SEM imaging combined with SEM-EDS analysis enables prediction of resistance of sorghum to storage insects

In developing tropical countries, 30-40% of stored grain can be destroyed by insects.  An important insect pest of stored sorghum (Sorghum bicolor (L.) Moench) is the maize weevil (Sitophilus zeamais Motschulsky).  Weevil larvae and adults feed on the sorghum kernels in the field and in storage.  To compare the rate of weevil damage for different varieties, weevils were placed in vials with sorghum kernels and checked every 3 weeks for 105 days for kernel damage and weight loss.  Seven varieties of these kernels were dehydrated, razor-sectioned, treated with iodine vapor, and carbon-coated for observation using a JEOL 6400 scanning electron microscope (SEM).  A sorghum variety developed in Mali (Seguifa) was sectioned, dehydrated, and embedded in resin.  The resin coated kernel was sectioned with an ultramicrotome below the razor-sectioning.  Improved resolution and sample integrity were noted for the embedded sample compared to the razor-sectioned samples. The microtomed block face was also vapor treated with iodine and carbon-coated.  Cut surfaces of all eight sorghum samples were observed with secondary and backscatter SEM and analyzed by energy dispersive spectrometry (EDS).   Because iodine binds to starch, the starch layer (aleurone) could be identified using backscatter and EDS spectra so measurements could be made from seed coat to aleurone layer.  The greater distance from the seed coat to aleurone layer was correlated to a lessened weight loss. This procedure allows a prediction of the resistance of newly developed sorghum varieties to weevil predation without incurring the delay of 105 days for the standard test trials.