Abstract
The biological control program against nodding thistle Carduus nutans commenced in New Zealand in 1972 with the introduction of the receptacle weevil Rhinocyllus conicus Seed predation by the larvae of this univoltine weevil did not bring about the desired reduction in nodding thistle populations and in 1992 a further seed predator was introduced the bivoltine gall fly Urophora solstitialis Possible interference between these two nichesharing agents may have resulted in lessthanadditive seed predation This recent study quantified the interactions between the two agents for the first time in New Zealand and found the mean number of gall flies per seedhead was reduced by 4693 in the presence of the receptacle weevil This suggests that the weevil may limit the gall flys populations thus reducing seed predation later in the nodding thistle flowering period The potential effects of this reduced attack on nodding thistle population growth rate were simulated using an existing matrix model
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