Alternative pest control methods for false codling moth, Thaumatotibia leucotreta (Lepidoptera: Tortricidae): exploring juvenile hormone analogs and entomopathogenic nematodes for integrated pest management

Thesis (MScConsEcol)--Stellenbosch University, 2023.

ENGLISH ABSTRACT: False codling moth (FCM) (Thaumatotibia leucotreta (Meyrick, 1913), Lepidoptera: Tortricidae) is a major phytosanitary pest of diverse crops in southern Africa, especially deciduous fruit and citrus. Here, I provide an overview of the significance of FCM in a South African context, a brief overview of current pre- and post-harvest control methods, and the factors contributing to the increased pressure towards alternative, pesticide-limited control. False codling moth control is currently reliant on integrated pest management (IPM) that employs multiple strategies in combination. To assess the potential inclusion of juvenile hormone analogs (JHA), fenoxycarb and methoprene, into current IPM programmes and/ or Sterile Insect Technique (SIT) programmes I investigated their effect on the development, cold stress resistance and flight performance in false codling moth under different constant temperatures in two experiments. In a third experiment, the concept of adding additional stressors to the system was investigated, by assessing the interaction of methoprene and fenoxycarb with a heat wave exposure on the susceptibility of FCM to the entomopathogenic nematode, Steinernema yirgalemense Nguyen, Tesfamariam, Gozel, Gaugler and Adams (Rhabditida: Steinernematidae). In the first experiment, I exposed fourth to fifth instar larvae to four concentrations of fenoxycarb and methoprene at 25 °C: 0.1 µg/µL, 1 µg/µL, 5 µg/µL and 10 µg/µL and compared these to a handling control. Larval and pupal development time, number of pupae formed, number of adults eclosed, and longevity were recorded daily. In adults that emerged, I also tested chill coma recovery time and spontaneous behaviour as a measure of low temperature stress resistance. At 25 °C (Experiment 1), both fenoxycarb and methoprene-exposed larvae had significantly lower pupation and emergence numbers compared to the control group, and overall mixed or weak effects on development, and no effect on longevity. Low temperature performance was not affected by either methoprene or fenoxycarb at 25 °C. Next (Experiment 2), to explore the impact of temperature on JHA effects, I exposed fourth to fifth instar larvae to either 1 µg/µL methoprene or 0.1 µg/µL fenoxycarb and maintained larvae under either 20 °C, 25 °C or 30 °C while recording the same parameters as described previously (Experiment 1), with the addition of flight performance tests in adults. Cooler temperatures significantly slowed development time from larvae to adults in both treated and control groups, and reduced the number of pupae in the fenoxycarb treated group. Adult emergence was reduced by methoprene at 30 °C, and by fenoxycarb at all temperatures. Temperature afffected chill coma recovery time and sponateous behaviour in fenoxycarb-treated individuals, with larvae kept at 20 °C having a faster response time than fenoxycarb-treated FCM at 25 °C and 30°C, as well as the control group at 30 °C. Temperature also increased response time for spontaneous behaviour at 30 °C compared to 20 °C and 25 °C. Temperature, but not JHA treatments, affected the flight ability of moths, with larvae kept at 30 °C producing significantly more dispersers compared to larvae kept at 25 °C. For the third experiment, fourth to fifth instar FCM larvae were exposed to a heat wave (38 °C) for two hours, allowed to recover at 25 °C for six hours, treated with either 1 µg/µL methoprene or 0.1 µg/µL fenoxycarb, and inoculated individually with 50 IJs/50 µL of in-vitro cultured entomopathogenic nematode (EPN) S. yirgalemense. Mortality was determined after 48 hours and dissections were carried out to verify infection performed after another 24 hours. Heat wave exposure did not significantly affect EPN susceptibility for either of the JHA treated groups. Infection was much lower than expected, based on the virulence of S. yirgalemense on FCM, and additional work is needed to form a better understanding of all the contributing factors. Overall, JHA had largely predictable and pronounced negative effects on life history traits and performance in false codling moth, making it a potential addition to integrated pest management programmes as a larvicide, but would not be beneficial to SIT mass-rearing programmes. Further research into the practicality of field application of JHA is essential, as well as the role of multiple simultaneous stressors (e.g., temperature and JHA) on the efficacy of biological control agents in the field.

