Behavioural Adaptations of the Generalist Herbivore Helicoverpa Punctigera (Lepidoptera: Noctuidae) with Respect to Primary and Secondary Hosts
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i Behavioural adaptations of the generalist herbivore Helicoverpa punctigera (Lepidoptera: Noctuidae) with respect to primary and secondary hosts Lachlan Craig Jones BSc (Hons) University of Queensland 2015 BSc University of Queensland 2014 A thesis submitted for the award of Doctor of Philosophy at the University of Queensland in 2020 School of Biological Sciences ii Abstract Generalist insects are defined by the extreme diversity of host plants they feed upon in nature. Typically, most individuals develop on only one or a few host species, however, which we term primary hosts. From this perspective, generalists are like specialists except for, at times, using plants outside their range of specialisation. I sought to understand the adaptations involved in host use patterns of generalist herbivores, focussing on behavioural mechanisms of host plant acceptance in adults and larvae. I began by analysing results from 178 studies (on 161 insect species) that tested the relationship between egg-laying across host species and the subsequent survival of the resulting offspring. For each study, I researched the native ranges of all plant and insect species that were tested, along with the range of host plants fed on by the study insect(s). This allowed me to divide the results into generalists and specialists, tested with their native or non-native host plants. I found that 83% of insects allocated eggs adaptively across their native hosts, with no differences between generalists and specialists or across insect taxa. Based on that background, the rest of the thesis focused on interactions between the generalist moth Helicoverpa punctigera and its host plants. This insect is endemic to Australia and mainly inhabits the dry interior regions, although spring migrations to sub-coastal agricultural regions has seen it become a significant seasonal pest on cotton, beans and various other crops. Although H. punctigera has been recorded on at least 170 plant species across 40 families, under natural conditions larvae are found predominantly on plants in the daisy (Asteraceae) and legume (Fabaceae) families, including 6-8 species of primary hosts on which larval incidence and abundance in the field is especially high. In Chapter 3 I tested how the larval performance, larval attraction and adult oviposition of H. punctigera differed across four native host plants, two of them primary and two secondary. I found that neither relative attraction of the larvae nor numbers of eggs laid across the four host plants matched larval survival on these plants. Primary hosts, determined through relative larval abundance across host species in the field, were not necessarily more attractive to ovipositing moths or feeding caterpillars, suggesting other important factors not captured in these tests influence attraction, survival and retention of insects on host plants in the field. Oviposition rates across host species correlated best with early stage rather than overall larval survival, whereas larval attraction to host plants was poorly correlated with survival at any stage. In Chapter 4 I investigated how generalists allocate most of their eggs to primary hosts while still accepting secondary hosts when primary hosts are scarce. This could occur through an oviposition threshold that is lower on primary hosts, a feedback loop where oviposition on primary hosts stimulates more subsequent egg- iii laying than secondary hosts, or that moths assess alternatives and choose primary hosts. I tested this by measuring moth flight time and counting how many landings preceded oviposition on a primary host relative to their equivalent performance on each of two secondary hosts. I also compared the rates at which subsequent eggs were laid on primary and secondary hosts during the first hour after initial oviposition, and overnight. I found that oviposition involves thresholds and feedback loops, rather than choice, allowing H. punctigera to specialise on a few plant species within its broader host range. In chapter 5 I observed larvae across individual plants in the field, finding that plants hosting larvae typically had more flowers than average. I also performed field experiments with potted primary and secondary host plants. I placed larvae on the ground equidistant from each plant or, in a second test, onto one of the plant species and left for 24 hours. Larvae starting from a neutral position typically move to the primary host. Nevertheless, a quarter of larvae that had been placed on a plant and left for 24 hours did move to the other host, regardless of which plant they had started on. This suggests caterpillars leave even highly acceptable plants essentially at random and likely feed on several individual plants over their developmental span, yet they are receptive to olfactory cues when approaching host plants. Generalists, like specialists, possess host recognition adaptations specific to a relatively narrow range of primary host plant species. Facultative use of secondary hosts by H. punctigera is achieved through their oviposition threshold and feedback mechanisms. These are likely adaptations to its arid environment, where host plant availability varies spatio-temporally. The mobile larvae can also recognise and locate host plants, probably by tracking plant volatiles. Besides improving understanding of behaviour and host plant associations, these results have possible management consequences, as Helicoverpa larvae in pigeon pea refuges could be exposed to transgenic insecticidal cotton if they move between pigeon pea and cotton plants. iv 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, financial support 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 higher degree by research 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 and have sought permission from co-authors for any jointly authored works included in the thesis. v Publications included in this thesis Jones LC, Rafter MA and Walter GH (2019) Insects allocate eggs adaptively across their native host plants. Arthropod-Plant Interactions 13: 181–191. Jones LC, Rafter MA and Walter GH (2020) Host plant acceptance in a generalist insect: threshold, feedback or choice? Behaviour 157: 1059-1089. Jones LC, Rafter MA and Walter GH (in press) Interactions of Helicoverpa punctigera larvae and adults with four native host plants relative to field use patterns. A manuscript accepted by Environmental Entomology on 6th November 2020. Submitted manuscripts included in this thesis Jones LC, Rafter MA and Walter GH (Resubmitted) Showier plants host more larvae: distribution and movement of generalist caterpillars among plants in the field. A manuscript resubmitted to Ecological Entomology on 21st August 2020. Other publications during candidature Journal articles resulting from Honours research: Jones LC, Foster BJ, Rafter MA and Walter GH (2018) Tiny insects against the weather – flight and foraging patterns of Frankliniella schultzei (Thripidae) not altered by onset of rainfall. Insect Science 25: 1119-1127. Jones LC, Rafter MA and Walter GH (2018) Colonisation of primary and secondary host plant species by Frankliniella schultzei thrips: a balance between attraction and repulsion? Arthropod-Plant Interactions 12: 321-328. Journal article resulting from a side project pursued during my PhD: Jones LC (2020) Maternal aggression and juvenile sociality in hibiscus harlequin bugs. Journal of Insect Behavior https://doi.org/10.1007/s10905-020-09751-z Conference abstracts: Jones LC, Rafter MA and Walter GH (2019) Insects allocate eggs adaptively when host plants are native. Oral presentation – Plant Herbivore Interactions Gordon Research Seminar (Ventura, CA USA) Jones LC, Rafter MA and Walter GH (2019) Host plant discrimination in a generalist insect – threshold, feedback or choice? Poster presentation – Plant Herbivore Interactions Gordon Research Conference (Ventura, CA USA) vi Contributions by others to the thesis My supervisors Gimme Walter and Michelle Rafter were involved in discussion of research aims and methods as well as critically revising drafts of the thesis chapters. Statement of parts of the thesis submitted to qualify