The importance of gall size for sea daisy gall midge parasitoids

The sea daisy gall midge, Asphondylia borrichiae (Diptera: Cecidomyiidae), was used as a model for understanding the selective advantage of galling in insects. This midge is attacked by four species of hymenopteran parasitoids. Parasitoid egg-laying may be inhibited by thick layers of gall tissue, but small galls may have few or small fly larvae. Asphondylia borrichiae was found on Borrichia frutescens (Asteraceae) and Iva frutescens (Asteraceae); it makes smaller, less fleshy galls on the latter plant. If gall size is important in determining parasitoid oviposition success, it follows that there should be different levels of parasitism on the two types of galls. Also, the size of galls on both hosts was evaluated as a source of information for parasitoids about available fly biomass. Parasitism rates were much higher on I. frutescens, but this result may have been due to the large population of A. borrichiae on this host. Gall size is positively correlated with the number of chambers and gall age in B. frutescens and the number of chambers in I. frutescens. However, all correlations were weak.