Influence of Female Age on the Reproductive Performance of Spermophagus niger Motschulsky and the Insecticidal Activity of Cymbopogon nardus (L.) Rendle Essential Oil

Abstract

Spermophagus niger L. is the main insect pest of stored Hibiscus sabdariffa seeds, causing significant damage through seed perforation. This study aimed to enhance our understanding of the biology and management of this pest by evaluating the influence of female age on reproductive performance, and assessing the insecticidal effect of Cymbopogon nardus essential oil (EO). The influence of females age was assessed through several reproductive and developmental parameters, including egg size, hatching rate, and development time, among other factors. The insecticidal activity of the EO was evaluated against both adults and freshly laid eggs. Increasing female age negatively affected offspring performance, resulting in smaller eggs, higher larval mortality, extended development time, and reduced adult size at emergence. Chemical characterization of C. nardus EO has shown a predominance of monoterpenes, with citronellal (23.61%), geraniol (11.63%), citronellol (9.63%), and D-limonene (1.99%). This EO exhibited significant insecticidal activity against adult S. niger , reducing both fertility and egg hatching rates. The median lethal concentration (LC₅₀) and the concentration causing 90% mortality (LC₉₀) values were estimated at 4.29 µL/L (95% CI: 3.96-4.62) and 10.52 µL/L (95% CI: 9.44-11.87), respectively. These results provide valuable insights into the biology of S. niger and demonstrate the potential of C. nardus EO as an alternative to chemical insecticides for protecting stored H. sabdariffa seeds.