Insect neurophysiology and botanical pest control “Stochastic resonance in insect hearing and alternative locust control” Doctoral thesis Presented to the Faculty of Natural Science of the University of Graz In Fulfilment of the Requirements for the Degree of Doctor rerum naturalium (Dr. rer. nat.) submitted by Zainab Ali Saad Abdelatti BSc, MSc at the Institute of Biology Neurobiology and Behaviour Under the supervision of Assoz. Prof. Dr. Mag. rer. nat. Manfred Hartbauer 1. Reviewer: Assoz. Prof. Dr. Mag. rer. nat. Manfred Hartbauer 2. Reviewer: Associate Prof. Dr. Alberto Pozzebon 2021 II For my mother’s soul III Zusammenfassung Orthopteren-Insekten bilden die wichtigste Gruppe pflanzenfressender Insekten. Dieses Insektentaxon fasziniert Forscher seit Jahrzehnten aufgrund der stereotypen akustischen Kommunikation und des verheerenden Schwarmverhaltens mancher Arten. Der Zweck des ersten ist das Anlocken von potentiellen Partnern, während das Schwarmverhalten durch eine hohe Bevölkerungsdichte ausgelöst wird und als Gegenmaßnahme zur Vermeidung von Nahrungsmittelknappheit und Kannibalismus angesehen werden kann. In meiner Doktorarbeit habe ich beide Themen an Modellarten der Ordnung der Orthopteren untersucht. Zuerst untersuchte ich die Signaldetektion unter lärmhaften Bedingungen bei der tropischen Laubheuschreckenart Mecopoda elongata, wo Männer periodische Chirps erzeugen, um Weibchen akustisch anzulocken. Im Lebensraum dieser zirpenden Art erzeugt eine geschwisterliche Mecopoda-Art kontinuierliche Trillergesänge mit hoher Intensität und mit einer ähnlichen spektralen Zusammensetzung im Vergleich zum Lockgesang der zirpenden Art. Daher ist das Hintergrundrauschen im natürlichen Lebensraum hoch, und ich habe die Hypothese getestet, ob der heterospezifische Gesang die Erkennung von leisen konspezifischen Chirps, die unterhalb der Hörschwelle sind, aufgrund eines Phänomens verbessern kann, das als stochastische Resonanz (SR) bezeichnet wird. SR ist allgemein als paradoxes Phänomen bekannt, das die Erkennung schwacher Signale unter verrauschten Bedingungen verbessert. Zweitens entwickelte ich ein neuartiges botanisches Pestizid auf Basis von Leinöl, um die Wüstenheuschrecke Schistocerca gregaria und die Wanderheuschrecke Locusta migratoria zu bekämpfen. Beide Arten können zerstörerische Schwärme bilden und eine große Bedrohung für die Landwirtschaft darstellen, wo immer sie auftreten. In der Tat entdeckte ich eine neuartige botanische Ölmischung, die gegen alle Stadien dieser Heuschrecken hochwirksam ist. Ich bestimmte die letale Konzentration von 50% (LC50) für diese botanische Pestizidformulierung und untersuchte eine mögliche toxische Wirkung auf nützliche und alternative Nichtzielarten. Schließlich testete ich die Hypothese, ob in Leinöl enthaltene Fettsäuren als schwarmstörendes Mittel für Wüstenheuschrecken wirksam sind. Zumindest bei Männchen scheint dies der Fall zu sein. In weiteren Experimenten bestätigte ich, dass Wüstenheuschrecken ein nekrophobes Verhalten zeigen. IV Abstract Orthopteran insects constitute the most important group of plant-feeding insects. This insect taxon has fascinated researchers for decades because of its stereotyped acoustic communication and devastating swarming behaviour. The purpose of the first is mate attraction, whereas the latter is triggered by high population density and can be seen as a countermeasure to avoid food shortage and cannibalistic interactions. In my doctoral thesis, I studied both of these topics in model species belonging to the order of Orthoptera. First, I studied signal detection in noisy conditions in the tropical bushcricket Mecopoda elongata, where males produce periodic chirps to attract females acoustically. In the habitat of this chirping species, a sibling Mecopoda species produces continuous trills at high intensity with a similar broadband spectral composition compared to the conspecific song of the chirper. Therefore, background noise is high in the natural habitat and I tested the hypothesis whether or not the heterospecific calling song may improve the detection of subthreshold conspecific signals due to a phenomenon that is termed stochastic resonance (SR). SR is generally known as a paradox phenomenon enhancing the detection of weak signals in noisy conditions by stochastic noise. Second, I developed a novel botanical pesticide based on linseed oil to control the desert locust Schistocerca gregaria and the migratory locust Locusta migratoria. Both species can form destructive swarms in their gregarious phase and pose a great threat to agriculture wherever they appear. Indeed, I discovered a novel botanical oil mixture that is highly effective against all stages of gregarious locusts. I determined the lethal concentration 50% (LC50) for