Spatial heterogeneity of gall formation in relation to chemotype distribution in Thymus vulgaris

The utilization of host plants by herbivorous insects depends on plant traits and physical environment. The purpose of the present work is to test the hypothesis that spatial variation in the presence of galls of the specialist fly Janetiellathymicola in natural populations of its host plant Thymus vulgaris differ in relation to spatial variation in chemotype presence. We quantified gall infection rates in 59 populations that differ in chemotype presence across a sharp ecological gradient in the South of France. We also quantified spatial aggregation of galls and plants and made a 3-year study of infection, biomass and plant survival in three populations. The proportion of galled plants was significantly higher in populations with non-phenolic chemotypes on deeper soils in sites with cold winter temperatures than in populations of phenolic chemotypes on stony soils in sites with mild winters. In a population with two non-phenolic chemotypes, galls were significantly more present on plants of the chemotype with the highest proportion of galls in the multi-population survey. In a 3-year study, galled plants had a significantly greater probability of being infected by a subsequent generation than non-galled plants. This probability declined with distance. Galls absorbed a significant proportion of vegetative biomass on a shoot, but their presence was unrelated to survival. Host plant chemistry, habitat characteristics and dispersal limitation conjointly affect this host–parasite interaction.