Choice of Feeding Sites, Growth and Survival by Crocidolomia Pavonana (F.) Frezzel Praise J

Choice of feeding sites, growth and survival by Crocidolomia pavonana (F.) Frezzel Praise J. Tadle BSc. MSc. A thesis submitted for the degree of Doctor of Philosophy at The University of Queensland in 2016 School of Biological Sciences Abstract The large cabbage moth, Crocidolomia pavonana (F.) (Lepidoptera: Crambidae), is one of the most significant pests of Brassica crops and has re-emerged as a serious pest of crucifer crops in tropical and sub-tropical Asia because of the control used against the diamondback moth, Plutella xylostella (L.) (Lepidoptera: Plutellidae). The damage caused by this pest can be significant because larvae devour young tissues of the plant at different stages and bore into the centre of the cabbage head at maturity and can damage the whole cabbage plant. Despite its economic importance the interaction of this pest with its host plant is not well understood. The current study showed that larval feeding by C. pavonana was influenced by the presence of cabbage in artificial diet, in laboratory bioassays. Of the four diets developed and tested, neonates survived and completed developmental stages (81%) on diet containing cabbage (Brassica oleracea cv sugarloaf) more so than on base diet (diet without cabbage), diet containing wheat grass or spinach powder. Neonates were able to locate diet with cabbage powder but not when cabbage was absent (i.e. base diet). Similarly, in an olfactometer test, neonates were strongly attracted to the odour of diet with cabbage compared with diet without cabbage. More than twice the number of feeding events was observed on filter paper disks impregnated with diet containing cabbage powder than on paper disks with base diet. The behavioural responses of neonates to diet with cabbage are linked to the glucosinolates present on cabbage plants. Sinigrin elicited feeding when added to diet without cabbage and also when applied to a non-host plant, Gossypium hirsutum (Sicot 71RRF). Neonates’ response to allyl isothiocyanate in a two- arm olfactometer was stronger than to sinigrin, which would explain larval attraction to damaged cabbage plants. These results indicate that sinigrin and allyl isothiocyanate provide important feeding and olfactory cues for C. pavonana larvae. Larval feeding and moth oviposition behaviour of C. pavonana was further studied using whole cabbage plants of two different ages (6 and 8-leaf stage) and this was compared to the response of a well-studied crucifer specialist, P. xylostella. Plant age did not significantly affect oviposition and feeding preference by C. pavonana whereas P. xylostella preferred younger plants for oviposition and larval feeding. The intra-plant distribution of eggs and feeding sites was similar for both species, but egg distribution showed a pattern different from feeding sites. Neonates that hatched from eggs deposited i on lower leaves moved towards the apical portion of the plant and established a majority of feeding sites on younger leaves. Preference for younger leaves was also observed on excised leaves, indicating architectural position was not the factor driving choice. Younger leaves are known to have a higher concentration of nitrogen content and proteins suitable for larval development, whereas lower leaves are considered not palatable because of fibrous content and low nutritional quality. Therefore, leaf age is an important factor affecting choice of feeding sites by the crucifer specialists. Sinigrin content, inferred from a separate study, did not explain feeding choice. The significant increase in the concentration of total aliphatic and indole glucosinolates with reduced leaf age does correspond with larval preference for younger leaves. Leaf wax and leaf toughness brought about by leaf age were studied as possible factors involved in leaf choice. Wax morphology, observed using scanning electron microscopy, differed between old and young leaves. The removal of leaf surface wax did affect C. pavonana neonates’ feeding choice, with greater preference for leaf disks with wax in situ, compared against leaf disks with wax removed, in a dual choice assay. Leaf wax extracted from either old or young leaves also induced feeding when added to artificial diet without cabbage. Leaf toughness, was not found to be a significant factor in feeding choice. The effect of sinigrin (allyl glucosinolate) and its hydrolysis product, allyl isothiocyanate on growth and survival of C. pavonana was investigated using artificial base diet and diet containing cabbage leaf powder, and compared with the response of P. xylostella. Consistent with other findings in this study, sinigrin promoted larval feeding for both crucifer specialists. Response to allyl isothiocyanate though, differed between C. pavonana