Which of the following insects is considered a severe pest in corn and commonly attacks ears?

The fall armyworm is regarded as a severe pest in corn primarily due to its tendency to feed on corn ears, which are critical to the yield of the crop. The larvae of the fall armyworm can cause significant damage as they consume kernels and potentially disrupt the development of the ear, leading to reduced yield and lower quality of the corn.

In contrast, the Mexican bean beetle primarily targets leguminous crops, such as beans, rather than corn. The green peach aphid is known for infesting various crops but does not typically cause the kind of direct damage to corn ears that the fall armyworm does. The scarab beetle, while it has an impact on some crops, primarily affects roots and does not focus on the ear of the corn. Thus, the fall armyworm is uniquely positioned as the severe pest specifically attacking corn ears among the options listed.