Management of pollination services in the Tasmanian Midlands : native and crop floral associations between scent, pollen and insect visitors

At a global scale, rates of crop production are outstripping the availability of crop pollination services. Furthermore, there are concerning declines in both managed and wild pollinators. In response to these issues, recent research highlights the contribution of native insects towards agricultural pollination and the importance of native vegetation in sustaining wild pollinator services. Concurrently, land clearing and the loss of resources supplied by native vegetation may be the greatest threat to native pollinator diversity.

In Australia, one of the first regions modified for agriculture, the Tasmanian Midlands have lost an estimated 83% of original habitat. Carrot seed (Daucus carota L.) export is a major commodity in this region, however rented honeybee hives are unable to meet crop pollination demands and thus limit the expansion of this industry. Revegetation with native flora offers a potential solution by increasing wild pollination services. The selection of suitable revegetation flora is hampered by the lack of a rapid method to compare floral traits and identify patterns between flora that may influence insect foraging.

This thesis will directly address these knowledge gaps. The overall thesis aim is to establish whether insect visits to native and crop flora in the Tasmanian Midlands are explained by either the scent or pollen traits of flowers. In order to achieve this, scent and pollen samples were analysed for a subset of extant Midlands native and crop flora. Simultaneously, insect visits were observed for the study region and taxa. In conclusion, the association between scent, pollen and insect visits was tested to uncover the influence of particular traits.

In Chapter 2, I examine scent as a floral trait and whether distinguishing scent profiles can be characterised for individual flora species through solid-phase microextraction (SPME) paired with gas-chromatography (GC-MS). Secondly, whether scent profiles for individual species group flora in known phylogenetic groups, implying the predominance of inherited volatiles, or whether flora group in patterns that may imply convergence due to pollinator selection pressures. Paired SPME and GC-MS was demonstrated to accurately collect representative and reproducible floral profiles, from which characteristic patterns could be isolated. Herein, the novel profiles of 15 Australian flora species are reported and contrasted against literature profiles of the commonly cultivated seed crops: Daucus carota L. and Brassica rapa L. Applying ordination and hierarchical clustering, both native and crop flora were found to generally group taxonomically as a result of shared, inherited volatiles. However, differences in shared and unique volatiles also highlighted evidence of past and ongoing evolutionary responses towards insect selection pressures.

In Chapter 3, I investigate pollen as a floral trait and whether distinct pollen fingerprints can be obtained for individual flora through paired atmospheric solids ambient pressure-mass spectrometry (ASAP-MS). Secondly, whether pollen fingerprints group flora according to known phylogeny or in patterns indicative of pollinator pressures. Only the second study to apply ASAP-MS to the characterisation of pollen, this method was successfully applied in the collection of distinct pollen fingerprints. The novel pollen fingerprints of 18 Australian plant species, seed crop Daucus carota L. and two naturalised Brassica rapa L. hybrids are reported. Employing ordination, Jaccard index correlation and hierarchical clustering, pollens were found to generally group in accordance with known taxonomy and exhibited an average correlation of 49%. Additionally, evidence of pollinator selection pressures and unique evolutionary convergence was observed between more distantly related species, such as the Fabaceae pea Dillwynia cinerascens and the Myrtaceae Eucalyptus amygdalina.

In Chapter 4, I sought to compare the visitation of major insect groups and their overlap between native and crop flora to determine potential revegetation flora species by mapping insect emergence and flowering sequence via a calendar. Insect visits were observed for 19 native Australian plant species, two naturalised Brassica rapa L. hybrids and the seed crop Daucus carota L. Beetle species constituted the highest taxa visiting native flora, flies and beetles dominated D. carota fields, whilst native and feral bees were most common to naturalised Brassica. Utilising ordination, Jaccard index correlation, and hierarchical clustering, insect visits broadly grouped flora taxonomically, with evidence of trends towards bee, fly or beetle preference across flora. Over two-thirds of the insect visitors to crop species overlapped with native flora, supporting the existence of a wild pollination service in this region that may be enhanced by revegetation. Comparison of insect preferences and emergence relative to flowering sequence allowed us to elucidate three sets of revegetation flora suitable for (i) maintaining existing biodiversity in the Midlands, (ii) enhancing wild pollination of D. carota crops, (iii) enhancing wild visitation of Brassica fields.

In chapter 5, my aim was to synthesise the results of the previous three chapters, and answer whether (i) either scent or pollen traits explain insect visitation, (ii) scent and pollen associate as floral traits. Testing association with Procrustes analysis and the Procrustean randomisation test, insect visits were found to associate with pollen as a floral trait and not scent. There was no statistical support for any association between pollen and scent traits of flowers. From these results I determined that pollen may provide an easily tested trait that can help to narrow down the selection of vegetation flora, when layered on top of information about flowering sequence and insect visitation.

In summary, this research addressed the overarching aim by finding that insect visits to native and crop flora in the Tasmanian Midlands are associated with the pollen composition of flowers. The ASAP-MS technique was demonstrated to provide vital fingerprinting data for pollen, allowing the analysis of pollen as a demonstrable trait to guide revegetation decision-making. The collection of scent as a floral trait was also easily obtainable by means of SPME GC-MS and merits further investigation. Lastly, three sets of sequentially flowering native flora were identified that are suitable for wild pollinator biodiversity in the Midlands region.