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
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- Date Issued: 2025-04-03
Thaumatotibia leucotreta (Meyrick) (Lepidoptera: Tortricidae) population ecology in citrus orchards: the influence of orchard age
- Authors: Albertyn, Sonnica
- Date: 2018
- Subjects: Cryptophlebia leucotreta , Population biology , Insect populations , Orchards , Insect nematodes , Entomopathogenic fungi
- Language: English
- Type: text , Thesis , Doctoral , PhD
- Identifier: http://hdl.handle.net/10962/62615 , vital:28213
- Description: Anecdotal reports in the South African citrus industry claim higher populations of false codling moth (FCM), Thaumatotibia (Cryptophlebia) leucotreta (Meyr) (Lepidoptera: Tortricidae), in orchards during the first three to five harvesting years of citrus planted in virgin soil, after which, FCM numbers seem to decrease and remain consistent. Various laboratory studies and field surveys were conducted to determine if, and why juvenile orchards (four to eight years old) experience higher FCM infestation than mature orchards (nine years and older). In laboratory trials, Washington Navel oranges and Nova Mandarins from juvenile trees were shown to be significantly more susceptible to FCM damage and significantly more attractive for oviposition in both choice and no-choice trials, than fruit from mature trees. Although fruit from juvenile Cambria Navel trees were significantly more attractive than mature orchards for oviposition, they were not more susceptible to FCM damage. In contrast, fruit from juvenile and mature Midnight Valencia orchards were equally attractive for oviposition, but fruit from juvenile trees were significantly more susceptible to FCM damage than fruit from mature trees. Artificial diets were augmented with powder from fruit from juvenile or mature Washington Navel orchards at 5%, 10%, 15% or 30%. Higher larval survival of 76%, 63%, 50% and 34%, respectively, was recorded on diets containing fruit powder from the juvenile trees than on diets containing fruit powder from the mature trees, at 69%, 57%, 44% and 27% larval survival, respectively. Bioassays were conducted to determine if differences in plant chemistry between fruit from juvenile and mature trees will have an impact on the susceptibility FCM to entomopathogenic nematodes (EPN), entomopathogenic fungi (EPF) and Cryptophlebia leucotreta granulovirus (CrleGV). No significant differences in the susceptibility of larvae reared on diets containing 15% fruit powder from juvenile and mature trees to EPN and EPF were recorded. Mortality of neonate larvae was significantly lower when placed on diets containing 15% fruit powder from mature trees (45% mortality) than diets containing 15% fruit powder from juvenile trees (61% mortality), after larvae ingested the lowest virus concentration tested, being 2 x104 OBs/ml. Data collected from field surveys showed significantly lower egg parasitism, virus infection of larvae and EPF occurrence in juvenile orchards than mature orchards. Egg parasitism was between 11% and 54% higher in mature orchards than juvenile orchards, with the exception of Mandarins during 2015, where egg parasitism was slightly higher in juvenile orchards, but not significantly so. A significantly higher proportion of larvae retrieved from mature orchards (7% of larvae) were infected with CrleGV than larvae retrieved from juvenile orchards (4% of larvae). A significantly higher occurrence of EPF was recorded in non-bearing and mature orchards, with 40% and 37% occurrence respectively, than in juvenile orchards, with 25% occurrence recorded. EPF occurrence in juvenile orchards increased significantly by 16% to 32% from the first to the third year of sampling. In contrast to results recorded in laboratory trials, similar or higher pest pressure in juvenile orchards than mature orchards did not always result in significantly higher levels of FCM damage under field conditions. FCM damage in juvenile orchards may have been lower than expected, as greater extremes of temperature and lower humidity were recorded in juvenile orchards, which would increase larval mortality. Results of this study showed that juvenile and mature orchards are significantly different and should be managed differently.
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- Date Issued: 2018
The population dynamics of an indigenous Psyllid Acizzia Russellae (Homoptera: Psyllidae )with special reference to the influence of the host plant Acacia Karroo
- Authors: Webb, John Warren
- Date: 1975
- Subjects: Homoptera , Jumping plant-lice , Insect populations , Insects -- Ecology , Acacia -- Karroo
- Language: English
- Type: Thesis , Doctoral , PhD
- Identifier: vital:5894 , http://hdl.handle.net/10962/d1013415
- Description: The biology and ecology of an indigenous, non-pest insect on an indigenous host plant were investigated. The study includes a taxonomic description of this new species, an account of its general biology, monitoring of the seasonal fluctuations in numbers of the psyllid and its hymenopterous parasitoids, and a study of various aspects of the host plant, including nitrogen levels, water stress, leaf hardness, and the effect of cutting, in relation to spatial and temporal differences in insect population numbers. Natural enemies, inter- and intra-specific competition had very little determining influence on the population numbers of the psyllid. Temperature and humidity had little direct effect, but may have influenced the population dynamics via its effects on the host plant. Seasonal patterns in psyllid numbers followed fluctuations in nitrogen levels; statistically valid correlations were obtained between leaf nitrogen and psyllid numbers on individual trees at various times. These findings were supported by the results of preliminary laboratory experiments employing different fertilizer treatments. No effect of water stress or leaf hardness was clearly discerned. Cutting of trees altered the characteristics of the subsequent regenerative growth so as to allow massive psyllid infestations to develop, thus showing the tremendous importance of the host plant in determining population levels in this insect. Preliminary investigations of the nature and mechanism of this effect were conducted, and its significance is discussed. The relevance of these findings to modern concepts of regulation in insect populations and to principles of pest management is discussed.
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- Date Issued: 1975