Diversity of THRIPS (Thysanoptera: Thripidae) On Export Vegetables, Their Sustainable Management and Phytosanitary Implications

Abstract

The horticulture industry contributes significantly to Ghana’s GDP through the cultivation and export of several exotic vegetables such as eggplant, chilli pepper, and tinder. These vegetables, mostly cultivated in the Southern part of the country are hosts to several insect pests such as thrips. Whilst some thrips species are biologically significant, others threaten the potential of Ghana to remain competitive in the export market through their ability to feed, transmit tospoviruses and physically contaminate exported consignments which often has phytosanitary implications. Efforts at controlling thrips populations with the current pest management approach of synthetic pesticides have proved futile due to their thigmotactic behaviour, lifecycle attributes and their minute size. It is critical in any pest management programme to correctly identify the species in question and combine knowledge of the biology and thermal requirements to improve on planning, timing and precision of pest management interventions. This thesis sought to understand the diversity and biology of thrips and look at pest management approaches that could guarantee the export requirements of thrips-free produce. Surveys were conducted in the Greater Accra, Eastern and Central regions on selected export vegetables from September 2019 to January 2021, to identify the thrips on export vegetables, their abundance and diversity between regions and crops. In each region, four vegetable export growing farms/localities were selected and thrips were collected from six export vegetables/location. In each crop, 40 plants were randomly selected in an X-pattern and thrips were collected into 60% alcohol for identification. A total of eight thrips species were identified including the South-East Asian thrips, Thrips parvispinus (Karny), the cotton thrips, Frankliniella schultzei (Trybom), the onion thrips, Thrips tabaci (Lindeman), the vespiform thrips, Franklinothrips vespiformis amongst others. Luffa acutangular supported a higher abundance of thrips, but diversity was greatest on Solanum melongena and Capsicum frutescens. Across the regions, the Central region had the greatest diversity of thrips with the invasive species, T. parvispinus, as the most dominant. This objective highlights the need for targeted control measures that are effective against the thrips species present to be developed, to enhance the quality and export potential of vegetables. As it is essential to keep tract of the population of the identified thrips species to understand the population dynamics and implement timely control measures, the species composition and population dynamics of thrips was monitored using blue, yellow, and white sticky traps on commercial eggplant and chilli farms in Tuba in two cropping seasons of 2020/2021. Each field was divided into 8 blocks and in each replicate, all trap colours representing three treatments were randomly tied on stakes at the centre for the respective crop. Data loggers were installed to record hourly climatic variables. Three thrips species (T. parvispinus, F. schultzei and T. tabaci) were identified from each of the commercial farms and the different species showed varied attractiveness to colour traps for both seasons, with white proving more attractive to T. parvispinus. The dynamics of the species varied significantly with the season and climatic factors (temperature and relative humidity) but not with the crop. Optimum temperatures and relative humidity of 28 ºC – 31 ºC and 60%-78 %, respectively showed a positive linear relationship between the trapped insects with temperature and RH whilst extreme environmental factors (35 ºC) negatively affected their abundance. Incorporating white sticky traps into the country-wide monitoring plan can improve the effectiveness of pest management interventions. To understand how the most important environmental factor, temperature, influence the biology of the invasive species, T. parvispinus, its growth and development was studied under six constant temperatures to develop a temperature-dependent phenology model for predicting future climatic trend. Mathematical functions were fitted to describe the relationship between temperature and demographic attributes of T. parvispinus using the “model builder” and “validation and simulation” components of the insect life cycle modelling software. The fitted mathematical functions were compiled to develop the overall phenology model which was used to simulate the life table parameters. Temperature influenced the biology of T. parvispinus, which had six life stages including the egg, two larval instars, propupa, pupa and adult stages. The development time decreased with increasing temperatures such that the longest egg-to-adult development time of 37.72 days occurred at 15 ºC, whilst the shortest of 8.84 days occurred at 35 ºC. Development rate was fastest at higher temperatures. The model predicted 25 ºC to 30 ºC as favourable temperatures for the reproduction of T. parvispinus, with maximum eggs of 54.83 at 30 ºC. The growth parameters such as the gross reproductive rate was maximum at 35 ºC with 45 offspring per generation, compared to 33.42 and 9.80 at 25 ºC and 15 ºC, respectively. This result provides for the first-time baseline optimal thermal requirements, useful in planning and timing pest management interventions for this pest in the context of climate change. To determine the efficacy of novel insecticides in suppressing thrips population, the variability in the population dynamics of thrips to pest management options and the economic feasibility of using pest management modules in thrips control, two field experiments were conducted in the Deciduous Forest and Coastal Savannah agroecological zones in the major and minor rainy seasons of 2020/2021. In the first experiment, eggplant seedlings transplanted on 5.4 x 5.4 m field plots were sprayed with novel insecticides Agroblaster®, Agroclean®, Viper, Akape® and Alphacep® after the thrips population reached a threshold of 1.5 thrips/leaf. Data on thrips numbers, damage, natural enemies, and other pests were collected one day before application (1DBS), and at 3, 5 and 9 days after each application (3DAS, 5DAS and 9DAS, respectively). The efficacy of treatments, within-plant distribution and yield data were assessed. The thrips population, damage and yield were lower on treated plots than the control plots and also in the Coastal Savannah than the Deciduous Forest zone for all seasons. Different treatments had different efficacies depending on the mode of action, location and season. Alphacep® was the best treatment at 3DAS and Agroclean® and Agroblaster® were best at 9DAS. Alphacep® reduced spiders’ abundance and showed slight to strong phytotoxicity, whilst other insecticides did not affect natural enemies and plants. Yield was highest in the Coastal Savannah zone. The result of the second experiment with four treatments (chemical intensive, less-risk, integrated control (IPM) and control) showed that the population dynamics of thrips varied across treatments and sampling time. The IPM treatment had the lowest abundance of thrips and other pests, the highest yield and cost-benefit ratio. This information can be used to select appropriate location-specific insecticides/options for use in vegetable production, increase yield and lower production cost. In conclusion, this thesis provides the first time a comprehensive study of thrips across different vegetable agroecosystems in Ghana, contributing to a better understanding of thrips biodiversity and highlighting the need for integrated pest management strategies tailored to each crop and region to effectively mitigate thrips infestations to meet sanitary and phytosanitary standards while preserving local biodiversity. Enhanced monitoring programs and capacity building efforts in thrips identification are recommended to support sustainable agricultural practices and safeguard Ghana's vegetable exports against thrips-related challenges.