HOST-PLANT ACCEPTANCE BY THE CARROT FLY: SOME UNDERLYING MECHANISMS AND THE RELATIONSHIP TO HOST-PLANT SUITABILITY Inauguraldissertation zur Erlangung der Würde eines Doktors der Philosophie vorgelegt der Philosophisch-Naturwissenschaftlichen Fakultät der Universität Basel von Thomas Degen aus Oberwil BL Basel, 1998 Genehmigt von der Philosophisch-Naturwissenschaftlichen Fakultät auf Antrag von Herrn Professor Dr. Thomas Boller, Frau Professor Dr. Martine Rowell-Rahier und Herrn Dr. Erich Städler Basel, den 29. April 1997 Prof. Dr. Stephen C. Stearns Dekan Ψ - la Bohème Psiladies, you don’t adore the perfume of roses as anyone who knows just your name inevitably supposes. Instead you better like to smell with your antennal noses the scents of those humble umbel bearers even in minute doses. They unwillingly attract you with these essences meant as defences against their adversaries, but you – malign – use all your senses to find your victims far or close: olfaction as well as vision, and no surprise if also taste is involved in your decision. But may the how of the latter still be mysterious to us the why of your choosiness is even more so – alas! Of course, as mothers you are supposed to bother first of all about your progeny’s nutritional need, as your child cannot easily move to another far away suitable host on which to feed. But mothers do not always know best, on some occasions you fail the test and lay some eggs beneath a root which is no good for your brood. But foliage is what you inspect and the access is only indirect to information about the part hidden in the soil, so it’s art to make the right selection based on your predilection. Yet, if your choice is good, there will be proper food for the future generation, the larvae in consideration. With their solid upper lip they’ll commence to rip the carrot’s skin at the root tip tip tip tip tip tip Dedicated to my Mother, my Father, my Sister and my little Niece I Contents Acknowledgements II Summary III Zusammenfassung V Part 1 General Introduction Chapter 1 Evolutionary aspects of insect-plant interactions 1 The mechanisms of host-plant selection 3 Host-plant selection in the carrot fly 4 Outline of the thesis 5 Part 2 Preliminary oviposition experiments Chapter 2.1 Some aspects of the oviposition behaviour of individual carrot flies 9 Chapter 2.2 Host-plant choice of adult carrot flies is not influenced by larval food 14 Chapter 2.3 T. Degen & E. Städler, An improved oviposition assay for the carrot fly 17 Part 3 Host-plant susceptibility to carrot fly attack Chapter 3.1 T. Degen, E. Städler & P. R. Ellis, Host-plant susceptibility to the carrot fly, Psila 22 rosae. 1. Acceptability of various host species to ovipositing females Chapter 3.2 T. Degen, E. Städler & P. R. Ellis, Host-plant susceptibility to the carrot fly, Psila 36 rosae. 2. Suitability of various host species for larval development Chapter 3.3 T. Degen, E. Städler & P. R. Ellis, Host-plant susceptibility to the carrot fly, Psila 55 rosae. 3. The role of oviposition preferences and larval performance Part 4 Non-chemical plant stimuli influencing host acceptance by adult carrot flies Chapter 4.1 T. Degen & E. Städler, Foliar form, colour and surface characteristics influence 65 oviposition behaviour of the carrot fly Chapter 4.2 T. Degen & E. Städler, Influence of natural leaf shapes on oviposition in three 81 phytophagous flies: a comparative study Part 5 Chemical plant stimuli influencing host acceptance by adult carrot flies Chapter 5.1 T. Degen, G. Poppy & E. Städler, Extracting oviposition stimulants for the carrot fly 85 from host-plant leaves Chapter 5.2 T. Degen & E. Städler, Oviposition of the carrot fly (Psila rosae) in response to 95 foliage and leaf surface extracts of various host-plant species Chapter 5.3 T. Degen, H.-R. Buser & E. Städler, Patterns of oviposition stimulants for the carrot 109 fly in various host plants Part 6 General Discussion Chapter 6 Semiochemicals and host selection by the carrot fly 122 The significance of non-chemical plant traits for host finding and acceptance 127 Comparative aspects and a tentative evolutionary outlook 128 Implications for applications 131 Curriculum vitae 136 II Acknowledgements First of all, my thanks go to my supervisor Erich Städler for suggesting an interesting and worthwhile subject for a Ph.D. thesis on a sometimes somewhat “tricky” beast with which he already had to tackle for his own thesis some thirty years ago. The advice that he gave me during all these years was very valuable, and I willingly tried to adopt if not all, then most of his propositions. Professor Thomas Boller and Professor Martine Rowell-Rahier kindly lent themselves to be in the jury committee for my thesis at the University of Basle. Walter Müller, director of the Swiss Federal Research Station Wädenswil, and Ernst Boller, head of the