The Effect of Root Volatiles on the Orientation Behaviour of Cockchafer Larvae in the Soil

Abstract

reproducible results throughout one season. However, over some period of time, a small part of the tested antennae showed similar responses to special compounds, which were anisol, 1.8-cineol, (1R)-camphor, 3-octanone and furanoid translinalooloxid.The third part of this study tested the behavioural response of cockchafer larvae on selected compounds. A dual choice test setup was designed to study the behavioural orientation of the belowground living larvae. The compounds, which were able to attract the larvae of M. hippocastani were anisol, 1,8-cineol, and terpinolene. A repellent effect was elicited by acetone (as a representative of the compounds with a high vapour pressure) and β-caryophyllene. General results: The emitted shoot volatiles of Quercus sp. and A. hippocastanum differ clearly from the emitted root volatiles. Electrophysiologically active compounds could (in a reproducible manner) only be detected in special time periods during the season, which suggests that seasonal and maybe also circadian rhythms play an important role in the complex system of impulse processing in cockchafer larvae. Distinct orientation behaviour of the larvae of M. hippocastani could be observed in dual choice tests with several compounds found in root volatile samples of Quercus sp. damaged by larval feeding. Attractive and repellent compounds could be identified. The antennal lobes (ALs, first central processing unit for olfactory information processing in the insect brain) of 3 rd instar of M. hippocastani contain a high number of glomeruli, which are regarded as the functional subunits of odour discrimination. Therefore, a highly developed odour discrimination ability of the cockchafer larvae is indicated.These findings are, to our knowledge, the first proof that larvae of M. hippocastani are able to perceive several volatiles emitted by roots of their host plants Quercus sp. in electrophysiological and behavioural tests. As a consequence, in dual choice tests they react with attractive or repellent behaviour. As a practical approach, these results could be used as a basis for semi-field studies towards pest control through attractive or repellent volatile compounds. However, in this context, additional investigations concerning the compatibility of the volatile composition (either single The Effect of Root Volatiles on the Orientation Behaviour of Cockchafer Larvae in the Soil iii compounds or volatile mixtures) and the carrier substance on floral and faunal organisms activity (especially on non-target organisms) are necessary. iv Zusammenfassung In der vorliegenden Dissertation wurde der Einfluss von Wurzelvolatilen auf das Verhalten von unterirdisch lebenden Waldmaikäferengerlingen Melolontha hippocastani untersucht. Diese Larven sind als ernstzunehmende Schadorganismen in Wäldern (z.B. Aufforstungsflächen von Eichenbeständen) sowie in verschiedenen Kulturen wie Wein-und Obstgärten, Wiesen und Weiden gefürchtet. Drei Aspekte der Chemischen Ökologie wurden näher beleuchtet, um zu einem besseren Verständnis der Verh...