Associational resistance (AR) occurs when a plant species experiences less herbivory when growing in the presence of other plant species than when growing in monoculture. Densities of the gall midge Asphondylia borrichiae Rossi & Strong on the coastal plant Iva frutescens L. are depressed in the presence of a second coastal plant species, Borrichia frutescens (L.). Previous studies suggested that hymenopteran parasitoids from Borrichia galls spill over onto Iva galls and reduce gall densities.
This study employs large-scale addition or near-complete removal of Borrichia from a series of spoil islands near the west central Florida coast. Densities of galls on Iva decreased where Borrichia was added, increased where Borrichia was removed and remained unchanged on islands where Borrichia abundance was not manipulated.
Relative to unmanipulated islands, the total parasitism rate on Iva galls and parasitism rate by Torymus umbilicatus, the parasitoid hypothesised to be primarily responsible for AR in the system, declined on islands where Borrichia was removed and increased on islands where Borrichia was added, supporting the idea of parasitoid-mediated AR.