Assessment of the Entomotoxic Potential of the Bacillus thuringiensis var. kurstaki HD-1-Based Biopesticide against Sahlbergella singularis (Hemiptera: Miridae): A Cocoa Mirid in Côte d'Ivoire

Abstract

The mirid Sahlbergella singularis Hagl. causes major production losses in cocoa trees in Côte d'Ivoire. To deal with this problem, farmers make excessive use of synthetic chemical insecticides to improve crop yield. In order to reduce the chemical pollution risk, this study was initiated to assess the efficacy of a Bacillus thuringiensis var kurstaki HD-1 (Btk HD-1)-based biopesticide against S. singularis. The study was carried out in the laboratory in Petri dishes on two L4 larvae using the triplet method and, in the field, on a homogeneous population of L4 larvae, using a scattered block design. Observations were made using four doses [D1 (10%), D2 (1%), D3 (0.1%), D4 (0.01%) (v/v)] of biopesticide versus a negative control (D0). The mortality rates and the severity of damage to cocoa organs (immature cocoa pods and twigs) caused by mirid bites were measured. Laboratory results revealed the effectiveness of the tested doses. D1 induced 100% mortality of L4 larvae, while D4, D3 and D2 caused 41.61, 47.55 and 69.41% mortality respectively, compared with 4.17% mortality in control (D0), on the fifth day of laboratory tests. In the field tests, in a natural environment, D1 and D2 provided more than 77% phytosanitary protection of cocoa organs against mirids. The LD50 values obtained on the fifth and seventh day of treatment were 0.038%, respectively for the laboratory tests and varied between 0.015 and 0.066% for the field tests. The demonstrated bioinsecticidal effect of Btk HD-1 is dose sensitive and increases with time. This biopesticide could be an alternative to the conventional chemical insecticides used to control cocoa mirids.