Susceptibility of pigeonpea and some of its wild relatives to predation by Helicoverpa armigera : implications for breeding resistant cultivars

Helicoverpa armigera (Hübner), the podborer, is a pest of considerable economic importance to crop production across Asia and Australia. The larvae cause significant damage to many crops including the cultivated pigeonpea, Cajanus cajan . We have conducted oviposition and pod feeding-damage studies with H. armigera on 5 varieties of C. cajan , 6 other Cajanus species, and Rhynchosia bracteata . The species could be divided into podborer-resistant [ C. scarabaeoides (ICPW 83), C. cajanifolius , C. sericeus , C. albicans , and C. platycarpus ], moderately resistant [ Rhynchosia bracteata and some varieties of C. cajan ], or susceptible [ C. cajan (ICPL 87)]. At 100 μg/g a methanol extract of the susceptible cultivar of Cajanus (ICPL 87) stimulated significantly more oviposition than a similar extract of the resistant species C. scarabaeoides , ICPW 83. Pod surface extracts of both ICPW 83 and ICPL 87 contained isoquercitrin, quercetin, and quercetin-3-methyl ether, although they were present at much lower concentrations in ICPW 83. A fourth compound, 3-hydroxy-4-prenyl-5-methoxystilbene-2-carboxylic acid (stilbene), was detected in a methanol extract of the pod surfaces of ICPL 87. A methanol extract of ICPL 87, from which the stilbene had been removed, stimulated oviposition when paired with the whole extract. These results are discussed in relation to the selection of varieties with morphological and chemical characters that can be used for developing cultivars less susceptible to podborer.