Analysis of the Biological Control of Mythimna separata (Lepidoptera: Noctuidae) by Apanteles ruficrus (Braconidae: Hymenoptera) in New Zealand

(1) Estimated levels of larval parasitism of the armyworm Mythimna separate (Walker) before and after the introduction of the parasitoid Apanteles ruficrus (Haliday) from Pakistan suggest that there has been an increase in average parasitism from c. 50-55% to c. 80%. Larval parasitism is not density dependent. (2) M. separate pupal parasitism is density dependent. (3) A. ruficrus parasitism reduces M. separate larval food consumption. The increased parasitism following the introduction of A. ruficrus is judged to have raised the armyworm population damage threshold in maize from c. 240-360 larvae m-2 to 400-530 larvae m-2. (4) The success of A. ruficrus in drastically reducing the pest incidence of M. separate is discussed in relation to its role in reducing food consumption as opposed to its lowering the average population levels of M. separate. (5) It is concluded that the ability of A. ruficrus to reduce the feeding potential of M. separate populations is probably the most important factor in its success as a biological control agent in New Zealand.