Effects of earthworm addition on the population dynamics of inoculated wheat aphids

Studies on interactions between plants and belowground organisms or between plants and aboveground organisms are numerous. However, research on the effects of belowground organisms on aboveground organisms via plant mediation remains limited. Soil organisms can influence interactions between plants and herbivores by altering plant growth and chemistry. Four treatments, including 0, 1, 2, and 4 Metaphire guillelmi individuals per pot, were applied, and wheat aphids were inoculated to investigate the effects of earthworms and aphids on the primary and secondary metabolism of wheat, as well as on the population dynamics of three aphid species (Schizaphis graminum, Sitobion graminum, and Rhopalosiphum padi) through indoor potted experiments. This study aims to provide theoretical support for understanding interactions among soil biota (earthworms), wheat, and aphids in agricultural ecosystems. The results indicated that earthworm addition at different densities increased the biomass, total nitrogen, and total carbon of wheat shoots and roots, whereas decreased root total carbon and the carbon-to-nitrogen ratio of wheat shoots and roots, while increasing the total flavonoid and total phenol content of wheat shoots. Aphid inoculation reduced the biomass, total carbon, and total nitrogen of wheat shoots and total nitrogen of wheat roots, whereas increased the total flavonoid and total phenol content of wheat shoots. The population dynamics of all three aphid species exhibited logistic S-shaped curves, and their population densities and environmental capacity (K) increased with rising earthworm density. Correlation analysis revealed that wheat shoot biomass, total flavonoid content, and total phenol content were positively correlated with the population density of the three aphid species. As earthworm density increased, the biomass, total carbon, and total nitrogen of wheat shoots also increased. Redundancy analysis showed that primary metabolites (soluble sugar, total carbon, and total nitrogen of wheat shoots) explained 48.83% of the variation in aphid population size, whereas secondary metabolites (total flavonoids and total phenols of wheat shoots) and soil physicochemical properties explained 11.91% and 31.90% of the variation, respectively. After aphid inoculation, earthworm addition mitigated the negative effects of aphids on the biomass and total carbon of wheat shoots. These results provide an important basis for understanding the interaction mechanisms among soil, plants, and pests.