Plant growth-promoting Bacillus suppress Brevicoryne brassicae field infestation and trigger density-dependent and density-independent natural enemy responses

Soil-dwelling Plant Growth Promoting (PGP) Bacillus live in intimate associations with plants; some species offer direct benefits via plant growth promotion while others confer protection against various pathogens. However, the roles of PGP Bacillus as elicitors of plant defences against agricultural pests and as a component of integrated pest management systems remain virtually unexplored. The effects of three major ubiquitous gram positive rhizobacteria; Bacillus cereus, Bacillus subtilis and Bacillus amyloliquefaciens were studied individually and in admixture on (i) calabrese (sprouting broccoli, Brassica oleracea) vegetative and reproductive growth parameters and (ii) the population dynamics of the specialist cosmopolitan pest, cabbage aphid (Brevicoryne brassicae) infestation, and its important natural enemies; the braconid endoparasitoid (Diaeretiella rapae), ladybird beetle (Coccinella septempunctata) and syrphid fly (all species). We found that all Bacillus treatments efficiently suppressed B. brassicae field populations in varying magnitudes. B. cereus and B. subtilis significantly increased the rates of parasitism by D. rapae, however, none of the other treated plants lured natural enemies, which responded in a density-dependent manner. Although the mixed Bacillus treatment significantly reduced root weight ratio, none of the Bacillus spp. treatments produced significant effects on calabrese growth. Taken together, PGP Bacillus may offer multiple plant benefits through suppressed pest infestation and increased percent parasitism in the field, with potential applications in integrated pest management.