Morphological and biochemical bases of resistance to spotted pod borer, Maruca vitrata (FAB.) (Lepidoptera: crambidae) in cowpea

Abstract

Cowpea, Vigna unguiculata (L.), is an important legume of the tropics and subtropics. It is used as a grain, vegetable and fodder crop. Cowpea is cultivated as a vegetable crop across an area of 6714 ha in Kerala (DES, 2014). However, the productivity of cowpea, at 625 kg ha-1 is very low, one of the reasons being infestation by insect pests. Among the different insect pests, the legume pod borer, Maruca vitrata (Fab.) is the most important one causing loss of up to 60 per cent in cowpea (Singh and van Emden, 1979). The larvae of the spotted pod borer, as the name implies, bore into buds, flowers and pods and feed on internal tissues. Their concealed nature protects the larvae from natural enemies as well as application of insecticides to a considerable extent, making management of the pest difficult. Exploitation of host plant resistance, which is one of the most effective and eco friendly pest management strategies, has hardly been attempted in case of cowpea. It was in this context that the present study „Morphological and biochemical bases of resistance to spotted pod borer, Maruca vitrata (Fab.) (Lepidoptera: Crambidae) in cowpea’ was undertaken in the Department of Agricultural Entomology, College of Horticulture, Kerala Agricultural University, Thrissur during 2014 - 2015. The objectives of the experiment were to evaluate degree of resistance to pod borer in selected accessions of cowpea, as well as to identify the morphological and biochemical bases of resistance in cowpea to pod borer. Forty eight accessions, obtained from different sources, were evaluated for resistance to the spotted pod borer, M. vitrata. Significant variation was observed between the genotypes to pod borer damage at bud, flower and pod stages. The extent of borer infestation varied from zero to 41.04 per cent. Five accessions, viz., EC 100092, IC 39945, IC 2918, EC 98668 and IC 52110 recorded zero per cent damage at all the three stages. Twenty - six accessions recorded damage varying from 0.1 to 9.29 per cent and were on par with each other. All the above accessions were rated as resistant. Seventeen accessions with borer damage ranging from 11.15 to 41.04 per cent were rated as susceptible. The variety Bhagyalakshmy recorded the highest damage of 32.5, 50.39 and 47.95 per cent for bud, flower and pod respectively. Analysis of morphological basis of resistance to pod borer indicated that pod wall thickness was significantly and negatively correlated with per cent damage. Similarly trichome density was also negatively correlated with pod damage, though the correlation was not significant. Length of flower stalk as well as length of trichomes was positively correlated with borer damage but the correlation was not significant. Significant positive correlation was observed between per cent damage and total protein content, moisture content, total sugars and reducing sugars. However, significant negative correlation was observed in peroxidase and polyphenol oxidase activity with per cent damage. The study confirmed variability among cowpea genotypes in response to pod borer infestation, indicating the potential for utilization of host plant resistance in the management of pod borer in cowpea. It also indicated that the resistance in cowpea to Maruca vitrata could be related to biochemical constituents of the plant, which, however, need to be confirmed.