Enhancing the effectiveness of the Thaumatotibia Leucotreta (Lepidoptera: Tortricidae) sterile insect technique release programme
- Authors: Githae, Michael Mathenge
- Date: 2025-04-03
- Subjects: False codling moth , Cryptophlebia leucotreta , Insect sterilization , Pests Integrated control , Insect populations , Parasitism , Fruit Diseases and pests
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/479897 , vital:78377 , 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. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2025
- Full Text:
- Date Issued: 2025-04-03
- Authors: Githae, Michael Mathenge
- Date: 2025-04-03
- Subjects: False codling moth , Cryptophlebia leucotreta , Insect sterilization , Pests Integrated control , Insect populations , Parasitism , Fruit Diseases and pests
- Language: English
- Type: Academic theses , Doctoral theses , text
- Identifier: http://hdl.handle.net/10962/479897 , vital:78377 , 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. , Thesis (PhD) -- Faculty of Science, Zoology and Entomology, 2025
- Full Text:
- Date Issued: 2025-04-03
Managing releases of Anagyrus vladimiri (Triapitsyn) to augment biocontrol of the citrus mealybug Planococcus citri (Risso) in South African citrus orchards
- Authors: Mommsen, Wayne Trevor
- Date: 2024-04-04
- Subjects: Citrus Diseases and pests South Africa , Citrus mealybug , Anagyrus vladimiri , Parasitism , Hyperparasite , Pests Biological control
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/434952 , vital:73118
- Description: In May 2019, South Korean inspectors rejected numerous grapefruit consignments from Letsitele, Hoedspruit and Onderberg in South Arica, because of live mealybug found on fruit. Growers expressed deep concern as mealybug management to a phytosanitary level was almost unattainable. Regular spray interventions for control of citrus black spot fungus, Phyllosticta citricarpa, and citrus thrips, Scirtothrips aurantii, cause repercussions in mealybug populations because they undermine the naturally occurring biocontrol complex. As part of an Integrated Pest Management (IPM) strategy, release of commercially produced parasitoids is common practice, to augment the naturally occurring beneficial insect populations. Prior knowledge of the harmful effects of insecticides on parasitoids is essential to IPM planning and the success of the biocontrol component in such a programme. Timing of augmentative releases to coincide with the phenology of citrus and the mealybug pest is also considered important for the successful establishment and control. Consequently, field trials were conducted to compare efficacy of early vs. late releases of Anagyrus vladimiri (Triapitsyn), an effective parasitoid of the citrus mealybug, Planococcus citri (Risso). Semi-field bioassays were conducted concurrently to determine the impact of various thripicides on A. vladimiri. The impact of sulfoxaflor, spinetoram, spirotetramat and prothiofos were rated harmless, as A. vladimiri mortality was lower than 25% after coming into contact with aged residues between 7 and 14 days old. October and November releases of A. vladimiri resulted in early parasitism and lowered peak-infestation of mealybug. January releases are possibly too late in grapefruit and lemon, open field, orchards, considering parasitism by A. vladimiri peaked in February. In mandarin orchards under net, percentage parasitism of 3rd instar mealybug increased a month later. Notably, at harvest, the difference in efficacy between treatments was not clear. This could be explained by high levels of natural parasitism observed in the treated and untreated orchards, which emphasises the importance of conservation biocontrol. In a second season, the proportion of hyperparasitoids captured (61%) from samples of mealybug-infested fruit was larger than the proportion of primary parasitoids, Anagyrus vladimiri, Coccidoxenoides perminutus (Girault) and Leptomastix dactylopii (Howard) (39%), which was far lower than the captures of eclosing primary parasitoids the previous season, which was 60%. The new discovery of Pseudaphycus sp. in citrus orchards in South Africa could be a key in explaining the uncontrollable levels of mealybug experienced and has drawn attention to a need for further understanding of ecological factors that influence biological control in citrus. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2024
- Full Text:
- Date Issued: 2024-04-04
- Authors: Mommsen, Wayne Trevor
- Date: 2024-04-04
- Subjects: Citrus Diseases and pests South Africa , Citrus mealybug , Anagyrus vladimiri , Parasitism , Hyperparasite , Pests Biological control
- Language: English
- Type: Academic theses , Master's theses , text
- Identifier: http://hdl.handle.net/10962/434952 , vital:73118
- Description: In May 2019, South Korean inspectors rejected numerous grapefruit consignments from Letsitele, Hoedspruit and Onderberg in South Arica, because of live mealybug found on fruit. Growers expressed deep concern as mealybug management to a phytosanitary level was almost unattainable. Regular spray interventions for control of citrus black spot fungus, Phyllosticta citricarpa, and citrus thrips, Scirtothrips aurantii, cause repercussions in mealybug populations because they undermine the naturally occurring biocontrol complex. As part of an Integrated Pest Management (IPM) strategy, release of commercially produced parasitoids is common practice, to augment the naturally occurring beneficial insect populations. Prior knowledge of the harmful effects of insecticides on parasitoids is essential to IPM planning and the success of the biocontrol component in such a programme. Timing of augmentative releases to coincide with the phenology of citrus and the mealybug pest is also considered important for the successful establishment and control. Consequently, field trials were conducted to compare efficacy of early vs. late releases of Anagyrus vladimiri (Triapitsyn), an effective parasitoid of the citrus mealybug, Planococcus citri (Risso). Semi-field bioassays were conducted concurrently to determine the impact of various thripicides on A. vladimiri. The impact of sulfoxaflor, spinetoram, spirotetramat and prothiofos were rated harmless, as A. vladimiri mortality was lower than 25% after coming into contact with aged residues between 7 and 14 days old. October and November releases of A. vladimiri resulted in early parasitism and lowered peak-infestation of mealybug. January releases are possibly too late in grapefruit and lemon, open field, orchards, considering parasitism by A. vladimiri peaked in February. In mandarin orchards under net, percentage parasitism of 3rd instar mealybug increased a month later. Notably, at harvest, the difference in efficacy between treatments was not clear. This could be explained by high levels of natural parasitism observed in the treated and untreated orchards, which emphasises the importance of conservation biocontrol. In a second season, the proportion of hyperparasitoids captured (61%) from samples of mealybug-infested fruit was larger than the proportion of primary parasitoids, Anagyrus vladimiri, Coccidoxenoides perminutus (Girault) and Leptomastix dactylopii (Howard) (39%), which was far lower than the captures of eclosing primary parasitoids the previous season, which was 60%. The new discovery of Pseudaphycus sp. in citrus orchards in South Africa could be a key in explaining the uncontrollable levels of mealybug experienced and has drawn attention to a need for further understanding of ecological factors that influence biological control in citrus. , Thesis (MSc) -- Faculty of Science, Zoology and Entomology, 2024
- Full Text:
- Date Issued: 2024-04-04
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