Plasticity of host finding behaviour of herbivorous insects in dependence of environmental factors
Final Report Abstract
Herbivorous insects face a very complex environment composed of multi-modal cues like visual appearance, odour and mechanical traits. The response to cues characterising different host plant species together with the insect’s performance on tissue of varying food quality may strongly depend on the experience of the individual. In this study, the plasticity of host finding behaviour and adjustments in performance were investigated in the oligophagous mustard leaf beetle, Phaedon cochleariae (Coleoptera: Chrysomelidae). This species uses various Brassicaceae species as hosts, which are characterised by glucosinolates as secondary plant compounds. The beetle species was used as a model organism to extend our knowledge on various aspects of host plant-herbivore interactions. The relevance of different cues in short-range orientation of adult beetles was evaluated in simple arena assays. Black and white contrasts evoked an attraction independent of stripe orientation suggesting that vegetation structure is of little importance. Green was highly attractive to both males and females, whereas yellow, which is described as super-normal stimulus for several herbivores, was less stimulatory. As leaves turn yellow during maturation and their quality decreases, yellow may indicate poor quality to P. cochleariae. The walking response was determined mainly by the dominant wavelength and less by colour intensity. Overall, females seemed to respond more sensitively to colours. The responses of adults to odours were tested in static olfactometers. The complex odour blends emitting from leaf tissue of several host plant species (Sinapis alba, Nasturtium officinale, Brassica rapa) attracted adult beetles. In addition, beetles orientated towards odours of non-host plant species (Bunias orientalis, Tropaeolum majus, both Brassicales). Amongst others, isothiocyanates as products of glucosinolate degradation may cause the attraction. Odours of other non-host plant species did not cause rejection but were ignored. Feeding choice assays were conducted to characterise stimulants and deterrents relevant for host plant recognition of adult P. cochleariae. Leaves of non-host and host species were extracted in solvents of different polarity and further fractionated by solid phase extraction. Methanolic extracts of all Brassicales had high stimulatory activity, in part due to the presence of glucosinolates, whereas the methanolic fraction of Plantago lanceolata was deterrent, due to iridoid glucosides. Further bioassay-guided fractionation was carried out by semi-preparative high performance liquid chromatography with B. orientalis, which currently spreads in Germany as a neophyte. The fraction of B. orientalis containing glucosinolates acted stimulatory, whereas the fraction with flavonoids deterred P. cochleariae. The specific composition of both stimulatory and deterrent compounds in B. orientalis may be responsible for the low acceptance of this plant species. Pre-feeding behaviour was filmed and behavioural steps analysed on host and non-host leaves. In accepted plant species, short test biting and short handling time were followed by long feeding durations. In contrast, unaccepted plant species evoked long test biting and only short feeding bouts. These steps are thus highly useful to predict plant suitability. Rearing on certain host plant species over several generations influenced the performance of P. cochleariae. The plant species identity affected the extent of the beetle’s adjustment to its particular host. Different developmental adjustments to various host plant species may be due to directional selection, eventually followed by stabilising selection, maternal effects and/or genetic drift. However, no evidence of imprinting was found. Furthermore, rearing history did not influence oviposition preferences of beetles even after 11 generations. All females preferred to oviposit on B. rapa, which may indicate a positive preference-performance relation or be based on the physical traits of this plant. The present study clearly showed that the oligophagous beetle P. cochleariae is highly plastic in its response to various plant cues of host and non-host species, whereby experience plays an important role.
Publications
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(2009) Differing acceptance of familiar and unfamiliar plant species by an oligophagous beetle. Entomologia Experimentalis et Applicata 131, 189-199
Kühnle A. & Müller C.
