Effects of Relative Host Plant Abundance, Density and Inter-Patch Distance on Associational Resistance to a Coastal Gall-Making Midge, Asphondylia borrichiae (Diptera: Cecidomyiidae)

Keith Stokes, Peter Stiling


Associational resistance (AR) is an emergent property of ecological communities and may play an important role in their assembly and structuring. Gall densities of the midge Asphondylia borrichiae Rossi&Strong (Diptera: Cecidomyiidae) on the coastal plant Iva frutescens L. (Asteraceae) are reduced in the presence of a second host, Borrichia frutescens (L.) (Asteraceae). In this system associational resistance is mediated by parasitoid natural enemies that emerge from Borrichia galls and attack galls on Iva, thereby reducing gall densities on Iva. We quantified distances between patches of Iva and Borrichia and the relative abundances of both plant species and predicted that gall densities would be reduced and parasitism rates elevated on Iva closer to Borrichia compared with more distant patches and on Iva occurring with relatively greater Borrichia abundance in comparison to reduced ratios of Borrichia to Iva abundance. Although gall densities were elevated on Iva more distant from Borrichia, as compared with Iva adjacent to Borrichia, parasitism rates were unaffected by patch distance in this system. Increasing relative abundance of Borrichia was found to significantly reduce gall densities on Iva, though parasitism rates on Iva galls were unaffected by Borrichia abundance. These results suggest other factors, e.g., environmental quality, host plant genotype, etc., may swamp out the effects of parasitoid—mediated AR in this system as Iva becomes more distant from Borrichia or as the abundance of Borrichia relative to Iva is reduced.

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