the botanical pesticide formulation and studied a possible toxic effect on beneficial and alternative non-target beetle species. Finally, I tested the hypothesis whether or not fatty acids contained in linseed oil act as a swarm disruptive agent for desert locusts. At least for males this seems to be the case. In additional experiments, I confirmed that males of desert locust show a necrophobic behaviour. V Content Subject Pages Zusammenfassung IV Abstract V List of Abbreviations VII Chapter 1: Synopsis 1 Main goals of the present thesis 7 Summary Chapter 2 8 Summary Chapter 3 9 Summary Chapter 4 11 References 13 Chapter 2: The heterospecific calling song can improve 19 conspecific signal detection in a bushcricket species Chapter 3: Plant oil mixtures as a novel botanical pesticide to 38 control gregarious locusts Chapter 4: Linseed oil affects aggregation behaviour in the desert 59 locust Schistocerca gregaria—A potential swarm disruptive agent Bibliography 79 List of Figures 92 List of Tables 94 Scientific contributions 95 Statutory Declaration 98 Acknowledgement 100 VI List of Abbreviations SR Stochastic resonance TN-1 T-shaped interneuron SPL Sound pressure level SNR Signal-to-noise ratio ISI Interspike intervals L. emulsion Linseed oil/bicarbonate emulsion FAO Food and Agriculture Organization of the United Nations WHO World Health Organization MMM Male-male mounting PAN Phenylacetonitrile VII Chapter 1: Synopsis The order Orthoptera (Eng. "straight wings") is one of the oldest lineages of insect species (Grzimek et al., 2004). It includes various hemimetabolous insect species, many of them are economically important, such as swarming locusts. Orthopterans are also famous for the airborne sounds they produce for the purpose of acoustic communication, with most grasshoppers calling during the day and katydids and crickets singing at night (Gwynne et al., 1996). Sound production and swarm formation are common behaviours in many orthopteran species (Parsons, 2010), thus insects of this order were considered important subjects for numerous scientific studies: singing orthopterans serve as model organisms for questions addressing intraspecific communication and swarming locusts as model organisms for the phenotypic transition that may cause a destructive plague (Parsons, 2010). Orthopterans are the dominant terrestrial herbivorous insects with chewing mouthparts and hind legs modified for rapid escape jumps (Naskrecki, 2013). Based on the size of the antennae, orthopterans are divided into two suborders: Ensifera and Caelifera. Ensiferans are long-horned insects including crickets, katydids, weta and their relatives. Most ensiferans are nocturnal species and females possess a long ovipositor (Ax, 2000; Grzimek et al., 2004). The Tettigoniidae (katydids or bushcrickets) is the largest ensiferan family and includes many acoustically communicating insect species. Caeliferans are short-horned grasshoppers including true grasshoppers, locusts and their relatives. Most caeliferan species are diurnal and females possess a short ovipositor (Ax, 2000; Grzimek et al., 2004). The majority of caeliferan species belong to the family Acrididae (field grasshoppers and locusts). Some of locust species are known for their devastating agricultural potential once they form large swarms. Singing katydids and crickets are fascinating and their songs have been appreciated in Japan and China for decades. In parts of China crickets are still sold as singing pets and their calling songs had a remarkable impact on art (Grzimek et al., 2004). The sound production of insects plays an important role in the attraction of mates, species recognition, defence of territory and intraspecific aggression (Gerhardt & Huber, 2002; Grzimek et al., 2004). Production of sound signals in orthopterans is commonly achieved by stridulation, which involves two parts of the body that are rubbed against each other. In acridids, stridulation organs are found on the inner edge of the femur of the hind legs and the outer surface of the forewings or the abdomen. Hearing organs of acridids are located in the first abdominal segment. In contrast, stereotyped singing of tettigoniids is achieved by rubbing both forewings against each 1 other and their ears are located in the tibia of forelegs (Ax, 2000; Grzimek et al., 2004). The majority of tettigoniid species live in the tropical regions (Naskrecki, 2013), where various organisms communicate acoustically at the same time which leads to a high level of background noise, especially after sunset. Acoustic communication in the context of mate attraction and mate choice leads to information exchange between a sender and indented (or unintended) receivers. In noisy situations, receivers face challenges to detect the signals