Abundance and diversity of safflower (Carthamus tinctorius l.) insects in Botswana

Abstract

Water scarcity has become a major challenge for Botswana farmers to increase their production despite various development initiatives introduced by the government. Safflower (Carthamus tinctorius l.) is a drought tolerant crop, with potential to serve as an alternative seed oil crop in Botswana. The plant is new in the country and still awaiting adoption by farmers. Literature has shown that safflower is vulnerable to some insect pests, hence hindering its maximum production. This study was carried out to provide a checklist and investigate the abundance and diversity of insect pests among safflower genotypes in Botswana. Insects survey was carried out in two seasons; summer and winter, on five different safflower genotypes; Gila, PI-537636, Kenya-9819, Turkey and Sina. Collection of data and specimens was done once a week and identification was carried out at the Botswana University of Agriculture and Natural Resources, Entomology Laboratory. Data collected were analysed using descriptive statistics (frequencies and percentages). Variation in insects abundance between genotypes and plant growth stages was determined by analysis of variance (ANOVA). Diversity indices were computed using Shannon diversity index, Sorensen’s index and Margalef’s richness index. Correlation analysis was used to establish relationships between population of insects, weather parameters and yield. Fifteen insect species belonging to fourteen families and eight orders were observed on safflower. Of these, 10 species were pests while the rest five, comprising four predators and one pollinator were beneficial. Order Hemiptera had the highest number of species in both seasons. Thrips tabaci and Amrasca biguttula biguttula were the most abundant insect species in summer and winter. Helicoverpa armigera and Aphididae species were identified as the most destructive pests of safflower in the current study. Insect pest populations fluctuated along safflower growth stages, but the most populated stage was flowering. The pests fed on all the upper parts (shoots) of safflower plants in the field, with leaves and capitula being the most affected parts of the crop. Even though insects viii were recorded in abundance, generally the impact of the pests did not significantly (P>0.05) differ between safflower yield from all the five genotypes. This was attributed to compensation ability of safflower plants. The highest diversity index in summer was recorded on genotype Sina (H’=1.47) and the lowest was recorded on PI-537636 (H’=1.32), while in winter the highest diversity was recorded on PI-537636 and Turkey (H’=0.94) and the lowest was H’=0.72 on Sina. The values suggested a non-significant difference in diversity of insects between genotypes. Sorensen similarity index also confirmed the similarity between genotypes as Sorensen similarity coefficient varied from 96% to 100% in summer and varied from 80% to 100% in winter. These findings agreed with the hypothesis of this study that there is no significant difference in abundance and diversity of insects between safflower genotypes. The overall impact of weather parameters on total population of insects were non-significant. Temperature indicated non-significant quadratic correlation with total population of insects in winter but positive and non-significant linear correlation with rainfall and relative humidity. In summer, there was a positive and nonsignificant linear correlation between temperature and total population of insects, a non-significant quadratic correlation with rainfall and positive curvilinear relationship with relative humidity.