Getting to know Trioza apicalis (Homoptera: Psylloidea) – a Specialist Host-Alternating Insect with a Tiny Olfactory System
Abstract: This thesis comprises ecological, morphological and physiological studies of the carrot psyllid Trioza apicalis (Homoptera: Psylloidea) with particular focus on olfaction. Data from inventories of winter habitats suggest that T. apicalis prefer Picea abies over Pinus sylvestris and Juniperus communis as winter hosts. Their migration range is at least I km, and we found no clear wind-borne patterns. No differences with regard to migratory capacity or preferences were noted between the sexes. We investigate behavioural phototaxis effects on host choice and settling behaviour. The results show that carrot psyllids are able to detect and react to minute differences in light intensity. We provide a thorough description of the antennal sensilla, using SEM and TEM, and describe five (possibly seven) plausible types of olfactory sensilla. Several sensillar types located in cuticular cavities are revealed. The total number of olfactory receptor neurons was estimated to be around 50, and the total neuron count including other modality axons was 70-80 at the base of the flagellum. Electrophysiological experiments revealed that the carrot psyllid can detect a variety of compounds present in extracts from both winter and summer host plants, as well as from extracts of male or female conspecifics. Only very minor differences between male and female response patterns were observed. Four active compounds were identified and tested in dose-response trials. One of them, nonanal, was found in the Trioza extracts, and is thereby the first suggested pheromone compound in a psylloid. A classification of the olfactory receptor neuron (ORN) types present in the sensilla investigated here revealed a significant redundancy, meaning the total number of ORN types in T. apicalis will be considerably fewer than 50. In a comparative study, we show that the antennal lobes (ALs) in T. apicalis and two aphid species lack morphologically distinct glomeruli. We propose that this is due to the exceptionally low number of afferent neurons. Nevertheless, the loosely aggregated organisation of axon terminals indicates targeting to discrete foci within the AL, which are supposedly equivalent to functional glomeruli.
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