Abstract
Phaseolus vulgaris is grown as a staple food source for humans and animals throughout the tropics and subtropics. Post-harvest losses due to Acanthoscelides obtectus and Zabrotes subfasciatus can be extensive, and breeders are looking for resistance in wild accessions which can be bred into commercial varieties. The bases of resistance to the two pest species are different, and the bases and mechanisms of resistance to Z. subfasciatus are discussed. Resistance to Z. subfasciatus is associated with the presence of arcelins, and concurrent absence of phaseolin. Arcelin 4 is found to be highly resistant to proteolysis by larval enzymes in vitro, and supplementation of meal from resistant varieties with phaseolin overcomes some of the antimetabolic effects; therefore the unavailability of essential amino-acids is proposed as a mechanism of resistance. A novel inhibitor, which inhibits Z. subfasciatus larval α-amylase, is also found in some resistant lines and is identified as being distinct from arcelin. A scheme is proposed to enable the resistance of a seed to attack by Z. subfasciatus to be predicted from the seed composition, using a formula which combines arcelin: phaseolin ratio with insect α-amylase inhibition, or the presence of arcelin 2 subunits.
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