Submitted to: Southwestern Entomologist
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 12/12/1995
Publication Date: N/A
Citation: N/A Interpretive Summary: The flowerhead weevil Rhinocyllus conicus is a successful biological control agent of musk thistle in the United States. It has been released and is established on this thistle in at least 25 states, of which about 15 states report at least partial control. We made a small release of this weevil in Louisiana in 1976 and another release in 1994. Our surveys indicate the weevil now occurs in four parishes in northwest Louisiana and infests up to 85% of the flowerheads at one site. Additional research will be conducted to determine its ability to control the thistle.
Technical Abstract: Adults of the biological control agent Rhinocyllus conicus Froelich (Coleoptera: Curculionidae) were released in Louisiana for control of musk thistle, Carduus nutans L. (Asteraceae). Two releases of about 450 to 500 weevils each were made in June 1975 in southern Red River Parish. On 16 July 1976, 6 sites were surveyed for Rhinocyllus conicus- infested flowerheads. Three sites were located in Red River Parish and one site each in Bossier, Caddo and DeSoto Parishes. Flowerhead infestations ranged from 0% in Bossier Parish to 43% at one site in Red River Parish. Mean weevil larvae per infested flowerhead ranged from 0 to 2.7. Flowerhead infestation was 12.5% at the 1975 release site. Infestations at two sites approximately 32 and 48 km from the 1975 release site indicate that the short-term dispersal capability of R. conicus may be greater than previously reported. On 20 May 1994, a 16-site survey was conducted in northwestern Louisiana. Five sites were located in Red River Parish, four in southern Caddo parish, four in northern Caddo Parish and three in Bossier Parish. Flowerhead infestation ranged from 0 to 84.2%, and mean weevil larvae per infested flowerhead ranged from 1.0 to 3.4. Sample Site 7 in the 1994 survey was nearest to the original 1975 release point. Here, 65% of the flowerheads were infested with a mean of 2.2 weevil larvae per infested flowerhead.