and P. xylostella. Allyl isothiocyanate was found to be toxic and lethal for P. xylostella but C. pavonana neonates fed and survived, and when assessed at 24 hours, had gained weight. Similarly, varying the concentrations of allyl isothiocyanate (1 to 10 µmol/g diet) did not significantly affected survival of C. pavonana neonates whereas 1 µmol/g diet caused maximal mortality for P. xylostella neonates. The very high survival of C. pavonana neonates on diet containing allyl isothiocyanate indicates that C. pavonana handles glucosinolate hydrolysis products in a way that differs from P. xylostella, and this finding points to a new or different mechanism for glucosinolate metabolism. ii Declaration by author This thesis is composed of my original work, and contains no material previously published or written by another person except where due reference has been made in the text. I have clearly stated the contribution by others to jointly-authored works that I have included in my thesis. I have clearly stated the contribution of others to my thesis as a whole, including statistical assistance, survey design, data analysis, significant technical procedures, professional editorial advice, and any other original research work used or reported in my thesis. The content of my thesis is the result of work I have carried out since the commencement of my research higher degree candidature and does not include a substantial part of work that has been submitted to qualify for the award of any other degree or diploma in any university or other tertiary institution. I have clearly stated which parts of my thesis, if any, have been submitted to qualify for another award. I acknowledge that an electronic copy of my thesis must be lodged with the University Library and, subject to the policy and procedures of The University of Queensland, the thesis be made available for research and study in accordance with the Copyright Act 1968 unless a period of embargo has been approved by the Dean of the Graduate School. I acknowledge that copyright of all material contained in my thesis resides with the copyright holder(s) of that material. Where appropriate I have obtained copyright permission from the copyright holder to reproduce material in this thesis. iii Publications during candidature No publications. Publications included in this thesis No publications included. iv Contributions by others to the thesis This thesis was conceived, designed and conducted by the candidate, Frezzel Praise J. Tadle with input from her supervisors Dr. Bronwen Cribb, Dr. Michael Furlong and Dr. Myron Zalucki. All the experiments and data presented in this thesis were performed and collected by the candidate, who analysed data and wrote the thesis. The significant contributions of others are summarised below: Chapter 2: The candidate’s supervisors, Dr. Bronwen Cribb and Dr. Michael Furlong initially suggested the idea for this chapter. The candidate, together with Dr. Bronwen Cribb and Dr. Michael Furlong developed the concept further and designed the experiments and analysis approach for this chapter. The candidate performed all the experiments, collected all the data, performed statistical analyses, and wrote the chapter. Chapter 3: The candidate, together with Dr. Bronwen Cribb, Dr. Michael Furlong and Dr. Myron Zalucki conceived the idea and designed the experiments and analysis approach, for this chapter. The candidate conducted all the laboratory experiments, data analyses and wrote the chapter. Chapter 4: The candidate jointly conceived and designed the experiment and analysis approach with Dr. Bronwen Cribb and Dr. Michael Furlong. The candidate performed all the experiments, collected data, performed statistical analyses, and wrote the chapter. Statement of parts of the thesis submitted to qualify for the award of another degree None. v Acknowledgements Foremost, I would like to express my deep appreciation and gratitude to my primary supervisor Dr. Bronwen Cribb for her untiring support to my PhD research, for her tremendous patience in guiding me, and the mentorship she has given to me. Her guidance helped me get through the rough road to finish my research and in writing this thesis. My sincere thanks is also extended to my co-supervisors, Dr. Michael Furlong for his valuable input in this thesis and help in designing an approach to the statistical analysis; Dr. Myron Zalucki for his useful suggestions and for editing my thesis manuscript. I am also grateful to the University of Queensland for the University of Queensland International Scholarship awarded to me, which provided financial support for my PhD. This scholarship has given me the opportunity to advance my professional career in the field of Entomology. I am also indebted to the Philippine Rice Research

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