A MODEL FOR THE EFFECT OF SPHECOPHAGA-VESPARUM-VESPARUM AS A BIOLOGICAL-CONTROL AGENT OF THE COMMON WASP IN NEW-ZEALAND

1. A model of intermediate complexity is described to predict and unde rstand the reasons for the ultimate impact of Sphecophaga vesparum ves parum (Hymenoptera: Ichneumonidae), introduced as a classical biologic al control agent of Vespula vulgaris (Hymenoptera: Vespidae) in New Ze aland. The model was parameterized as far as possible from independent field data then fitted to the observed performance of the parasitoid over the first 5 years after release. 2. Wasp nest densities monitored over 5 years and seven sites in beech (Nothofagus spp.) forests avera ged 12.2 ha(-1), with a maximum of 33 ha(-1). These are among the high est Vespula densities in the world. A Ricker model accounted for chang es in nest density from year to year, giving a maximum ratio of increa se in nest density of 3.3 per year and overcompensating density depend ence at high densities, probably caused by queen competition and nest usurpation in spring. 3. The parasitoid has established at low levels in two sites, one of which has been studied in detail. Here parasitism levels (% autumn nests parasitized) have remained around 5% for 5 yea rs, with a slight suggestion of an upward trend. 4. The model suggests that ultimate parasitism levels depend almost entirely on the parasit oid's effective ratio of increase, R, defined as the maximum number of spring adults produced per spring adult (spanning several intermediat e summer generations). Ultimate suppression of wasp nest densities dep ends on R, the rate of increase in parasitism within a year, and the m ortality of parasitized early spring nests. The initial rate of build- up of parasitism additionally depends on the pattern of emergence of p arasitoid cocoons, which may extend over 4 years. 5. R for S. vesparum vesparum at the site where it has established appears to be about 1.3 -1.6, which is close to the lower limit of 1 for persistence. Such val ues suggest an ultimate suppression of wasp nest density and level of parasitism of about 10% and 25%, respectively. 6. The low value of R a t this site and the parasitoid's limited likely impact, appear to be d ue to a combination of delayed emergence of overwintering cocoons, low overwintering cocoon survival, and low production of cocoons per para sitized autumn nest. Additionally, a reduced or variable attack rate m ay contribute to the parasitoid's lack of establishment at other sites , possibly due to poor synchrony between spring emergence of wasp quee ns and adult parasitoids.7. A more successful parasitoid species or ec otype would need to have a higher R value, which requires cocoon emerg ence after 1 year rather than 2 or more, and/or higher values for the above parameters, particularly overwintering survival of cocoons. In a ddition, it would have to: act after the spring host density dependenc e; cause a high (> 80%) mortality of parasitized spring nests; and ide ally also reduce queen output from autumn nests. An alternative to an agent causing mortality to spring nests would be a microbial one which significantly reduced queen fertility without impairing her competiti ve ability. 8. A 50% reduction in queen output from autumn nests, due to parasitism by S. vesparum vesparum, contributes little to host supp ression because of its timing relative to host density dependence. If an alternative agent reduced wasp densities to a greater extent, the a dditional contribution of reduced queen output would become relatively more significant as density declined and the density dependence becam e less intense.