Diversity And Some Aspects Of The Bioecology Of Aphids On Cabbage And Their Association With A Novel Necrotic Disease

Abstract

Agriculture serves as a significant source of livelihood for many small-scale farmers, who account for the greatest share of agricultural production. Of the numerous varieties of exotic vegetables grown in Ghana, cabbage (Brassica oleracea var. capitata) is widely grown to meet the high demand of the ever-growing urban market, due to the many health benefits associated with its consumption. However, the devastating impacts of pests such as aphids and diseases on cabbage have resulted in significant yield losses. Aphids achieve this fate by developing resistance to many insecticides by rendering them ineffective, heavy infestations, development of sooty mould and transmission of viral diseases. Management interventions have been largely dependent on calendar spray regimes, with little or no knowledge on pest species and bioecology. While these interventions are an integral component of an integrated pest management approach on cabbage, identifying pest species, understanding their bioecology is important in the development and success of these pest management interventions. The current study sought to understand the diversity and bioecology of aphids on cabbage, and their role in the aetiology of a novel necrotic disease in Ghana. Surveys were conducted from 2019 to 2020, in nine regions of Ghana across five agroecological zones, noted for the production of cabbage to determine aphid species associated with cabbage and their distribution. Their regional abundance and spatiotemporal variation across agroecological zones were also assessed. In each region, cabbage farms in three districts were selected for the survey. Sampling was done by randomly sampling 20 plants per farm and aphids were counted, and some collected into vials containing 70% alcohol for identification. Two aphid species were identified to occur on cabbage, i. e. Lipaphis erysimi pseudobrassicae and Myzus persicae, with the former being the more abundant in all the regions. The highest aphid distribution and abundance was recorded in the Southern part of the country. There was significant variation in the population of these aphids across the different agroecological zones, with the highest recorded in the Deciduous Forest and Coastal Savannah zones. Aphid abundance was influenced by agroecological zones, vegetative cover and climatic conditions. The distribution of L. e. pseudobrassicae and M. persicae, in Ghana could not be ascertained by manually sampling all cabbage growing regions, hence, the rationale for the use of species distribution models to understand their spatial patterns and the underlying contributing climatic factors to map their suitable habitats. Occurrence data collected during the survey was used to determine aphid spatial distribution. A tree-based supervised machine learning algorithm, Random Forest was used to infer the underlying contributing factors, and to map the habitat suitability of both aphid species. Temperature and isothermality were the most important weather variables that affected the distribution of L. e. pseudobrassicae and M. persicae. The Southern Ghana was most suitable for the occurrence of both aphid species. Species distribution models can inform researchers about particular ecological adaptations of aphids and other insect pests in targeted areas, which can optimize efficiency and cost in pest control programmes.