The parasitoid Microctonus hyperodae Loan ( Hymenoptera: Braconidae) was introduced into New Zealand to control the weevil Listronotus bonariensis ( Kuschel) ( Coleoptera: Curculionidae), a major pest of graminaceous plants. Four experiments were conducted to investigate the effects of various pesticides that are commonly used in the pastoral environments of L. bonariensis and M. hyperodae. Topical applications of aqueous solutions prepared from commercial formulations of five herbicides were not toxic, but the surfactant Silwett L-77 increased M. hyperodae mortality relative to the water-treated controls. Laboratory assays showed that M. hyperodae adults were susceptible to chlorpyrifos residues on pasture foliage following application of the insecticide to field plots at >= 5 g a. i. ha-1. Maintenance of L. bonariensis on ryegrass in the laboratory showed that treatment of the food plants with chlorpyrifos at >=96 g a.i. ha-1 reduced L. bonariensis survivorship and ultimately reduced M. hyperodae prepupal emergence from those hosts. Initially, mortalities of non-parasitized L. bonariensis were significantly greater than for parasitized L. bonariensis. Maintenance of parasitized L. bonariensis on diflubenzuron-treated ryegrass plants arrested M. hyperodae larval development in the host and ultimately reduced prepupal emergence of M. hyperodae from those hosts. Despite the arrested development of M. hyperodae, the mortality of L. bonariensis hosts was increased. Adult M. hyperodae successfully reared from hosts maintained on diflubenzuron ( 12.5 g a. i. ha-1) treated food plants had reduced reproductive potential. The consequences of pasture management strategies that employ pesticides are discussed in relation to biocontrol of L. bonariensis by M. hyperodae. none