Assessment of Polyphenols and Saponins as Secondary Metabolites from Different Cocoa Genotypes on Attractiveness and/or Repellency towards the Brown Cocoa Mirid Sahlbergella singularis under Laboratory Conditions

Abstract

The Brown cocoa mirid (Sahlbergella singularis) is the most economically damaging insect pest of cocoa in West and Central Africa, including Cameroon. Chemical insecticides are currently the primary means of controlling this pest; however, their extensive use poses significant risks to the environment, non-target organisms and human health. Consequently, sustainable, environmentally friendly and cost-effective alternatives are needed for mirid management. Among these alternatives, host-plant resistance, including the identification and deployment of resistant and/or tolerant cocoa genotypes, represents a promising approach for reducing mirid damage and improving the long-term sustainability of cocoa production systems. However, knowledge of the direct role of cocoa secondary metabolites, particularly saponins and polyphenols, under single exposure to mirids remains limited, although these compounds are reported to be key components of plant defence against herbivores. From September 2023 to February 2024, secondary metabolites, including polyphenols and saponins, were extracted from twelve cocoa genotypes, and their effects on mirid attractiveness or repellency were evaluated. Food preference was evaluated using microassays designed to measure cocoa attractiveness and antixenosis responses to mirids. Fourth- and fifth-instar larvae of Sahlbergella singularis were selected for these assays because they are easier to manipulate than the more delicate early instars and, unlike adults, are less likely to evade handling through flight. The mirids used in the study were obtained from a colony maintained in the insectarium of IRAD, Nkoemvone, and were reared according to the protocol described by Babin et al. (2008). After the extraction and fractionation of plant material in the Laboratory of Pharmacochemistry and Natural Products of the University of Yaoundé I, attractiveness/repellency tests for the two targeted secondary metabolites were performed using the protocol of McDonald et al. (1970). The microtest results showed that mean mirid feeding lesions, expressed as host-plant attractiveness, varied significantly among the tested cocoa genotypes, ranging from 2.70 ± 1.10 (genotype T60/887) to 15.03 ± 1.80 (genotype IMC60 × SNK605). At the same concentration (0.1 g/mL), saponins from different cocoa genotypes were more repellent to mirids, with positive mean repellency values ranging from 93.33 ± 11.55 (class V for UPA143 × SNK64) to 66.67 ± 11.55 (class IV for T79/501 and T60/887), whereas polyphenols were more attractive to mirids, as indicated by negative repellency values for all cocoa genotypes (class 0), ranging from -26.67 ± 15.40 (IMC60, T60/887 and IMC60 × SNK16) to -60.00 ± 20.00 (T79/501 × SNK13). Understanding the direct role of these compounds in cocoa defence processes is necessary to support breeding programmes for cocoa resistance to mirids, particularly S. singularis.