Collective defence in non-myrmecophilous aphids and its evolutionary origin.

Group-living insects increase their defensive capabilities by cooperatively fighting against attackers. Due to their high growth rate, aphid colonies are especially attractive to predators and parasitoids, which promoted the evolution of various defence mechanisms. In non-myrmecophilous aphid species colonies defence is accomplished by virtue of physical and chemical defence with the potential to deter or even repel aphid enemies. In some aphid species, such as Aphis nerii and Uroleucon hypochoeridis, collective defence in the form of synchronous abdomen twitching and leg kicking can be observed in various defence contexts. However, the functional role of this kind of collective physical defence (CPD) is still not clear and preliminary observations suggest CPD to cause a repelling effect against insects approaching from the air. In consequence, CPD may reduce the threat associated with kleptoparasitic wasps, ladybird beetles, lacewings, predatory midges and hoverflies. A high predation pressure exerted by these natural enemies suggests that CPD may have evolved in parallel multiple times in aphid species lacking a common ancestor. A major aim of this project is to investigate a possible repelling effect mediated trough CPD against insects approaching aphid colonies from the air and will be achieved by stereo imaging technology. Manipulation of colony size and aphid mobility is expected affect the repelling effect in behavioural experiments. Since only a certain number of individuals contribute to CPD elicited through visual stimulation, passive individuals have to be regarded as defectors. The number of active and passive individuals observed in behaviour studies allows calculating an evolutionary stable equilibrium of both players in a theoretical modelling approach. Additionally, stimuli triggering CPD from outside and cues synchronising collective defence within colonies will be studied in playback experiments. Such stimuli include particle oscillations, such as those found in the proximity of flying insects, and substrate vibrations generated either by approaching insects or by twitching individuals themselves. Finally, the evolution of CPD will be reconstructed in a phylogenetic approach encompassing different species belonging to the subfamily of Aphidinae. CPD is not the only collective behaviour that can be observed in U. hypochoeridis colonies. One can also observe collective dispersal during attacks of ladybird beetle larva as well as group migration at a critical stage of host desiccation. This project aims ruling out those cues eliciting aphid dispersal and coordinating collective migration. Alarm pheromones are possibly involved in colony arousal and will be studied in an analytical approach as well as in a behaviour assay. Additionally, we study substrate-borne vibrations generated by aphids in the course of colony disintegration and colony re-assembly. Results obtained in this project will shed light on the functional role of CPD in non-myrmecophilous aphids and will provide insight into proximate mechanisms coordinating collective defence, collective migration as well as colony re-formation in U. hypochoeridis colonies. This study will be complemented by a phylogenetic approach that aims to reveal the evolution of CPD in the subfamily Aphidinae.

Due to their high growth rate, plant lice have great potential as plant pests. Because they occur in large quantities, they also have numerous natural enemies, which they encounter individually or collectively through various defense strategies. The defensive reactions range from mechanical defenses, which are characterized by kicking with the hind legs and twitching with the abdomen, to chemical defense using sticky substances. In order to combat lice as effectively as possible with the help of biological agents, it is necessary to thoroughly investigate the defensive repertoire and defense strategies of various species of louse. Oleander lice often show collective mechanical defense responses after visual and vibratory stimulation. Behavioral tests with these lice showed that higher tones with a pitch of about 2- 4 kHz trigger defense movements even at very low intensities. Since natural enemies do not produce such high frequencies, it is believed that this frequency tuning of the sensorium has arisen in a different behavioral context (e.g., mating behavior). The comparison of different defensive behaviors between different species from Austria and Scotland has shown that the chemical defense is the most common form of defense reaction and that this is usually accompanied by mechanical defense reactions. A collective mechanical defense by synchronously twitching colony members was previously known only from a few species and could be found in the course of this project in a total of 19 species. A comparison between Scottish and Austrian species has revealed that in Scotland lice colonies rarely enter into cooperation with ants to settle the ant defense with sugar-sweet juices (honeydew). In order to be able to compare the behavioral repertoire between different species of louse, it is necessary to determine the specific species. However, the species identification of louse species is very difficult and often erroneous with classical morphological methods. Therefore, much effort has been put into this project to determine the louse species by genetic markers. For this, several genetic sequences had to be tested in order to find those gene segments that allow a reliable species identification. For this purpose certain gene sections of mitochondria, the power plants of the cells, as well as of the nucleus itself were analyzed. With these results, the basis has been created to identify the most common louse species by their genes. By means of gas chromatography and mass spectrometry, the chemical composition of the defense secretions of various species of louse was investigated and revealed that in addition to beta-farnesene, a common alarm substance, there is also a relatively high concentration of a currently unknown chemical component in various louse species, namely tetradecyl acetate. It is therefore possible that this volatile substance also plays a role in alarming the colony. However, behavioral tests could not confirm this assumption so far. Chemical profiles of volatile substances from lice extracts were also prepared and it was found that different species have very different chemical compositions. However, as there was already a great variability in the chemical composition of volatiles within a species, these profiles cannot be used for the chemical species identification of lice.