Experimental validation of the soil fertility hypothesis in a natural Mediterranean tri‐trophic system

Host plant resources are a major issue in gall‐insect ecology. Soil fertilization may affect both plant quality and indirect defenses, as well as the prey quality of herbivores for predators. The soil fertility hypothesis (SFH) claims that gall‐inducing insects infest more plants growing on poor than on fertile soils. Until now this was studied and supported only by observations in natural tropical biomes. We tested its validity in a field experiment in a Mediterranean region, using four aphid species (Hemiptera: Aphididae, Eriosomatinae) that induce galls in wild Pistacia atlantica Desf. (Anacardiaceae) trees, and their main enemies. The impact of fertilization was examined in control trees that were not fertilized (‘poor soil’) and others that were fertilized with NPK for either one or two consecutive growing seasons. Average internode length in apical shoots of fertilized trees was longer than in control trees, following SFH expectations. We did not detect a different number of aphid species in each tree, contrary to SFH predictions. Fertilization lowered the number of galls, the gall survival, and the mean number of aphids inside the galls, as expected by the SFH. All the galls suffered increased attacks by enemies in fertilized trees, as expected by the SFH. In conclusion, following this field experiment, the SFH should not be rejected as a possible framework in research on galling‐insect ecology. Fernandes & Price (1991) proposed the soil fertility hypothesis to explain that insects create more surviving galls in trees growing on poor soils, emphasizing the role of the natural enemies in this process. Until now this hypothesis was verified only by observations in tropical biomes. Our study experimentally demonstrates that this hypothesis can be useful to understand the distribution and abundance of insects inducing galls in plants in other biomes as well.