Successful Biological Control of the Musk Thistle in Oklahoma Using the Musk Thistle Head Weevil and the Rosette Weevil

Rhinocyllus conicus Froelich, the musk thistle head weevil, was released in northeastern Oklahoma beginning in 1991 for biological control of Carduus nutans L., the musk thistle. By 2001, weevils had been released in 34 counties. Trichosirocalus horridus (Panzer), the rosette weevil, was released in six counties in 1998; and additional introductions in 2000 and 2001 brought the total number of county releases for this species to 29. Release areas were surveyed in 2000 and 2001 to determine the level of weevil establishment. Head weevils were recovered from 30 of 34 counties. Thistle populations were considered well infested if 30% of the heads had >4 larvae or pupae present. In 63% of the counties, ≥25% of the sites were well infested. Thistle densities had been reduced by 25–90% in 13 counties in Oklahoma where weevils had been released. Rosette weevils were recovered in three of the six original release counties and in one county where no releases were made until 2001. A combination of head and rosette weevils in the rosette weevil recovery areas provided a synergistic reaction with thistle density reductions occurring faster than in sites where only head weevils were released. Head weevils are established in Oklahoma and are effectively reducing thistle infestations. Rosette weevils also are established in several of the 1998 release areas.