- Title
- Enhancing the effectiveness of the Thaumatotibia Leucotreta (Lepidoptera: Tortricidae) sterile insect technique release programme
- Creator
- Githae, Michael Mathenge
- Subject
- Uncatalogued
- Date Issued
- 2025-04-03
- Date
- 2025-04-03
- Type
- Academic theses
- Type
- Doctoral theses
- Type
- text
- Identifier
- http://hdl.handle.net/10962/479897
- Identifier
- vital:78377
- Identifier
- DOI 10.21504/10962/479897
- Description
- The false codling moth, Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae), is a key phytosanitary pest in the South African citrus industry. Due to its cryptic nature and its endemic presence in sub-Saharan Africa with a wide array of host plants, and eradication is not possible. However, the sterile insect technique (SIT) has been incorporated into an Area-Wide Integrated Pest Management (AW-IPM) programme to induce widespread population suppression. The successful implementation of the SIT programme required a series of well-researched phases, with one key phase being the determination of an effective overflooding ratio, previously established at 10:1. Despite this, various studies of T. leucotreta SIT programmes have reported higher release ratios than 10:1 in orchards, necessitating an investigation into their efficacy. This thesis aimed to understand the effects of higher release ratios, different combinations of sterile and fertile T. leucotreta of both sexes, compatibility of SIT and the egg parasitoid Trichogrammatoidea cryptophlebiae, and the pre-release mating levels during production and release stages to improve the effectiveness of the T. leucotreta SIT programme. This study initially investigated the impact of different ratios of sterile and fertile adults on fruit damage, sterile male competitiveness, and population growth in laboratory cages using Washington Navel oranges. Sterilised to fertile T. leucotreta adults at ratios of 0:1 (control), 10:1, 20:1, 40:1, and 60:1 were placed inside insect-rearing cages and allowed to mate, oviposit and infest the fruit. Treatment cages receiving sterile T. leucotreta produced significantly fewer damaged fruit, larval entries, and F1 adults compared to the control. The number of damaged fruit, larval entries, and F1 adults negatively correlated with the increase in the overflooding ratio of sterile to fertile T. leucotreta. Control cages had significantly higher fecundity and fertility compared to treatment cages. The 60:1 ratio exhibited the lowest per generation rate of increase (<1× from the parental [P1] to the F1 generation) compared to the 10:1 ratio (current release ratio). The effects of different combinations of both treated (T) and untreated (U) male (M) and female (F) adult T. leucotreta: UM×UF (control), TM×UF, UM×TF, TM×TF, and UM×UF×TM×TF on fruit damage, mating competitiveness, and per-generation rate of increase were tested. The treatments were housed in insect-rearing cages containing Navel oranges and allowed to mate, oviposit, and infest the fruit. Treatment cages with both treated male and female T. leucotreta had significantly fewer damaged fruit, larval entries, and emerged F1 adults compared to the control cages, except for the UM×UF×TM×TF combination. Similarly, control cages and UM×UF×TM×TF treatments had significantly Higher fecundity and fertility compared to other treatments. The TM×UF combination exhibited the lowest rate of increase per generation (<0.57× from the parental [P1] to F1 generation). A field cage study was conducted to evaluate the effects of various overflooding ratios and different combinations of sterile and fertile male and female T. leucotreta. However, the results were limited, and inconclusive due to collection of insufficient data, as the fruit infestation level was low. This could be attributed to the low quality of the moths released or effects of environmental variables on the moths. A laboratory study explored the susceptibility of T. leucotreta eggs resulting from various pairings of sterile and fertile moths to parasitism by Trichogrammatoidea cryptophlebiae. The ratios of sterile to fertile T. leucotreta used were: 0:1, 10:1, 20:1, 40:1, and 60:1. The resulting eggs were then exposed to T. cryptophlebiae for parasitism, and the parasitism rates of newly laid (24 h), 48 h and 72 h old eggs were evaluated. Overall, eggs from all ratios were suitable for T. cryptophlebiae development and acceptable for oviposition. Significantly higher number of parasitised eggs were recorded between the control (0:1) and ratios 40:1 and 60:1 at 48 h old eggs. Additionally, a higher proportion of flying T. cryptophlebiae emerged across the ratios, with a higher proportion of female-to-male sex ratio. Pre-release mating levels were studied during the production and release stages, divided into three stages: moth eclosion, irradiation, and release. A significantly higher number of spermatophores and percentages of mated female T. leucotreta were recorded at the eclosion and irradiation stages in January. Similarly, in May, a significantly higher number of spermatophores and percentages of mated female T. leucotreta were recoded post-irradiation and release stages in the Sundays River Valley region (SRV). Overall results indicated more spermatophores and percentages of mated female T. leucotreta at the irradiation and release stages in the SRV region. In conclusion, the study demonstrated that a release ratio exceeding 40:1 and different combinations of sterile and fertile T. leucotreta, especially the TM×UF combination, has a suppressive effect against T. leucotreta and integrating SIT with T. cryptophlebiae, shows potential for enhancing the effectiveness of the T. leucotreta SIT programme. Additionally, the mating competitiveness of sterile insects in dual-sex releases can be improved by controlling the level of pre-release matings.
- Description
- Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2025
- Format
- computer
- Format
- online resource
- Format
- application/pdf
- Format
- 1 online resource (195 pages)
- Format
- Publisher
- Rhodes University
- Publisher
- Faculty of Science, Zoology and Entomology
- Language
- English
- Rights
- Githae, Michael Mathenge
- Rights
- Use of this resource is governed by the terms and conditions of the Creative Commons "Attribution-NonCommercial-ShareAlike" License (http://creativecommons.org/licenses/by-nc-sa/2.0/)
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| Thumbnail | File | Description | Size | Format | |||
|---|---|---|---|---|---|---|---|
| View Details Download | SOURCE1 | GITHAE-PHD-TR25-61.pdf | 1 MB | Adobe Acrobat PDF | View Details Download |