Räumliche Konkurrenz der Larvalparasitoide Habrobracon hebetor (Say) (Hymenoptera: Braconidae) und Venturia canescens (Gravenhorst) (Hymenoptera: Ichneumonidae) um die Mehlmotte Ephestia kuehniella Zeller (Lepidoptera: Pyralidae)

The Mediterranean flour moth Ephestia kuehniella (Lepidoptera: Pyralidae) is a suitable host for the hymenopteran parasitoids Habrobracon hebetor (Hymenoptera: Braconidae) and Venturia canescens (Hymenoptera: Ichneumonidae). These two larval parasitoids differ in the following biological traits: H. hebetor is an idiobiontic, gregarious ectoparasitoid, and V. canescens is a koinobiontic, solitary endoparasitoid. Previous laboratory data obtained in small-scale experiments (Petri-dish) showed that moth larvae parasitised by V. canescens were paralysed by H. hebetor, i.e. the latter parasitoid out-competed V. canescens completely. However, field observations in Central European bakeries and mills showed that both species occur simultaneously. Moreover, V. canescens seems to be more common than H. hebetor in these stored-product environments harbouring populations of E. kuehniella. These observations led to the hypothesis that spatial scale affects the competition between the two parasitoids. Keeping the host-parasitoids ratios constant, experimental chambers with three different volumes were tested, with a maximum volume of 8m3. Hosts were exposed in four patches per room. In trials conducted in small boxes, the increase of parasitism by increasing numbers of female H. hebetor was confirmed. It was shown that the presence of V. canescens did not affect parasitism by H. hebetor under these conditions. With increasing numbers of female H. hebetor in the box, the numbers of emerging adult E. kuehniella decreased. The additional presence of V. canescens drastically reduced the numbers of E. kuehniella emerging in all trials. Spatial effects on competition were detected by comparison of experimental boxes and cages with different volumes. With increasing volume, the percentage of V. canescens emerging was constant, and the number of H. hebetor progeny was decreasing. Patch exploitation was decreasing in H. hebetor with increasing volume, but not in V. canescens. The data suggest that V. canescens is more competitive in large rooms where H. hebetor fails to completely find and exploit host patches.