The results indicated that C. chinensis
showed varied response to cowpea grains of different accessions in terms of number of eggs laid, number of adults emerged, percent adult emergence, mean development period (MDP) and growth index (GI). Similarly, there were differential responses in terms of PSWL due to insect infestation. The results of the egg counts revealed that there were statistically significant differences among cowpea accessions (F= 2.38; P
<0.01). Number of eggs laid by C. chinensis
ranged from 50.3 to 146.3 eggs per 20 seeds with minimum on IC 519720 followed by IC 399004 (59.8), indicating that, these accessions were least preferred for oviposition (Table 1). Most preferred accession for egg laying was IC 257844 (146.3). The results in terms of egg counts on different accessions indicated the existence of variability among the accessions that affect oviposition by C. chinensis
. The study of Lephele et al., (2012)
explained that the physical barrier may either limit access into the grain or make it unsuitable for oviposition. Variations in the rate of oviposition due to physical or mechanical characteristics of seeds of cowpea accessions were reported by Amusa et al., (2014)
and Tripathi et al., (2015).
Significant differences were noticed among cowpea accessions in terms of AE, PAE and PSWL (Table 1). AE ranged from 17.00 to 20.75 adults / 20 seeds. Minimum was recorded in IC 628780 (17.00) and IC 436845 (17.00) and maximum was in IC 582853 (20.75). PAE ranged from 13.22 to 41.94 per cent. Minimum PAE was recorded in IC 257844 and maximum was in IC 519720. Lowest PSWL was recorded in IC 582853 (28.38), while highest was recorded in IC 519699 (46.63).
Growth index (GI) is an important parameter of insect growth and development and is a criterion for comparing the growth responses of insects to different plants (Howe, 1971)
and is widely used by various researchers to identify resistance in various legume crops to bruchid infestation (Jackai and Asante, 2003
; Tripathi et al., 2015; Mohamed et al., 2019; Kpoviessi et al., 2020; Satheesh Naik et al., 2021).
Genotypes with a low GI are considered as resistant and those with a high GI are considered as susceptible. GI values significantly differed among cowpea accessions ranging from 0.47 to 1.40. Lowest GI was recorded in IC 257844 while, highest was recorded in IC 519720. On the basis of GI, out of 27 cowpea accessions, only one accession namely, IC 257844 which was collected from Komeru village, Vizianagaram District andhra Pradesh was found to be moderately resistant (Fig 1) while, seven accessions were classified as moderately susceptible, 11 were susceptible and eight were highly susceptible (Table 1). In the similar line of works, Tripathi et al., (2015)
reported that out of 52 cowpea accessions screened based on GI, only two accessions viz
., Pusa Komal and IC 328859 were resistant to C. chinensis.
Fig 1: Moderately resistant cowpea accession, IC 257844.
Table 1: Reaction of cowpea genotypes to C. chinensis.
In the current study, it was found that none of the accession was found be to immune or resistant to C. chinensis.
The present findings are in conformity with Tripathi et al., (2020),
who screened 103 cowpea accessions based on biological parameters and found that none of the accessions was found to be immune but only two accessions were found moderately resistant to C. maculatus
. Similarly, Sarkar and Bhattacharyya (2015)
also found that none was found resistant to bruchids in greengram genotypes. Globally too, very few cowpea accessions are reported as resistant to bruchids. Screening of more than 15,000 cowpea accessions at International Institute of Tropical Agriculture (IITA), Nigeria, revealed only three land races, TVu11952, TVu11953 and TVu2027 to be moderately resistant to C. maculatus (Srinives et al., 2007).
Present study indicated that cowpea accessions varied in physical seed characters viz
., seed shape, seed texture, seed coat colour, seed length, seed width, seed thickness and 100 seed weight. Based on seed shape, accessions were categorized into ovoid, rhomboid, kidney shaped and globuse. Seed length ranged from 5.1 mm (CoVu702) to 8.0 mm (IC 519762) and width ranged from 4.33 mm (CoVu702) to 6.00 mm (IC 399004). Seed roundedness ranged from 3.22 mm (CoVu702) to 4.7 mm (IC 399004). 100 seed weight (g) ranged from 7.0 g (IC 436897) to 14.2 g (IC 582853). Cowpea genotypes exhibited wide variation in seed coat colour (mild brown, apricot buff, mild grey, white, deep red, black, brown and red). Seed coat texture was smooth for all accessions. This could be one of the reasons for the susceptibility of all accessions to the beetle in the present study, as Mohamed et al., (2019)
reported that cowpea seeds with smooth seed texture were more preferred for egg laying, per cent weight loss and per cent adult survival. Based on correlation analysis, it was found that seed physical characters did not show significant relationship with any of the insect biological characters. This is in conformity with the findings of Tripathi et al., (2020),
who reported that seed physical characters like colour, shape, texture and size had no direct influence on the resistance or susceptibility to bruchids. Therefore, an absolute relationship could not be established. Correlation studies between PSWL and insect biological parameters (Table 2) indicated that PSWL had no significant correlation with them. However, it had significant negative correlation with seed length (r=-0.433), seed width (r=-0.452), seed thickness (r=-0.549) and 100 seed weight (r=-0.668). It indicates that bigger the grain size lesser is the seed weight loss due to bruchid attack.
Table 2: Correlation matrix of growth parameters of C. chinensis and seed physical parameters of cowpea accessions.
To conclude, this study demonstrated that none of the accession showed either immune or resistant reaction to bruchid. However, based on GI, one accession (IC 257844) was found to be moderately resistant (MR). This could be used in breeding programme for development of resistant cowpea cultivars. Future line of works including large number local land races, crop wild relatives that are available in the genebank are to be evaluated in order to find sustainable and durable sources of resistance against bruchids. Further, this study suggests that the tested cowpea accessions could not be stored without appropriate control means for reducing damage and weight loss by bruchids infestation.