The present study investigates the effect of root volatiles on the orientation behaviour of cockchafer larvae Melolontha hippocastani in the soil. Cockchafer larvae are known as severe pests in agriculture and forestry. The present study adresses three relevant aspects of their chemical ecology linked to food choice belowground: Volatile organic compounds (VOCs) of several potential host plant roots and shoots are investigated and analysed by using gas chromatography mass spectrometry (GC-MS). The obtained volatile patterns of damaged roots differ clearly from the undamaged ones. Emitted shoot volatiles differ clearly from the emitted root volatiles. Electrophysiological methods are employed to record sensory reactions of the detached larval antennae to several of the previously identified compounds. Volatile emissions of Quercus sp. are investigated in detail. By using electroantennography (EAG), changes in the receptor potential elicited by odour stimuli are recorded. It turns out that seasonal and maybe also circadian rhythms appear to play an important role in cockchafer larvae antenna physiology. The behavioural response of cockchafer larvae is tested on selected compounds. A dual choice test setup is designed to study the behavioural orientation of the belowground living larvae. Attractive and repellent compounds in host plants are identified. Preparative methods show the antennal lobes (ALs, first central processing unit for olfactory information processing in the insect brain) of 3rd instar of M. hippocastani to 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 volatiles emitted by roots of their host plants Quercus sp. in electrophysiological and behavioural tests.
Beeinflussen verschiedene Volatilenmuster die Wirtspflanzenwahl unterirdisch lebender Insekten? Zum besseren Verstandnis der Orientierung und Fraspraferenz von Maikaferlarven Melolontha hippocastani FABR. (Coleoptera: Scarabaeidae) im Boden wurden Wahltests durchgefuhrt. Den Larven von M. hippocastani wurden Karotten (Daucus carota ssp. sativus) und Eichenwurzeln (Quercus sp.) zur Auswahl angeboten. Die Duftstoffe von Karotte und Eichenwurzel wurden auf Aktivkohle gesammelt und mit Gaschromatographie - Massenspektroskopie untersucht (GC-MS). Unverletzte Karotten sowie Eichenwurzeln unterschieden sich in ihren Volatilenmustern deutlich voneinander. Daruber hinaus konnten Unterschiede im Volatilenmuster unverletzter, mechanisch verletzter sowie angefressener Wurzeln nachgewiesen werden.
In many insect species olfaction is a key sensory modality. However, examination of the chemical ecology of insects has focussed up to now on insects living above ground. Evidence for behavioral responses to chemical cues in the soil other than CO2 is scarce and the role played by olfaction in the process of finding host roots below ground is not yet understood. The question of whether soil-dwelling beetle larvae can smell their host plant roots has been under debate, but proof is as yet lacking that olfactory perception of volatile compounds released by damaged host plants, as is known for insects living above ground, occurs. Here we show that soil-dwelling larvae of Melolontha hippocastani are well equipped for olfactory perception and respond electrophysiologically and behaviorally to volatiles released by damaged host-plant roots. An olfactory apparatus consisting of pore plates at the antennae and about 70 glomeruli as primary olfactory processing units indicates a highly developed olfactory system. Damage induced host plant volatiles released by oak roots such as eucalyptol and anisol are detected by larval antennae down to 5 ppbv in soil air and elicit directed movement of the larvae in natural soil towards the odor source. Our results demonstrate that plant-root volatiles are likely to be perceived by the larval olfactory system and to guide soil-dwelling white grubs through the dark below ground to their host plants. Thus, to find below-ground host plants cockchafer larvae employ mechanisms that are similar to those employed by the adult beetles flying above ground, despite strikingly different physicochemical conditions in the soil.
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