Oviposition behaviour and host selection of different German strains of the egg parasitoid Trichogramma aurosum Sugonjaev and Sorokina (Hymenoptera: Trichogrammatidae) were examined on eggs of five Lepidopteran hosts (Cydia pomonella (L.), Spodoptera littoralis (Boisd.), Helicoverpa armigera (Hübner), Agrotis segetum (Schiff.) and Lobesia botrana (Den. and Schiff.)). The parasitization behaviour of individual female wasps was examined in choice and non-choice tests. Single female wasps were observed for 90 min. using a rectangular grid. Results from the choice test revealed that 75–90% of C. Pomonella eggs attacked by T. aurosum strains were successfully parasitized. Values for L. Botrana and for A. segetum ranged between 40–80% and 40–70%, respectively. No significant preference was found between C. Pomonella eggs and host eggs of both L. botrana and A. segetum. Time needed by the female wasps to drill on eggs of L. Botrana was shorter than the time needed for drilling on C. Pomonella eggs. This could be due to differences in egg chorion thickness and, presumably, differences in chorion hardness. During direct observation of the parasitism behaviour it was noticed that all strains spent about 20% of the observation time for drilling on either C. Pomonella or L. Botrana eggs, 30–60% on resting, 4–15% on cleaning, < 4% on walking and < 1% on feeding. Results from the non-choice test showed that a significantly higher number of C. pomonella and L. botrana eggs were parasitized in comparison to the other hosts offered. The presence of hair-like structures deposited on eggs of S. littoralis and a
thick egg chorion in H. armigera seems to represent a physical barrier that impedes successful parasitization.