The current status of Argentine stem weevil (Listronotus bonariensis) as a pest in the North Island of New Zealand

Abstract

Recent sampling in Waikato and Taranaki shows that Argentine stem weevil (Listronotus bonariensis) can still cause major damage to susceptible ryegrass pastures despite the introduction of the parasitoid, Microctonus hyperodae, in 1991. Percentage of tillers with L. bonariensis larval damage in diploid and tetraploid perennial ryegrass and Italian ryegrass in January and February were between 11% and 68%. High egg numbers were noted on occasion. Although there is evidence that M. hyperodae reduces host populations, parasitism levels vary considerably between years in Canterbury and are low in Otago and Southland where high populations of L. bonariensis have recently been recorded. Factors that may be lessening the impact of M. hyperodae are considered. These include asynchronous generations of the parasitoid and its host, host behavioural adaptations, compensatory oviposition and inhibition of flight in parasitised individuals leading to low levels of parasitism in new pastures.