Pod wall trichomes and resistance of two wild cowpea, Vigna vexillata, accessions to Maruca testualis (Geyer) (Lepidoptera: Pyralidae) and Clavigralla tomentosicollis Stål (Hemiptera: Coreidae)

Abstract

AbstractExperiments were conducted to investigate the role of pubescence in the resistance of two wild cowpea varieties, TVNu 72 and TVNu 73, to feeding and damage caused by two cowpea pests, Maruca testulalis (Geyer) and Clavigralla tomentosicollis Stål. The fine structure of the trichomes was also studied by electron microscopy in order to try to determine the role played by trichomes in the resistance of these varieties to both pests. Feeding and development were deterred in both insects on pods of TVNu 72 and TVNu 73 with or without trichomes compared to the susceptible variety, IT84E-124. In M. testulalis, the same effect was observed on seeds and flowers. However, M. testulalis larvae fed and developed better, as indicated by the higher growth index values, when trichomes were removed. Growth index estimates showed a 13-fold decrease when trichomes were left intact on both TVNu 72 and TVNu 73 compared to the cultivated susceptible control. It was concluded that resistance to M. testulalis was based on trichomes in the first instance (as a first line of defence) and phytochemicals. Survival of C. tomentosicollis nymphs was not affected by trichome removal even though adult feeding damage was slightly increased. This is explained in line with previous reports which indicate that phytochemicals may be the principal factor in the resistance of TVNu 72 and TVNu 73 to this species. Glandular and non-glandular trichomes were found to be present on both the cultivated and wild cowpea. Trichomes in the two types of cowpea differ significantly only in trichome number (susceptible cultivated cowpeas have more) and non-glandular trichome length (those on wild cowpea are ca 20 times longer). Trichome length and angle to pod surface seemed to be more important than density per se.