Occurrence and severity of the weeds Anthriscus caucalis and Torilis nodosa in pyrethrum

Pyrethrum [Tanacetum cinerariifolium (Trevis.) Sch.Bip.] is a short-lived, perennial horticultural crop grown for the production of pyrethrins. Weed management in pyrethrum is heavily reliant on chemical use and has unintentionally selected for a range of species. One of the major concerns to the pyrethrum industry is the presence of Apiaceae species, Anthriscus caucalis M. Bieb and Torilis nodosa (L.) Gaertn. Increased occurrence and severity of infestation of these species in pyrethrum is a threat to the industry through reduced crop yields and increased costs for their control. To determine the severity of this threat, pyrethrum crops were surveyed for the presence and level of infestation of A. caucalis and T. nodosa over 2 years. More than 30% of all pyrethrum crops had either 1 or both species present, with A. caucalis the more prevalent of the 2 species. Although the level of infestation was generally low, 23 pyrethrum crops had a severe or very severe infestation of the species in 2001. The severity of infestation was generally low, although older crops were found to have a higher level of infestation than newly established crops. All pyrethrum crops, which had undergone 4 annual harvests, were found to have Apiaceae weeds present. This work has documented the severity of occurrence of these species and highlighted the need to develop strategic control procedures to reduce their immediate impact and potential threat.