Zoology and Herbology Department, allowed me to carry out my project at a place with an extraordinarily well-equipped infrastructure. My dissertation project was funded by a scholarship of the Kanton Baselland, which was temporarily supple- mented by grants of the Roche Research Foundation and the Werenfels-Fonds (Freiwillige Akademische Gesellschaft Basel). The Dissertationenfonds of the University of Basle and the Basler Studienstiftung paid for a substantial part of the printing costs. Financial support by the Swiss Federal Research Station helped me to get through times of “dire straits” towards the final stages of what might sometimes have appeared to become a never-ending story. Guy Poppy acquainted me with the microwave-assisted hexane extracts, a method that turned out to be very helpful in my attempts to solve a part of the problems on which I was working. Hans-Ruedi Buser generously devoted some of his precious time to me, introduced me to the mysteries and miracles of GC-MS and patiently explained everything to me in such a detailed manner that even an awkward biologist like me got the impression to understand at least something. I thank all my colleagues in the staff of the Research Station and the many guests, which we were able to welcome in our group, for providing a comfortable ambience and for everything that they contributed to the success of my project. Of course, my colleagues in the “Städler” group, most notably Cornelia Kesper and Robert Baur, deserve to be specially mentioned in this context. The very special thanks go to our dear German “immigrant” and – I dare to say it – my best friend at the Station, Carsten Hippe, not only for his scientific input, but for sharing my lot as a “foreigner” exiled in Zurich, for his company and for letting me sometimes win a snooker frame against him. I was very happy that I could share the apartment during most of my stay at Wädenswil with Rolf Wacker, another colleague from the Station. This was a really good flat-share experience, and I enjoyed it very much. Also, I do not forget to credit the merits of my good old friend Daniel “Mäuse” Mäusezahl without whose “initial ignition” – believe it or not – I actually would not have started this thesis: from little acorns grow big oaks. Finally, I want to express my great thankfulness to my parents Anna and Walter and to my sister Elisabeth for their love and for their permanent readiness to support me in every possible way. III Summary The larvae of the carrot fly, Psila rosae (F.) (Diptera: Psilidae), attack the roots of numerous wild and cultivated plants belonging to the family Umbelliferae (= Apiaceae). Since they have only a limited capacity to move within the soil and to find suitable host plants on their own, their survival depends largely on the host choice by the ovipositing female flies. In a previous study, characteristic allelochemicals in the leaf surface of carrots were shown to elicit oviposition. However, these oviposition stimulants could not explain the preference of the carrot fly for particular carrot cultivars. In this thesis I therefore expanded the investigations, which had so far focused on carrot, Daucus carota, the economically most important host plant, to a wider range of host species differing considerably in susceptibility. This study was intended to be a further step towards the identification of the physical and chemical plant traits that account for the host-plant preferences of the carrot fly. This should eventually result in a better understanding of the mechanisms of host-plant resistance to this pest insect. As a basis for the subsequent investigations into chemical plant cues mediating host acceptance, I determined the preference hierarchy of the carrot fly for 30 umbelliferous host-plant species respectively varieties and 6 non-hosts. To this end, foliage of the test plants was presented to caged populations of carrot flies in dual and multiple choice oviposition assays together with leaves of a standard plant, the susceptible carrot cultivar “Danvers”. Only two species, Anthriscus cerefolium (garden chervil) and Carum carvi (caraway), proved to be more acceptable than the standard plant, about half of the species received significantly fewer eggs. Pimpinella major (greater burnet saxifrage) was the least preferred umbelliferous species and was ranked within the non-hosts. Differences in post-alighting, pre-ovipositional behaviour of the female flies on the leaves, i.e. in the proportion of exploratory runs leading to oviposition, were responsible for a major part of the variation recorded in the number of eggs laid. A nearly identical set of plant species was in addition tested for the suitability to support larval development. I collected the pupae and non-pupated third instar larvae that had grown on potted plants inoculated with a constant number of eggs.