Non-host larvae negatively impact persistence of the entomopathogen Beauveria bassiana in soil

Distribution and pathogenicity of Beauveria bassiana in soil with earthworm action and feeding

Earthworm action and feeding have an important impact on a variety of microorganisms in the soil. However, the effects of the earthworm on Beauveria bassiana, a common entomopathogenic fungus in the biological control of pests, have been little studied. In this study, the epigeic earthworm species Eisenia fetida (Savigny) was selected to evaluate its impact on B. bassiana TST05 including its distribution in soil and its pathogenicity to target insects. By testing B. bassiana TST05 distribution, biomass in soil, viable spore germination rate, and pathogenicity to insect larvae after passing through the earthworm gut, the results showed that the activity and feeding of E. fetida promoted the B. bassiana TST05 diffusing downwards in the soil, while decreasing active fungal spores. After passing through the earthworm gut and excretion, the living B. bassiana individuals still had activity and pathogenicity to insects. The germination rate of the viable fungal spores was 15.09% and the infection rate to the insect larvae of Atrijuglans hetaohei Yang reached 62.35%, 80.95% and 100% after infection at 7 d, 10 d, and 14 d, respectively. The results showed that action and feeding of earthworms promoted the distribution of B. bassiana TST05 in soil, but decreased B. bassiana viable spores. This study is important for understanding the interaction between earthworms and B. bassiana in soil and for guiding the scientific application of B. bassiana in the biological control of pests.

Effect of earthworm Eisenia fetida epidermal mucus on the vitality and pathogenicity of Beauveria bassiana

Beauveria bassiana is one of the most widely studied and used entomopathogenic fungus as biopesticide. In the biological control of pests, B. bassiana will persist in the soil after application, and will inevitably contact with earthworms, especially the epigeic earthworm species. So, what are the effects of earthworm and its epidermal mucus on the activity of B. bassiana? We employed the epigeic earthworm Eisenia fetida, B. bassiana TST05 strain, and the insect Atrijuglans hetaohei mature larvae to study the impact of earthworm epidermal mucus on the vitality and pathogenicity of B. bassiana to insect. Methods included scanning electron microscope observation, detection of spore germination, fungal extracellular enzyme activity, and infection testing to A. hetaohei. The results showed that the B. bassiana spores may attach to the cuticle of E. fetida but they could be covered by the epidermal mucus and became rough and shrunken. After treatment with the epidermal mucus, the spore germination and extracellular enzymes of B. bassiana was significantly inhibited. Inoculation of A. hetaohei larvae with a mixture of B. bassiana and mucus showed that the mucus could reduce the pathogenicity of B. bassiana to the insect, resulting in a slower disease course and lower mortality. It was concluded that the epidermal mucus of the earthworm E. fetida can inhibit the activity of B. bassiana, as well as the infectivity and pathogenicity of fungus to target insects. However, after treatment with epidermal mucus the surviving B. bassiana still had certain infectivity to insects. This is of great significance for the application of B. bassiana in biological control of pests.

The nesting preference of an invasive ant is associated with the cues produced by actinobacteria in soil

Soil-dwelling animals are at risk of pathogen infection in soils. When choosing nesting sites, animals could reduce this risk by avoiding contact with pathogens, yet there is currently little evidence. We tested this hypothesis using Solenopsis invicta as a model system. Newly mated queens of S. invicta were found to nest preferentially in soil containing more actinobacteria of Streptomyces and Nocardiopsis and to be attracted to two volatiles produced by these bacteria, geosmin and 2-methylisoborneol. Actinobacteria-rich soil was favored by S. invicta and this soil contained fewer putative entomopathogenic fungi than adjacent areas. Queens in such soil benefited from a higher survival rate. In culture, isolated actinobacteria inhibited entomopathogenic fungi, suggested that their presence may reduce the risk of fungal infection. These results indicated a soil-dwelling ant may choose nest sites presenting relatively low pathogen risk by detecting the odors produced by bacteria with anti-fungal properties.

Insect pathogens are widely distributed in soil. Soil-dwelling insects must overcome challenges arising from pathogens in soil. Here we report that a soil-dwelling ant may choose nest sites with lower pathogen infection risk, specifically the ant can sense the cues of some actinobacteria that can inhibit the growth of the pathogens. By choosing the sites with higher abundance of some actinobacteria, the ant can get a higher survival rate. The ant and some actinobacteria thus coordinate a specialized adaptive strategy of infection risk management, enabling the ant population to grow.