Olfactory and visual cues in host-finding in the Burnet moth, Zygaena trifolii

Abstract

Zygaena trifolii (Esper) moths are strongly attracted to the flowers of Dipsacaceae, such as Knautia arvensis (L.) Coulter and other scabious-like flowers. This attraction is particularly obvious in habitats where other nectar sources are readily available. It may thus be assumed that during the life cycle of Burnet moths, scabious flowers offer resources that are of special significance. We report on the physiological significance of volatile compounds of K. arvensis flower-scents to Z. trifolii moths, as tested by their antennal response and their attractiveness in the field. Methods Investigations were made by testing the EAG-activity of five fractions of the crude extract of K. arvensis, of synthetic compounds and of the synthetic female sex pheromone. Volatile compounds of K. arvensis were collected using the CLSA-technique (Grob, 1973). The five fractions were obtained by preparative gas chromatography. Pure synthetic samples formed the reference stimuli. EAG experiments were carried out with freshly excised antennae by following the procedure described by Van der Pers (1981). The chemical composition of volatile compounds in the scent of K. arvensis was identified by capillary gas chromatography and mass spectrometry. Field tests were carried out using coloured cardboard discs of 35 mm 0, which differed in the spectral composition of the colour, providing a nectar source and rubber caps, which bore the test compounds. We recorded only the complete feeding reactions (proboscis reaction). Results and discussion Fractions 1, 2 and 3 were clearly dominated by cc-pinene, 1,8-cineole and verbenone, while the rest of the odour contained only small amounts of multicomponent mixtures. The verbenone of fraction 3 was shown to be the (-)-enantiomer of a very high optical purity. The average quantity of (-)-verbenone in crude extracts is about 200 ng/50 flower heads. The tests with the crude extracts of K. arvensis and synthetic compounds clearly showed that an olfactoric stimulus is provided in the flower scent, resulting in great biological activity at the electrophysiological level, and in field tests. This high electrophysiological activity led to the hypothesis, that K. arvensis flowers contain olfactoric stimuli for the feeding behaviour of Burnet moths. Our field data demonstrate that the attraction of wild flying Z. trifolii moths to artificial flower heads is dependent on both a visual stimulus and the scent stimulus provided by from the crude extract. In the absence of either, settling behaviour and the feeding reaction were not observed. Changes in the spectral composition of the colour immedi- ately led to a break in the biological reaction. These findings agree with the concept of Schneider (1986), that insect orientation in the field does not depend on a single stimulus, but on a number of different stimuli that are registered and evaluated at the same time.