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Comparison of closely related species can elucidate adaptive differences in species characteristics. The present study compares the effect of experience on the host-finding behaviour of two Cotesia (Hymenoptera: Braconidae) parasitoid species that differ in their degree of specialization. After multiple experiences with host larvae, females of the generalist parasitoid Cotesia glomerata showed a clear preference for volatiles from Pieris brassicae-infested Brussels sprouts leaves over P. rapae-infested Brussels sprouts leaves in two-choice tests ('preference learning at herbivore level'). A single experience with a host did not lead to such preferences.Experience of adult C. glomerata with different P. brassicae-infested cabbage varieties or nasturtium (Tropaeolum majus L.) led to preferences for the experienced plant-host complex in most cases ('preference learning at plant level'). No effect of rearing plant (early adult learning) on plant preference was found.In contrast to the generalist C. glomerata, females of the specialist C. rubecula did not show preference learning at the herbivore level. At the plant level, experience with different P. rapae-infested cabbage varieties in no case resulted in a difference in preference between treatments. The results support the hypothesis that learning plays a more important role in searching in generalists than in specialist parasitoids. The behaviour of the generalist C. glomerata was more easily changed by experience than that of the specialist C. rubecula.

The role of volatile stimuli in the long‐range host‐searching behaviour of the specialist parasitoid Cotesia rubecula Marshall (Hymenoptera: Braconidae) was studied. Components from the plant‐host‐complex Brussels sprouts (Brassica oleracea L. var. gemmifera (DC.) Schulz. cv. ‘Titurel’)‐Pieris rapae L. (Lepidoptera: Pieridae) were compared for their attractiveness in dual choice tests in a windtunnel. Stimuli from cabbage plants that were mechanically damaged or damaged by P. rapae caterpillars were more attractive to this parasitoid species than stimuli emitted by the host larvae or their faeces. Parasitoids preferred leaves from the plant‐host‐complex over artificially damaged leaves. Undamaged cabbage plants were the least attractive to the foraging females. These results indicate that in‐flight searching behaviour of C. rubecula is guided by plant‐derived information and that for this specialist species more reliable and specific host‐derived cues play a minor role at longer distances.

SUMMARYThe fl-galactosidase gene (lacZ) of Escherichia coli was inserted in phase with the coding sequence of the Autographa californica nuclear polyhedrosis virus (AcMNPV) late-expressed Mr 10000 (pl0) gene. The fusion gene was inserted into the AcMNPV genome by cotransfection of a recombinant plasmid pAcR159Z, consisting of the EcoRI P fragment-containing pBR325-derived plasmid pAcR159 and the lacZ insert inthe p 10 gene, and wild-type AcMNPV DNA. Infection of Spodopterafrugiperda cells by the resulting recombinant AcMNPV/p 10Z-2 showed high level expression of a p 10-lacZ fusion protein, but no synthesis of pl0. Therefore, the pl0 gene is dispensable for virus replication and the pl0 promoter is effective in driving the expression of foreign genes. Cells infected with AcMNPV/pl0Z recombinants resembled those infected with wild-type AcMNPV in the amounts of polyhedrin synthesized and polyhedra formed, although pl0 was absent. The nucleus and cytoplasm of AcMNPV/pl0Z-2-infected cells lacked the fibrous structures that are associated with pl0 in wild-type AcMNPV-infected cells. Instead, large granular structures were observed that were found by immunogold labelling to contain the lacZ gene product. The electron-dense 'spacers', thought to be precursors of the polyhedron membrane, were absent from cells infected by the recombinant virus and the polyhedra did not have a membrane. The recombinant AcMNPV/pl0Z-2 was at least twice as virulent for second instar S. exigua larvae than was wild-type AcMNPV. The increased virulence of the recombinant is an important property for the control of insects.

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