Biological control properties of microbial plant biostimulants. A review

Plant biostimulants, sometimes referred to as biofertilisers or plant enhancers, are ingredients stimulating plant nutrition processes independently of the product's nutrient content with the sole aim to improve the agronomic performance of a plant. Many of these biostimulants contain microorganisms. Although most of these microorganisms are supposed to only promote plant growth, some have plant protection properties. We reviewed commercial microbial plant biostimulants with regard to their potential effects on insects. This revealed 483 different products and 245 microorganisms registered as microbial plant biostimulants in Hungary, Switzerland, Spain, France, Indonesia, and/or Canada (181 +/- 157 products, 64 +/- 27 species per country). Among the products, 82% contained bacteria (133 +/- 106 products), 63% contained fungi (77 +/- 59) and 14% contained protista including algae (23 +/- 24). About 1/3rd of products contained mixes of either bacteria, fungi, and/or protista; and 48% contained more than one microorganism. About 53% of products (137 +/- 121) contained microorganims that had been reported to have insecticidal properties and 36% of species (23 +/- 9), although the underlaying mechanisms often remain unknown. About 67% of products (149 +/- 133) contained microorganisms reported to defend a plant from insects, and 54% of species (35 +/- 10). The most common biostimulant microorganisms with reported insecticidal effects were strains of Rhizophagus irregularis, followed by Bradyrhizobium japonicum, Rhizobium leguminosarum, Bacillus megaterium, B. subtilis, B. amyloliquefaciens, B. licheniformis, Penicillium bilaiae, B. pumilus and Ascophylum nodosum. In conclusion, growers may profit from, but should be made aware of the multiple effects of microbial plant biostimulants.