Effect of pubescence in cowpea resistance to the legume pod borer Maruca testulalis (Lepidoptera: Pyralidae)

Abstract

Pubescence (trichomes) in wild and cultivated cowpeas, Vigna vexillata and V. unguiculata, adversely affected oviposition, mobility, and food consumption and utilisation by the legume pod borer, Maruca testulalis (Geyer), in tests conducted with TVnu 72 (wild, highly resistant and highly pubescent), TVu 946 (semi-wild, moderately resistant and pubescent) and IT82D-716 (cultivated, highly susceptible and pubescent). In free-choice and no-choice tests, significantly ( P < 0.05) fewer eggs were laid on 3-week-old potted plants of TVnu 72, the highly pubescent and wild species; this species also received fewer eggs on abaxial and adaxial leaf surfaces. Ovipositional activity on stipules, stem, branches, terminal shoots, adaxial and abaxial leaf surfaces of the susceptible cultivar, IT82D-716 was higher ( P < 0.05) than on TVu 946 and TVnu 72. More eggs were laid on the abaxial surfaces than on the adaxial surfaces of leaves for all tests conducted, except for the TVnu 72 no-choice test. Significant negative correlations ( r = −0.99 ∗∗ and r = −0.99 ∗∗ ) were found between mean number of eggs laid and length and density of non-glandular trichomes. There was a significant negative correlation ( r = −0.99 ∗∗ ) between larval velocity and length of non-glandular trichomes on pod surface. Similarly, significant negative correlations ( r = −0.85 ∗ and −0.89 ∗ ) were found between larval velocity and density and length of non-glandular trichomes on leaf surface. Pods of IT82D-716 and TVnu 72 were the most — and least — preferred for settling by larvae of M. testulalis, respectively. There was a significant positive correlation ( r = 0.99 ∗∗ ) between larval penetration time on pods and length of non-glandular trichomes. Weight loss, number of feeding punctures and number of larvae on whole pods of TVnu 72 were lower and significantly different ( P < 0.05) from IT82D-716 and TVu 946. This study showed that it would be advantageous to use TVnu 72 in a breeding programme to incorporate pubescence into high-yielding commercial cultivars for resistance against M. testulalis and possibly other major pests.