Effects of land cover on parasitism of the red admiral, Bassaris gonerilla (Lepidoptera: Nymphalidae)

Eckehard Brockerhoff *1, Inge van Halder 2

1 Scion, PO Box 29 237, Christchurch 8540, New Zealand
2 INRA, Pierroton, France

The red admiral, one of New Zealand’s few butterflies, feeds exclusively on nettles, primarily tree nettle, Urtica ferox. According to previous studies, two introduced parasitoids, the Australian ichneumonid Echthromorpha intricatoria and the European pteromalid Pteromalus puparum, can be responsible for remarkably high rates of parasitism. There are also concerns that admiral populations appear to be declining. Because both introduced parasitoids are apparently primarily associated with Lepidoptera occurring in agricultural and other non-forest areas, we hypothesised that admiral parasitism is affected by the vegetation surrounding nettle patches. We tested this by investigating pupal parasitism of admirals in eight study areas with different land cover across Banks Peninsula. In each study area we collected pupae from nettles that were either adjacent to agricultural land or mainly surrounded by native forest. This enabled us to test both the effects of land cover in the wider landscape context and the effects of the local vegetation around nettles. The overall parasitism rate was ca. 75%, however, parasitism varied considerably among sites, and reached 100% at several sites. Parasitism was not consistently related to the amount of non-forest in the wider landscape, and both parasitoids penetrated areas with high forest cover. However, pupae from forest sites, where nettles were entirely surrounded by forest, had considerably lower or no parasitism. This suggests that the vegetation directly surrounding nettle patches influences parasitism, although these findings could also be related to density dependence effects because admirals were generally rarer inside forest than at forest edge sites nearby.

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