Earthworms Enhance Maize Resistance to Aphids From Bottom to Top via Increasing Soil <i>Bacillus</i> Abundance

Soil microorganisms can enhance plant anti-herbivore defence. Earthworms and fertilisers significantly influence soil microbial communities. However, the association between earthworms and plant anti-herbivore defence, together with alterations in soil microbial composition under different fertiliser conditions, remains ambiguous. This study investigates the effects of earthworms (Metaphire guillemi) on the resistance of the corn leaf aphid (Rhopalosiphum maidis) and the soil bacterial community under organic and chemical fertiliser applications. The results showed that earthworms markedly enhanced maize resistance to aphids only with the application of organic fertiliser, this effect was not obvious in sterilised soil, suggesting an association with soil microorganisms. Moreover, earthworms altered the composition of the soil bacterial community and decreased its diversity. This reduction was accompanied by an enrichment of Bacillus when earthworms and organic fertiliser were applied. Bacillus (three strains) was isolated and demonstrated to help maize defend against aphids. Additionally, the levels of flavonoids and total phenols were significantly elevated following the introduction of earthworms under the organic fertiliser treatment, as well as Bacillus inoculation. This study elucidates how earthworms improve plant resistance to herbivores through soil microorganisms, providing an empirical basis for reducing chemical pesticides by applying earthworms and organic fertilisers.