AFRIKAANSE OPSOMMING: Valskodlingmot (FCM) (Thaumatotibia leucotreta (Meyrick, 1913), Lepidoptera: Tortricidae) is 'n fitosanitêre pes van verskeie gewasse in Suider-Afrika, insluitend sagtevrugte en sitrus. In hierdie tesis verskaf ek 'n oorsig van die belangrikheid van valskodlingmot in 'n Suid-Afrikaanse konteks, gee 'n kort oorsig van huidige voor- en na-oes beheermetodes, sowel as die faktore wat bydra tot die verhoogde druk op alternatiewe, plaagdoder-beperkte beheer. Die beheer van valskodlingmot maak staat op geïntegreerde plaagbestuur (IPM) wat verskeie strategieë in kombinasie gebruik. Om die potensiële insluiting van jeughormoon-analoë (JHA), fenoksikarb en metopreen, in huidige IPM-programme en/of Steriele Insek Tegniek (SIT) programme te bepaal, het ek in twee verskillende eksperimente die effek van JHA op die ontwikkeling, kouestresweerstand en vlugprestasie van valskodlingmot onder verskillende konstante temperature ondersoek. In ʼn derde eksperiment, is die konsep van addisionele stressors in die sisteem ondersoek. Valskodlingmot is eers blootgestel aan ʼn hittegolf, behandel met metopreen en fenoksikarb en toe blootgestel aan die entomopatogene nematode, Steinernema yirgalemense Nguyen, Tesfamariam, Gozel, Gaugler en Adams (Rhabditida: Steinernematidae) om die interaksie tussen die hittegolfblootstelling en die JHA op die vatbaarheid van valskodlingmot te bepaal. In die eerste eksperiment het ek larwes van die vierde en vyfde stadium aan vier konsentrasies van fenoksikarb en metopreen: 0.1 µg/µL, 1 µg/µL, 5 µg/µL en 10 µg/µL blootgestel by 25 °C en dit vergelyk met 'n hanteringskontrole. Larwe- en papie-ontwikkelingstyd, die aantal papies wat gevorm is, die aantal volwassenes wat uitgebroei het en langlewendheid is daagliks aangeteken. In oorlewenede volwassenes het ek ook koue koma herstel tyd (CCRT) en spontane gedrag (SB) bepaal as 'n maatstaf van lae temperatuur stres weerstand. By 25 °C (Eksperiment 1), het beide fenoksikarb en metopreen-blootgestelde larwes aansienlik laer verpoping en uitbroeigetalle gehad in vergelyking met die kontrolegroep, en algehele gemengde of swak uitwerking op ontwikkeling, en geen effek op langlewendheid nie. Lae temperatuur prestasie is nie deur metopreen of fenoksikarb by 25 °C beïnvloed nie. Volgende (Eksperiment 2), om die impak van temperatuur op JHA-effekte te ondersoek, het ek larwes van die vierde en vyfde stadium blootgestel aan óf 1 µg/µL metopreen óf 0.1 µg/µL fenoksikarb en larwes onder óf 20 °C, 25 °C of 30 °C gehou, terwyl dieselfde parameters aangeteken was as voorheen beskryf (Eksperiment 1), met die byvoeging van vlugprestasietoetse by volwassenes. Koeler temperature het ontwikkelingstyd van larwes na volwassenes in beide behandelde en kontrolegroepe aansienlik vertraag, en het die aantal papies in die fenoksikarb-behandelde groep verminder. Volwasse uitbroei getalle is verminder deur metopreen by 30 °C, en deur fenoksikarb by alle temperature. Temperatuur het koue koma herstel tyd en spontane gedrag beïnvloed in fenoksikarb-behandelde individue, met FCM wat gehou is by 20 °C met 'n vinniger reaksietyd in vergelyking met fenoksikarb-behandelde FCM by 25 °C en 30 °C, sowel as die kontrolegroep by 30 °C. Temperatuur het ook reaksietyd vir spontane gedrag versnel by 30 °C in vergelyking met 20 °C en 25 °C. Temperatuur, maar nie JHA nie, het die vlugvermoë van motte beïnvloed. Larwes wat by 30 °C gehou was het aansienlik meer verspreiders produseer in vergelyking met larwes wat by 25 °C gehou was. Vir die derde eksperiment is FCM-larwes van die vierde en vyfde stadium vir twee uur aan 'n hittegolf (38 °C) blootgestel, toegelaat om te herstel by 25 °C vir ses ure, behandel met óf 1 µg/µL metopreen óf 0.1 µg/µL fenoksikarb, en toe individueel blootgestel aan 50 IJs/50 µL in-vitro gekweekte S. yirgalemense. Mortaliteit is na 48 uur bepaal en disseksies is uitgevoer om infeksie te verifieer wat na nog 24 uur uitgevoer is. Hittegolfblootstelling het nie EPN-vatbaarheid vir enige van die JHA-behandelde groepe beduidend beïnvloed nie. Infeksie was baie laer as wat verwag is, gebaseer op die virulensie van S. yirgalemense op FCM, en bykomende werk is nodig om 'n beter begrip van al die bydraende faktore te vorm. In die algemeen het JHA grootliks voorspelbare en ‘n merkwaardige negatiewe uitwerking op lewensgeskiedenis-eienskappe en prestasie in valskodlingmot gehad, wat dit 'n potensiële toevoeging tot geïntegreerde plaagbeheerprogramme as larwe-doder kan maak, maar sal nie voordelig wees vir massa-teling in ‘n SIT program nie. Verdere navorsing oor die praktiese toepassing van JHA in die veld is noodsaaklik, asook die rol van verskeie gelyktydige stressors (bv. temperatuur en JHA) op die doeltreffendheid van biologiese beheermiddels in die veld.

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