Evaluation of Bacillus Thuringiensis Isolates’ Potential in the Management of Pod Borers (Lepidoptera: Noctuidae) Infesting Pigeon Peas

Abstract

Pod borer infestation, especially Helicoverpa armigera, is a key constraint affecting pigeon pea (Cajanus cajan) production. Farmers find it difficult to adopt the use of synthetic pesticide because pigeon pea is a subsistence crop pesticides are expensive and there is danger of pesticide resistance development. The objectives of this study were to i) evaluate the response of pod borer against Bacillus thuringiensis (Bt) isolates in the laboratory and ii) determine the efficacy of Bacillus thuringiensis cry gene against the pigeon pea pod borer. Soil samples from pigeon pea growing farms within Machakos and Makueni Counties were isolated with a selective technique using nutrient broth mixed with 0.25M Sodium Acetate (C2H3NaO2). The Bacillus thuringiensis isolates were grown on nutrient agar plates to establish colony features and were subjected to gram reaction to ascertain the Bacillus thuringiensis morphological characteristics. Molecular detection of ten (10) Bacillus thuringiensis isolates was done using PCR with specific primers for Cry1 and Cry2 genes. The isolates were tested at different dilutions (10-1, 10-2, 10-3, 10-4, 10-5, and 10-6) for bioassay and bio efficacy experiment in the field to determine the virulence against pigeon pea pod borer. Second and third instar larvae used for bioassay were reared in ITOC- ICIPE. Out of the 10 characterized isolates, two isolates, J7 and J10, with strong bands of Cry1 and Cry2 genes and were selected for the bioassay. The same isolates at the effective dose rates determined in the laboratory were used to test their bio efficacy potential in controlling pod borers in the field. In the laboratory, Bacillus thuringiensis Cry1 and Cry2 significantly (P<0.05) caused higher mortality of 2nd and 3rd instar larvae of pod borer compared to control. Bacillus thuringiensis Cry1 significantly (p<0.05) caused higher mortality to 2nd and 3rd instar larvae of pod borers compared to Bt Cry2. In the field, Bt Cry1, Bt Cry2 and the mixture of Bt Cry1 and Bt Cry2 at concentrations 10-3 and 10-4 significantly (p<0.05) reduced the number of damaged pods in the field compared to the control. Among the treatments, Bacillus thuringiensis Cry1 recorded the least number of damaged pods in the field and was significantly (p<0.05) different from the mixture of Cry1 & Cry2 and Cry2 alone. These results demonstrate the potential of locally isolated Bacillus thuringiensis Cry1 and Cry2 to manage pod borer infestation of pigeon peas in the field. Further evaluation and development can be done to provide an alternative method of managing the pod borers for sustainable production of pigeon peas.