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CAT Scan

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Cowpea Seed Resistance to Callosobruchus maculatus: A New Look

It is hard to describe the behavior of insects living within seeds because most of their life cycle occurs out of sight. However, by means of Computed Axial Tomography (CAT) scan machines we were able to visualize the behavior and feeding patterns of insects living within seeds.

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Photo of a cowpea bruchid among cowpea seeds

Photo by Matt Tarver

The cowpea bruchid, Callosobruchus maculatus (F.) (Chrysomelidae: Bruchini), is a worldwide pest of stored cowpea grain (Vigna unguiculata (L.) (Walp.), annually causing tens of millions of dollars of damage. 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 scan imaging we visualized 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 airspace separating the two seed halves.

Figure A: Line drawing of the sections of a typical cowpea seed, with the left being a side view and the right a cross view. Figure B: Micro-CAT scan image of a cowpea seed with a bruchid larva evident in the cotyledon.

Micro-CAT scans comparing susceptible and resistant cowpea seeds