Factors Affecting Distribution of the Gall Forming Midge Rhopalomyia californica (Diptera: Cecidomyiidae)

Abstract

The cecidomyiid midge, Rhopalomyia californica Felt, forms galls on the perennial evergreen shrub Baccharis pilularis De Candolle, which is found in coastal regions throughout central California. R. californica , however, is not found in the more southern part of its host's range and is essentially absent south of Santa Barabara. Reasons for this restricted distribution were examined by field experiments at 2 sites: Santa Barbara and Big Creek, which is located 290 km to the North, well within the area where galls are widespread and abundant. No differences in plant suitability for oviposition were detected between the 2 sites when the midges were protected from predators, but midges responded differently to the plants, each midge producing more galls at Santa Barbara but each gall containing, on average, fewer larval chambers. Mortality caused by predators (or other sources of egg or early-larval mortality) was significantly higher at Santa Barbara than at Big Creek, reducing both the number of galls produced per female and also the average number of chambers per gall that was produced. Mortality caused by parasitoids was also significantly higher at Santa Barbara than at Big Creek. There was no evidence for spatial density dependence at either site. When combined with realistic parasitism rates and likely fecundities, such mortality rates result in a negative growth rate over the period studied for galls at Santa Barbara but a positive growth rate at Big Creek, providing a possible reason for the restricted distribution of the midge.