Abstract
The univoltine leaf miner Chromatomyia fuscula Zetterstedt is a Scandinavian cereal pest. We wanted to compare the phenology of C. fuscula in southern Norway with that of its most important natural enemies: 15 parasitoids of the families Eulophidae and Pteromalidae (Hymenoptera: Chalcidoidea). The use of two Malaise traps in an organically-grown spring barley field and its boundary through 6 seasons (1992-1997) also allowed us to compare these two habitats and to observe the effect of harvesting on the parasitoid activity without interference from pesticides. C. fuscula overwinters as an adult and oviposits in May/June. Few specimens of the next generation, emerging in the crop, were caught in the boundary traps, suggesting the fly hibernates elsewhere. In contrast, the F1 generation of the parasitoids was caught in considerable amounts both in the crop and boundary. The abundance of parasitoids was highest in July/August; in the crop it usually started decreasing well before harvesting; in the boundary it peaked two weeks or more after harvesting. The results suggest that many parasitoids (especially females) move from the crop to the boundary (or beyond) before harvesting. In both habitats parasitoid species richness usually increased until harvesting, and thereafter decreased. The pooled parasitoid female proportion was 0.36; in crop and boundary it was 0.30 and 0.66, respectively, and the majority of species had a higher proportion of females in the boundary than in the crop. The phenology of two of the most common parasitoids is presented: The pupal parasitoid Cyrtogaster vulgaris Walker (Pteromalidae) had a high activity in the boundary, also very early (females only) and late (both sexes) in the season. The larval parasitoid Diglyphus begini (Ashmead) (Eulophidae) was less active early and late in the season, and had a much smaller boundary activity than C. vulgaris. Both sexes were present throughout the season. The annual sex ratio of D. begini was density dependent, being highly male biased in the two years with highest catches. In C. vulgaris neither density nor habitat explained the sex ratio. D. begini probably overwinters inside the mine as a preadult, having one generation on C. fuscula in the crop and another one in an alternate host away from the habitats sampled here. C. vulgaris overwinters as fertilized females in the border habitat.
Highlights
The leaf miner Chromatomyiafuscula (Zetterstedt) is a regular pest of cereals in Scandinavia (Andersen, 1991)
In a seven-year project we studied C. fuscula and its parasitoid complex in an organically-grown spring barley field in southern Norway
The project is described in Hagvar et al (1998), who pre sented the annual total Malaise catches of the adult leaf miner and 15 of its parasitoids over a period of six years, plus the annual composition of the parasitoid complex in the barley crop and its boundary
Summary
The leaf miner Chromatomyiafuscula (Zetterstedt) is a regular pest of cereals in Scandinavia (Andersen, 1991). The answers may contribute to a better under standing of how to preserve and enhance natural enemies of pests in general (Barbosa, 1998). In this context, the function of boundary habitat has received considerable attention (e.g. in Altieri, 1994; Ekbom et al, 2000), but the effect of harvest on the beneficial fauna is less studied, and we are not aware of any previous work on the effect of harvesting on adult parasitoid populations in cereals
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