Life history and host specificity of Rhopalomyia n. sp. (Diptera : Cecidomyiidae), a potential biological control agent of scentless chamomile

The life history and host specificity of Rhopalomyia n. sp., a multivoltine gall former and potential agent for the biological control of scentless ch amomile [Tripleurospermum perforatum (Merat) Lainz, Asteraceae], was invest igated at field sites in Eastern Austria and under seminatural conditions i n Switzerland from 1995 to 1997. Rhopalomyia n.sp. produced 4 generations p er year in Switzerland and induced galls in different meristematic tissues of T. perforatum, including the apical meristems of rosettes and bolting pl ants, leaf axils, buds, and flowers. Galls formed in axillary buds or flowe rs were usually single-chambered whereas the majority of galls induced in a pical meristems consisted of several chambers (up to 80), each containing 1 larva. Gall volume increased with the number of chambers per gall. The gal l midge was attacked by 1 parasitoid species (Aprostocetus n. sp.) in the f ield. The probability of parasitoid attack was higher in multichambered gal ls than in single-chambered galls, whereas percentage parasitism per gall w as either unaffected by gall size or declined with an increasing number of chambers per gall. Gall midge females contained on average 92.5 eggs, of wh ich 20-96% were laid, depending on temperature. Females in culture produced an average of 61 offspring, and the gall midge population increased up to 150-fold during 2 generations. A threshold temperature of 7.02 degrees C wa s calculated, and 478.5 DD were necessary to complete development from egg to adult. A total of 51 plant species and varieties in 6 tribes in the fami ly Asteraceae were offered in no-choice and multiple-choice oviposition and development tests under seminatural conditions. Development to adult was r estricted to the genus Tripleurospermum, and T. perforatum was the preferre d host plant in all tests. Rhopalomyia n, sp. is a multivoltine species wit h the potential for rapid population increase. Its ability to induce galls on all plant stages, especially to attack the rosettes of scentless chamomi le in late summer and early spring, should supplement the effect of 2 univo ltine weevil species already released. Permission for field release of Rhop alomyia n. sp. in North America was granted in October 1998.