Behavioural adaptations of the generalist herbivore Helicoverpa punctigera (Lepidoptera: Noctuidae) with respect to primary and secondary hosts

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-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