also varies. That is why local trials and good-quality products matter.
Sources
- du Jardin, P. "Plant biostimulants: definition, concept, main categories and regulation." Scientia Horticulturae 196, 3–14 (2015).
- European Union. Regulation (EU) 2019/1009 laying down rules on the making available on the market of EU fertilising products.
- Glare, T. R. and Moran-Diez, M. E. (eds) Microbial-Based Biopesticides: Methods and Protocols. Springer, 2016.
- Lacey, L. A. et al. "Insect pathogens as biological control agents: back to the future." Journal of Invertebrate Pathology 132, 1–41 (2015).
- Li, J., Van Gerrewey, T. and Geelen, D. "A meta-analysis of biostimulant yield effectiveness in field trials." Frontiers in Plant Science 13, 836702 (2022).
- OECD. Guidance on the Regulation of Biopesticides and Semiochemicals. OECD Series on Pesticides, various.
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