Aspects of the biology of the receptacle weevil, Rhinocyllus conicus (Froehlich) relating to its role as a biocontrol agent of nodding thistle, Carduus nutans L.

Abstract

Observations were carried out on the lifecycle of Rhinocyllus conicus (receptacle weevil) and growth forms of Carduus nutans (nodding thistle) from Nov. 1988 to June 1990 at Galatea in the central Bay of Plenty and on the coast at Whakatane. The dates of first emergence of adults in the spring, disappearance of the last adults in early summer, reappearance of adults in mid -summer and disappearance of the last adults before winter were noted. Oviposition in number and relative to thistle growth stage plus larval numbers in and their effect upon inflorescences was observed. Adult seasonal appearance and anatomical changes were noted macroscopically and investigated by microscopic dissection. Adult weevils were shown to disappear from the field in mid-summer and to hibernate from June to September-October. Hibernation sites are hard to find, and remain an unsolved problem. The phenology of nodding thistle in relation to the life-cycle of R. conicus was compared. The effect of livestock grazing and weather patterns on insect behaviour were recorded as were the feeding preferences of adult weevils.