Abstract:
Cowpea, Vigna unguiculata, is a popular food crop in the tropics and subtropics. The crop is particularly susceptible to pests and diseases, limiting its yield. Cowpea cultivars that are resistant to pests in the field and in storage are becoming increasingly significant. Five improved cowpea varieties were screened (‘Asontem’, ‘Golinga’, Kirk house, ‘Padi Tuya’ and ‘Wang Kae’) for their susceptibility to field and storage insect pests. The field experiment was laid out in a Randomized Complete Block Design with five treatments and four replicates. The insect pests encountered were identified and recorded. Mean yield parameters such as number of pods per plant, weight of pods per plant, length of pod, number of seeds per pod, number of loci per pod, seed weight per pod, 100-seed weight, and total yield were assessed. In the minor season, ‘Golinga’ recorded the highest number of pods per plant, while ‘Padi Tuya’ had the least number of pods per plant. Meanwhile, ‘Asontem’ recorded the highest yield of 71.7 g in the minor season, while ‘Padi Tuya’ recorded the least value of 60.4 g. In the major season, ‘Wang Kae’ recorded the highest number of pods per plant, while ‘Padi Tuya’ recorded the least number of pods per plant. ‘Asontem’ again recorded the highest total yield of 74.1 g while ‘Padi Tuya’ recorded the least mean total yield of 59.2 g.
Correlation between insect infestations and yield of the varieties in the field was determined. Aphis craccivora infestation was negatively correlated (r = - 0.2368, p > 0.05) with total yield in the major season. Megalurothrips sjostedti infestation was negatively correlated with the total yield (r = - 0.0784, P > 0.005) in the major season, and was also negatively correlated with the total yield (r = - 0.4232, P > 0.01) in the minor season. Maruca vitrata infestation was positively correlated with the total yield (r = 0.0636), P > 0.05) in the major season, while it was negatively correlated with total yield (r = - 0.3767, P >0.01) in the minor season.
The laboratory experiment was laid out in a Completely Randomized Block Design with five treatments and three replicates. There was significant repellence of Callosobruchus maculatus by the different concentrations (1g/mL and 2g/mL) of the five cowpea extracts. All five cowpea varieties recorded C. maculatus mortalities after 4 days of introducing the adult insects onto the seeds. The five cowpea varieties recorded some levels of damage for the six-week period after introduction of C. maculatus. C. maculatus infestations and seed weight during storage were determined after the six-week period. The median development period of the C. maculatus in the five cowpea varieties ranged from 17 to 28 days, with four out of the five varieties (‘Asontem’, ‘Golinga’, ‘Kirkhouse’ and ‘Wang Kae’) classified as moderately resistant to insect infestation, and one (‘Padi Tuya’) classified as susceptible to insect infestation. Qualitative and quantitative phytochemical analysis on the cowpea varieties confirmed the presence of different levels of phenols, flavonoids, tannins, and alkaloids.