Indirect effect of earthworms on wheat-aphid interactions

Abstract

Studies on interactions between plants and belowground organisms, or between plants and aboveground organisms, are numerous and highlight their importance in understanding ecosystem functioning. However, these two compartments are rarely studied together, despite the direct link formed by the plant itself. Soil organisms can influence interactions between plants and herbivores by changes in plant growth and chemistry. Under controlled conditions the influence of earthworms and aphids on wheat was investigated. The effect of earthworms on aphid reproduction was also investigated, to demonstrate the indirect plant-trait-mediated effects. We showed a significant effect of both above and belowground organisms on wheat. Interestingly, nitrogen content was higher in the presence of aphids (1.63 ± 0.07 %,) or earthworms alone (1.58 ± 0.04 %) and even higher in presence of both (2.07 ± 0.07 %). Moreover, we showed a strong effect of earthworms on aphid reproduction rates which are almost twice as high as in the absence of earthworms where we observed a reproduction rate of 32 individuals per day compared with 57 in the presence of earthworms. The presence of aphids led to a 16 % reduction in above-ground biomass compared with the control (1.83 ± 0.08 g vs 2.19 ± 0.09 g for the control), while the presence of earthworms led to an 46 % increase (3.20 ± 0.12 g vs 2.19 ± 0.09 g), and the presence of both organisms had no significant effect on the above-ground biomass of wheat compared with the control (2.39 ± 0.08 g vs 2.19 ± 0.09 g). These results could have implications for pest management, as they indicate that the presence of earthworms compensate the negative effect of aphids on plant biomass, probably by acting on nutrient uptake and plant defensive traits. By addressing the effects of earthworms on both the pest performances and plant traits, we highlight the need for further research on the indirect effects of below-ground compartments on pest performance. This study builds towards a better understanding of the interaction between soil, plant and pests and highlights the importance to study the pest management with a holistic view of agroecosystems.