Establishment of Rhinocyllus conicus Froelich on Carduus macrocephalus in Texas

The number of countries in Texas with infestations of an exotic thistle, Carduus macrocephalus (Desf.) (Asteraceae), increased from one to twenty-eight between 1975 and 1993. The weevil Rhinocyllus conicus Froelich was released as a biological control agent on C. macrocephalus in Ellis County, Texas, in 1984 and Kerr County, Texas, in 1987. A population of the weevil established at the Kerrville site by 1992 and studies to evaluate its effectiveness were conducted in 1992 and 1993. In 1992, female weevils laid 606 eggs per plant, and larvae infested 89% of the flowerheads. In 1993, females laid 346 eggs per plant, and infested 92% of the heads. Seed production of infested plants was reduced 81% in 1992 and 90% in 1993 compared with that of uninfested plants. The number of eggs and larvae was greatest in the terminal heads and sequentially decreased in lateral, sublateral, and axillary heads. Weevils migrated 2.5 km from the study site in 1992 and 5.3 km from the site in 1993. In 1992, additional releases of R. conicus were made in Kerr, Gillespite, and Menard counties. This represents the first release and establishment of a biological control agent on Carduus in Texas.