Potential of integrating entomopathogenic and endophytic fungal based biopesticides for sustainable management of the South American tomato pinworm, Phthorimaea absoluta (Meyrick) (Lepidoptera: Gelechiidae)

Abstract

The South American tomato pinworm, Phthorimaea absoluta (Meyrick) (Lepidoptera: Gelechiidae) is one of the most devastating insect pests of tomato and other solanaceous crops (nightshade and potato) in Africa, yet control options are limited with the management of the pest heavily relying on indiscriminate use of synthetic insecticides leading to adverse effects on the environment and health of humans. Entomopathogenic fungi (EPF) offer an effective and viable alternative to synthetic insecticides, as they cause significant epizootics in the target host populations through an inundative approach and can also be used as plant endophytes whose presence within host plants are beneficial to the host. Even though promising beneficial effects due to endophytes are reported, the mechanism of plant-endophyte-P. absoluta communication is still poorly understood. Besides, the efficacy of three Metarhizium anisopliae (Metchnikoff) Sorokin fungal strains (ICIPE 18, ICIPE 20 and ICIPE 665) has been previously reported as the most potent isolates with high potential as biocontrol agents of adult P. absoluta. However, the interactions between their performance and abiotic factors that could affect their efficacy in the field need to be determined for an effective selection of the most virulent fungal isolate(s) best suited for mass-production prior to formulations and field deployment. This study assessed the efficacy of 18 candidate entomopathogenic- and endophytic-fungal isolates with the aim of integrating them into a sustainable management programme to control the pest. Chapter one provides an overview of current knowledge on the ecology and management practices of P. absoluta, and it identifies potential research gaps. Chapter two assessed the thermotolerance and virulence of the three candidate M. anisopliae isolates against adult P. absoluta under different temperature regimes. The findings revealed that over 90% of conidia germinated at 20, 25 and 30 °C while no germination occurred at 15 °C. In addition, growth of the three isolates occurred at all temperatures but was slower at 15, 33 and 35 °C compared to 20, 25 and 30 °C. Optimum temperatures for mycelial growth and spore production were 30 and 25 °C, respectively. Furthermore, ICIPE 18 produced 34.87 and 58.96% higher amount of spores than ICIPE 20 and ICIPE 665, respectively. The highest mortality of P. absoluta moths occurred at 30 °C for all the three isolates and was 91, 90 and 78% for ICIPE 18, ICIPE 20 and ICIPE 665, respectively. While the LT50 values were 30.69 and 45.63% for ICIPE 18 and ICIPE 20, respectively, significantly lower at 25 °C and 34.87% and 32.72% for ICIPE 18 and ICIPE 20, respectively, lower at 30 °C than those of ICIPE 665. Subsequently, Logan-4 and Logan-1 models gave the best fit to the mortality data to model the virulence of ICIPE 18 and ICIPE 20, against adult P. absoluta using the Entomopathogenic Fungi Application (EPFA) software. Spatial prediction revealed suitable locations for ICIPE 18 and ICIPE 20 deployment against P. absoluta in Kenya, Tanzania, and Uganda. Chapter three investigated the endophytic properties of 15 fungal isolates on tomato and nightshade and evaluated their insecticidal activity against adult and immature stages of the pest. The results showed that twelve isolates were endophytic to both host plants with varying colonisation rates. Hypocrea lixii F3ST1 and T. asperellum M2RT4 colonised more than 85% of all the plant tissues of both host plants while B. bassiana ICIPE 706 colonised 60, 40 and 15% of roots, stems and leaves of tomato, respectively; and 70, 35 and 15% of roots, stems and leaves of nightshade, respectively. Trichoderma atroviride F2S21 successfully-colonised 100, 100 and 75% of roots, stems and leaves of tomato plant respectively, and 100, 95 and 55% of roots, stems and leaves in nightshade, respectively. Tomato and nightshade host plants endophytically-colonised by Trichoderma asperellum M2RT4, Beauveria bassiana ICIPE 706 and Hypocrea lixii F3ST1 outperformed all the other isolates, significantly reducing the number of eggs laid, mines developed, pupae formed, and adults emerged. Trichoderma asperellum M2RT4 endophytically-colonised tomato plants recorded the lowest number of eggs (30.0 ± 4.51 eggs), followed by Beauveria bassiana ICIPE 706 (31.25 ± 5.88 eggs), Hypocrea lixii F3ST1 with (63.25 ± 2.66 eggs) compared to (111.0 ± 13.32 eggs) in the control. Upon egg hatching, T. asperellum M2RT4-endophytically-colonised tomato plants recorded the lowest number of mines (24.0 ± 5.4 mines) compared to (107.33 ± 13.32 mines) in the control. The lowest number of eggs was laid on T. asperellum M2RT4 endophytically-colonised nightshade plants (33.25 ± 3.97 eggs) compared to (109.33 ± 23.31 eggs) in the control. The lowest number of mines (24.5 ± 5.55 mines) was recorded on T. asperellum M2RT4 endophytically-colonised nightshade plants compared to the control (107.33 ± 23.31 mines). In endophytically-colonised tomato plants, fewer P. absoluta pupae (20.75 ± 4.05 pupae) were produced in B. bassiana ICIPE 706 followed by T. asperellum M2RT4 (21.25 ± 5.22 pupae) which were significantly different from the control (103.67± 12.55 pupae). Phthorimaea absoluta adult emergence varied significantly among the fungal isolates, where the highest number of moths (148.0 ± 24.57) emerged from Fusarium proliferatum F2S51 endophytically-colonised tomato plants, followed by the control (101.67 ± 11.46) while the lowest number (17.0 ± 6.34 moths) was recorded on T. asperellum M2RT4 endophytically-colonised tomato plants. The highest number of pupae was obtained in the control (102.33 ± 22.93 pupae) and the lowest (19 ± 4.12 pupae) was recorded in T. asperellum M2RT4 endophytically-colonised nightshade plants. Further, the number of adults that emerged from the control (99.33 ± 22.98 moths) was significantly higher than the lowest number (15.5 ± 3.2 moths) that was obtained in T. asperellum M2RT4 endophytically-colonised nightshade plants. Furthermore, the survival of exposed adults and F1 progeny was significantly reduced by Trichoderma sp. F2L41 and B. bassiana isolates ICIPE 35(4) and ICIPE 35(15) compared to other endophytic isolates. Chapters four and five unravelled the underlying chemical and molecular mechanisms by which the presence of the most potent endophyte Trichoderma asperellum M2RT4 within tomato host plant affects P. absoluta host selection and herbivory, respectively. Chemical analysis revealed the emission of methyl salicylate in endophytically-colonised tomato plant while for non-colonised infested plants monoterpenes were significantly higher. Phthorimaea absoluta females were attracted to monoterpernes including α-pinene, 2-carene, and β-phellandrene but repelled by methyl salicylate. Additionally, it was found that upon herbivory, T. asperellum M2RT4 modulates tomato plant chemistry through the production of (Z)-jasmone thus activating both salicylic and jasmonic acid defense pathways. Transcriptome analysis showed that in planta colonisation of tomato plant by T. asperellum M2RT4 activates tomato host plant defense pathways through the expression of N-methyltransferase. The findings not only reveal suitable locations for deployment of EPF-based biopesticides against P. absoluta in East Africa, but also contribute to a better understanding of host plant-P. absoluta interactions mediated by endophytic fungi, which might potentially be used in the development of effective biopesticides and their integration into a sustainable P. absoluta IPM management programme.