The aboveground and belowground compartments of terrestrial ecosystems, which are generally separated in interaction studies, are nevertheless linked by their dependence on the same plant. Soil organisms can influence interactions between plants and above-ground herbivores by changes in plant growth and chemistry.

In the present study, under controlled conditions the direct influence of earthworms and aphids on wheat traits (biomass; C and N content) was investigated. The effect of earthworms on aphid reproduction was also investigated, in order to demonstrate the indirect plant-trait-mediated effects.

The results showed a significant effect of both above and below-ground organisms on plant traits. Indeed, wheat biomass and carbon and nitrogen contents were influenced by the presence of earthworms and aphids, suggesting that earthworms can promote aboveground and belowground plant growth by promoting resource acquisition. Interestingly, the nitrogen content was significantly higher in the presence of aphids or earthworms alone and it was even higher when the two organisms were present together. Moreover, the results showed a strong effect of earthworms on aphids, with a reproduction rate almost twice as high as in the absence of earthworms.

These results could have implications for crop 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, despite a higher aphid reproduction rate in the presence of earthworms probably linked to nitrogen content. By addressing the effects of earthworms on both the pest performances and plant traits, this study highlights 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 herbivores and highlights the importance to study the pest management with a more holistic view of agroecosystems.

A factorial greenhouse experiment was performed using wheat under four conditions: absence/presence of earthworms, and absence/presence of aphids. A total of 60 pots (4x15 pots, height = 21.8 cm; diameter = 18.7 cm) was filled with soil. The study soil was a Haplic Luvisol and was sampled in an agricultural field. After being air-dried, the soil was sieved at 4 mm. Gauze was attached at the bottom of each pot to prevent earthworms from escaping. On 3 March 2023, five untreated wheat seeds, Triticum aestivum L. (Fructidor variety) were planted. Plants were watered twice a week and kept in a greenhouse with ambient temperature (an average of 12 °C at night and 25 °C by day, for 61 days), humidity, and light. Once the seeds had germinated and reached the early tillering stage, the plants were thinned to two plants per pot.

On 4 April 2023, aphids and earthworms were inoculated. Fifteen pots were prepared for each treatment: 15 pots without earthworms nor aphids; 15 pots with aphids but without earthworms; 15 pots with earthworms but without aphids; 15 pots with both earthworms and aphids. Each pot was randomly distributed in the greenhouse. Replication statement is summarised in Table 1. Two or three Lumbricus terrestris L. (Lumbricidae) (corresponding to a biomass of 10 g) were placed in each of the 15 aphid-free pots and the 15 pots with aphids. Similar earthworm densities have been reported for agroecosystems, though with considerable variation (Dulaurent et al. 2023). Earthworms were purchased at Decathlon® (Live fishing bait, Canadian worms). Six aphids of the species Sitobion avenae Fabricius (Aphididae), were placed on the plant leaves using a brush, in each of the 15 earthworms-free pots and the 15 pots with earthworms. Aphids reared on wheat were provided by Crisop society (Koppert Ervibank®). Nylon bags (100×50 cm) with 0.05×0.05 cm mesh (Kweekzak 100×50 cm from Vermandel Entomologie Speciaalzaak) were used to contain the aphids in the pots. To homogenize the experiment, nylon bags were placed around all the pots, with and without aphids, and all the pots were regularly randomly moved around the greenhouse.

At the end of the experiment (2 May 2023), all the wheat was harvested, and the roots separated from the shoots. All shoot samples were left for 48 hours at 8 °C to facilitate aphid counting to estimate a simplified reproduction rate (corresponding to the final number of aphids over the total number of days, 28 days). Shoots and roots were dried for 48 h at 50 °C directly after aphid counting; weighted to obtain dry biomass; and finally, ground to fine powder before further chemical analyses. Shoot and root C and N contents were quantified with a gas chromatography in an automatic elemental analyser (FlashSmartTM; Thermo Fisher Scientific) using approximately 0.0060 g of dried powder.