Use of micro-CAT scans to understand cowpea seed resistance to Callosobruchus maculatus

The cowpea bruchid, Callosobruchus maculatus (F.) (Coleoptera: Chrysomelidae: Bruchini), is a worldwide pest of stored cowpea grain [Vigna unguiculata (L.) Walp. (Leguminosae)], causing tens of millions of dollars' worth of damage annually. One method of managing this pest involves planting cultivars whose seeds are resistant to bruchids. Despite extensive research, we still do not know the mechanism by which TVu 2027, the original resistant cowpea line, resists C. maculatus. Using micro-CAT (computerized axial tomography) scan imaging, we visualized the feeding patterns of bruchids living and growing within cowpea seeds. We present evidence that an interior zone in TVu 2027 seeds is responsible for the high mortality and developmental delays experienced by avirulent larval bruchids. We observed that both virulent as well as avirulent bruchid strains have different feeding patterns in TVu 2027 seeds compared to susceptible seeds. It appears that the resistance factor is most concentrated in a zone in each cotyledon adjacent to the air space separating the two seed halves.