Biology and behaviour of New Zealand ladybirds : insights into the establishment success of introduced species : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Zoology at Massey University, Albany, New Zealand
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Date
2021
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Massey University
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Abstract
What factors affect the successful establishment of introduced species? Our understanding of establishment processes is limited and concentrated on mean trait differences. However, recent research suggests that high levels of variation in functionally important traits facilitate establishment success. Ladybirds are generalist predators that serve as an excellent model system to study establishment success as they are widely introduced as biocontrol agents against agricultural pests. In my PhD, I examined the phenotypic (morphological, behavioural and physiological) variation and phylogenetic relationships between introduced and native ladybirds in New Zealand. Contrasting the differences between successfully established introduced species and native species in recipient ecosystems provides insights about the mechanisms that drive establishment success and a foundation for future research into introduction biology.
Firstly, I developed a framework to study establishment processes using ladybirds as a model system. I identified fundamental gaps in research into unsuccessful species introductions, dispersal abilities and the quantification of both inter- and intraspecific variation in ladybirds. This provided the conceptual foundation for my subsequent research comparing inter- and intraspecific variation in the morphology, behaviour and physiology of New Zealand ladybirds.
Morphologically, I found that introduced ladybirds had wider bodies than native ladybirds. Further, introduced species had greater intraspecific variation in size than native species. Introduced ladybirds with wide bodies and high intraspecific variation may have a larger prey range compared to native ladybirds, thereby increasing the probability of successful establishment. Behaviourally, I found differences between species in their general activity levels and responses to simulated predator attacks but there was no difference between species in their intraspecific variation. Physiologically, there was inter- and intraspecific variation in survival during desiccation resistance assays.
Overall, body size appears to influence traits associated with establishment success. There was a strong positive correlation between body size and general activity levels and survival time during desiccation resistance assays in each species. I found that the widely dispersed introduced ladybird Halmus chalybeus had significantly higher levels of intraspecific variation in size and colour compared to other species of ladybirds. This suggests that high intraspecific variation may indeed increase establishment success and dispersal.
Finally, the molecular phylogeny I constructed strongly supported the monophyly of Coccinellidae and subfamilies Microweiseinae and Coccinellinae and moderately supported the monophyly of tribe Epilachnini and paraphyletic tribes Coccinellini, Scymnini, Chilocorini and Coccidulini. However, there was no indication that the successful establishment of introduced species of ladybirds increases with relatedness to native species in New Zealand.
This is the first study to compare phenotypic variation between introduced and native ladybirds in New Zealand and the first molecular phylogeny to include native species of New Zealand ladybirds. My thesis presents indirect support for the hypothesis that high levels of intraspecific variation increase establishment success in introduced species. I propose that assessing phenotypic variation in introduced species is key to understanding establishment processes and thereby improving the effectiveness of biocontrol and conservation programmes.
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Keywords
Ladybugs, Behavior, Introduced insects, New Zealand