Reduced consumption and use of pods of Vigna species (Leguminosae) by Maruca vitrata (Lepidoptera : Pyralidae)

The consumption and use of pods of some Vigna (Family: Leguminosae) species by Maruca vitrata F. (Lepidoptera : Pyralidae) were investigated in the laboratory under an ambient temperature of 25 ± 2 °C and 60 ± 20 % relative humidity. The effect of morphological and anatomical characteristics of the pods on their nutritional status was also assessed. Larval mass gain on pods of cowpea cultivars, IT84S 2246 and TVu 13731, was significantly higher than on pods of wild cowpea, TVnu 863, cowpea cultivar, IT91K 180, V. oblongifolia, TVnu 42 and V. vexillata, TVnu 72. The relative consumption rate (RCR) and relative growth rate (RGR) were significantly lower on TVnu 72 pods compared with other plants. Approximate digestibility (AD) was significantly higher on pods of IT91K 180, TVnu 42 and TVnu 863 compared with other plants. There was no difference between plants for efficiency of conversion of ingested food (ECI) and efficiency of conversion of digested food (ECD). Strong, significant positive correlations were found between mass gain and ECD (r = 0.90), RCR and RGR (r = 0.89), as well as RGR and ECI (r = 0.84). The density of glandular trichomes on pods of TVnu 72 was significantly higher than on pods of IT84S-2246 and TVnu 863. The density of nonglandular trichomes was similar among plants, but these trichomes were significantly longer on pods of TVnu 72 than on other Vigna species. There were strong, significant negative correlations between the length of the nonglandular trichomes and RCR,RGR and ECI. Of the 3-4 different layers of cells found in the pod wall of the Vigna species, thickness of the fourth (innermost) layer (endocarp) was significantly negatively correlated with RGR and ECI. Similarly, the thickness of the entire pod wall was significantly, negatively correlated with larval mass gain. Our findings suggest that long, non-glandular trichomes and thick pod walls reduce consumption and use of pods of Vigna species by M. vitrata

Pod surface characteristics in wild and cultivated Vigna species and resistance to the coreid bug Clavigralla tomentosicollis Stal. (Hemiptera: Coreidae)

Several Vigna species were used to determine the role of pod trichomes and pod toughness in the resistance of cowpea to feeding damage by the coreid bug Clavigralla tomentosicollis Stal. The scanning electron microscopy study revealed the presence of glandular and non-glandular trichomes on the pod wall of all test genotypes. The cultivated genotypes TVu 1890, TVu 3354 and IT84S-2246 of the V. unguiculata ssp. unguiculata showed significantly lower (P<0.05) densities of glandular trichomes than accessions of the wild Vigna species (TVnu 72, TVnu 151, and TVnu 707). All pods were similar with respect to the density and length of non-glandular trichomes. The two wild accessions TVnu 151 and TVnu 707 of the V. unguiculatassp. dekindtiana had low pod, strength similar to that of the susceptible genotype IT84S-2246, and also showed high seed damage levels comparable to that of this susceptible genotype. These accessions of the subspecies dekindtiana contrasted with the wild and resistant accession TVnu 72 of the V. vexillata species which suffered minor genotypes damage in spite of its low pod strength. The association between high pod strength and low seed damage was found only in the two cultivated genotypes TVu 1890 and TVu 3354. Our results suggest that tough pod wall and high density of glandular trichomes can be combined to achieve enhanced resistance to C. tomentosicollis in cultivated Vigna genotypes