Prioritising biological control agents for release against Sporobolus pyramidalis and Sporobolus natalensis (Poaceae) in Australia
- Authors: Sutton, Guy Frederick
- Date: 2021
- Subjects: Grasses -- Diseases and pests , Bruchophagus , Wasps , Alien plants -- Biological control -- Australia , Sporobolus -- Biological control -- Africa , Sporobolus -- Biological control -- Australia , Insects as biological pest control agents -- Australia , Insects as biological pest control agents -- Africa
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/172445 , vital:42201 , 10.21504/10962/172445
- Description: Sporobolus pyramidalis Beauv. and S. natalensis (Steud.) Th. Dur. and Schinz. (giant rat’s tail grass) (Poaceae), invade rangelands and pastures in eastern Australia, costing the livestock industry approximately AUS$ 60 million per annum in grazing losses. Mechanical and chemical control options are costly and largely ineffective. Biological control is viewed as the most promising control option, however this management strategy has largely been avoided for grasses, due to their perceived lack of suitably host-specific and damaging natural enemies. In this thesis, the prospects for using biological control against S. pyramidalis and S. natalensis in Australia was assessed, in light of these potential challenges. Climate matching models were used to identify high-priority geographic regions within the plants’ native distributions to survey for potential biological control agents. High-priority regions to perform surveys were identified by modelling the climatic suitability for S. pyramidalis and S. natalensis in sub-Saharan Africa (i.e. their potential native ranges’), and climatic compatibility with regions where biological control is intended in Australia. High-priority regions for S. pyramidalis included: (1) coastal East Africa, ranging from north-eastern South Africa to Uganda, including south-eastern DRC, (2) some parts of West Africa, including inland regions of the Ivory Coast and western Nigeria, (3) northern Angola and (4) eastern Madagascar, and for S. natalensis included: (1) eastern South Africa, (2) eastern Zimbabwe, (3) Burundi, (4) central Ethiopia and (5) central Madagascar. Prospective control agents collected from these regions have the highest probability of establishing and proliferating in Australia, if released. In surveys of the insect assemblages on S. pyramidalis and S. natalensis in the climatically-matched region of eastern South Africa fifteen insect herbivores associated with the grasses were identified. Insect feeding guild, geographic distributions, and seasonal abundances suggest that three stem-boring phytophagous wasps, Tetramesa sp. 1, Tetramesa sp. 2 and Bruchophagus sp. 1 (Hymenoptera: Eurytomidae), have potential as control agents. Species accumulation curves indicated that additional surveys in South Africa are unlikely to yield additional potential control agents. Field host-range surveys of 47 non-target grass species in South Africa showed that Tetramesa sp. 1, Tetramesa sp. 2, and Bruchophagus sp. 1, were only recorded from S. pyramidalis and S. natalensis. Integrating field host-range with phylogenetic relationships between plant species indicated that no native Australian Sporobolus species or economic crops and pastures are expected to be attacked by these wasps. All three wasp species are predicted to be suitably host-specific for release in Australia. Three other endophagous herbivores attacked non-target native African Sporobolus species that share a close phylogenetic relationship to native Australian Sporobolus species, and therefore, demonstrate considerable risk of non-target damage. These species should not be considered as potential control agents. Under native-range, open-field conditions, Tetramesa sp. 1 caused an approximately 5-fold greater reduction in plant survival and reproductive output than Tetramesa sp. 2 and Bruchophagus sp. 1. Tetramesa sp. 1 in combination with Tetramesa sp. 2 did not significantly increase the level of damage, while Bruchophagus sp. 1 may decrease the efficiency of Tetramesa sp. 1, if released in combination. Tetramesa 1 is therefore the most promising candidate agent. Prioritising potential agents using predicted efficacy allowed otherwise equally suitable prospective agents to be prioritised in a strategic manner. Prioritising which natural enemies to target as biological control agents is a complex task. Field host range and damage assessments in the native range may provide more realistic data than typical studies performed under artificial conditions in a laboratory or quarantine. Moreover, it could assist practitioners in prioritising the most suitable agent(s) at the earliest stage in the programme as possible. This study demonstrated that grasses are suitable targets for biological control as they can harbour host-specific and damaging natural enemies.
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- Date Issued: 2021
Interactions between fig wasps and their host figs
- Authors: Nefdt, Rory John Charlton
- Date: 1990
- Subjects: Wasps , Mutualism (Biology)
- Language: English
- Type: Thesis , Masters , MSc
- Identifier: vital:5718 , http://hdl.handle.net/10962/d1005404 , Wasps , Mutualism (Biology)
- Description: Fig trees (Ficus spp.) and fig wasps (Hymenoptera: Agaonidae) are partners in an intimate mutualism. The trees provide ovules in which wasp larvae develop while the wasps pollinate the flowers and are therefore indispensible for fig seed production. Agaonid fig wasps oviposit down the styles of fig flowers and it has generally been accepted that they were unable to reach the ovules of "long" styled flowers , which would produce seeds, thus maintaining an evolutionary stable mutualism. African fig species were found to have unimodal style length frequencies, with no separation into long and short styled flowers. In several species the ovipositors of their associated agaonids were long enough to reach the majority of ovules. The number of foundress agaonids entering a fig influenced fig seed set and therefore was an important factor regulating the proportion of flowers producing seeds or pollinators. In the two Ficus species that were studied, entry of more than three agaonid foundresses into one fig resulted in competition for limited oviposition sites and less female - biased offspring sex ratios. It is hypothesised that sequential laying of male eggs followed by female eggs, under variable oviposition site limitation, results in sex ratio adjustment, as predicted by local mate competition theory. Evidence in support of this hypothesis is presented. A number of non - pollinating torymid and pteromalid fig wasps also oviposit into each fig species. The sycophagines and sycoecines oviposit down the styles from inside the fig inflorescences like their agaonid counterparts, while other species insert their ovipositors through the wall of the fig from the outside. Like the agaonids, sycophagines were characterised by being pro - ovigenic, with numerous fully developed eggs at emergence. Sycoecines were able to re - emerge from figs they had oviposited in and lay their eggs in more than one fig. They had short ovipositors, allowing access to a smaller proportion of flowers than agaonids or sycophagines. Externally ovipositing fig wasps were syn-ovigenic, able to develop eggs as adults and invested more energy and time during each oviposition event. Differences in the ovipositor lengths of these species did not segregate their oviposition sites spatially, and therefore does not reduce competition between species. Attack by parasitoids and inquiline fig wasps from the exterior did not constitute a selection pressure against agaonids ovipositing in ovules closer to the periphery of the fig's surface, as predicted by Michaloud's enemy-free-space hypothes is. It cannot therefore explain the preference shown by ovipositing agaonids for shorter styled flowers.
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- Date Issued: 1990