Does varying cowpea spacing provide better protection against cowpea pests?

Insect pest incidence, abundance, damage and grain yield of four cowpea pests: aphids, Aphis craccivora Koch, bean flower thrips, Megalurothrips sjostedti (Trybom), legume pod borer, Maruca vitrata (Fabricius) and the pod-sucking bug (PSB) complex were measured across a range of plant spacing using two cowpea varieties, a breeding line (IT86D-715) which is semi-erect and a popular spreading cultivar grown in the sahel [Mousa Local]. The Nelder fan design was used to produce varying plant spacings (from 0.55 to 1.50 m) in a continuum. Severity of aphid infestation increased with increase in plant spacing, but severity of thrips damage to floral buds decreased with increase in plant spacing up to 1.2 m. Pod damage by M. vitrata was higher on IT86D-715 at closer spacing. The incidence and damage by PSBs in both varieties did not vary significantly (P<0.95, df=15 and P<0.18, df=15, respectively) with variation in plant spacing, but their damage contributed to the reduction of total grain yield. Pod yield increased with fewer insects as plant spacing increased from 0.8 to 1.5 m. Optimum plant spacing was different for the four insect pests. Aphids were less of a problem when plant spacing was less than 0.8 m, whereas a spacing of 1.0-1.5 reduced flower thrips and the pod borer.