Endophytic fungi for the management of spotted pod borer, Maruca vitrata Fab.(Lepidoptera:Crambidae) in cowpea.

Abstract

The spotted pod borer, Maruca vitrata Fab. (Lepidoptera: Crambidae) is a major constraint in the production of legumes because of its wide host range, distribution and destructiveness. In cowpea, the loss due to pod borer infestation varies from 20 to 60 per cent, often reaching upto 80 per cent in severe cases. Application of pesticides can hardly be recommended in cowpea as the pods are harvested at alternate days. Biological control of M. vitrata using entomopathogenic microorganisms has received considerable attention in recent times. Previous studies have demonstrated the potential of entomopathogenic organisms to colonise an array of plants and confer protection from insect pests. In this context, a study entitled “Endophytic fungi for the management of spotted pod borer, Maruca vitrata Fab. (Lepidoptera: Crambidae) in cowpea” was undertaken with an objective of isolation and characterization of endophytic entomopathogenic fungi in cowpea, standardization of method of inoculation of entomopathogenic fungi in cowpea, and evaluation of selected endophytic fungi for management of the spotted pod borer, M. vitrata. Purposive sampling surveys were conducted in the major cowpea growing areas of Kozhikode, Thrissur, Kottayam and Thiruvananthapuram districts of Kerala. A total of 235 endophytic fungal isolates were obtained from the cowpea plant samples collected from 40 locations. This comprised of 103 isolates from roots, 63 from stems, 31 from leaves, 33 from pods and five from flowers. Nine accessions of cowpea with different levels of resistance were also screened for the isolation of fungal endophytes. All the accessions were found to harbor endophytic fungi and yielded 32 isolates. Among these accessions, Palakkadan thandan payar yielded the maximum number of seven endophytic fungi. Lola and Mysore local had the lowest number of isolates. In contrast to the results of survey, the leaves of cowpea plant harbored more endophytic fungi than other plant parts. A total of 267 isolates were obtained from survey samples and cowpea accessions. Three isolates were found to be pathogenic to M. vitrata. They were identified as Fusarium oxysporum (EEF 1) and two isolates of Purpureocillium lilacinum (EEF 4 and EEF 64) through morphological and molecular characterization. These isolates were evaluated along with Beauveria bassiana (NBAIR strain) for their bioefficacy against M. vitrata. The best two organisms in the bioefficacy studies, viz., B. bassiana (NBAIR strain) and P. lilacinum (EEF 4) were used for standardizing the inoculation technique for endophytic colonization in cowpea plant. Three different methods of inoculation viz., seed, soil and foliar inoculation were evaluated for identifying the best method for colonization of entomopathogenic fungi in cowpea plants. The effect of fungal colonization on infestation of spotted pod borer in cowpea was further assessed under field condition. Foliar inoculation with B. bassiana registered the lowest mean pod damage of 12.53 per cent. B. bassiana applied as foliar application resulted in the highest marketable pod yield of 152.83g plant-1 , followed by P. lilacinum as foliar application (149.33g plant-1 ). Based on the polyhouse and field studies, it was inferred that foliar application of B. bassiana was found to be the best treatment against the target pest, M. vitrata. In addition, the results of the current study suggested that that endophytic colonization in cowpea had little adverse impact on plant growth and yield. Foliar application of B. bassiana, which was identified as the most effective treatment against pod borer in the previous studies, was selected for comparative evaluation with the diamide insecticide, flubendiamide against M. vitrata. Cowpea plants treated with both flubendiamide and B. bassiana had significantly less number of infested pods than control, with 8.41 and 15.05 per cent infestation respectively. Control plants recorded the highest infestation of 21.28 per cent. Mean marketable pod yield showed significant difference between flubendiamide (166.14g plant-1 ) and foliar inoculation of B. bassiana (155.14g plant-1 ). Both these treatments also had significantly higher marketable pod yield compared to control plants (139.29g plant-1 ). In conclusion, the present study revealed that use of B. bassiana as an endophyte could be a useful tool in integrated pest management of pod borer in cowpea. However, further studies are needed to understand the mechanisms through which the endophytes protect plants from herbivores and promote plant growth and yield.