Given the high moth catches in pheromone traps, particularly in the West Tennessee counties bordering Kentucky, there are certainly some non-Bt corn fields that are (or will be) infested with damaging populations of southwestern corn borer (SWCB).  You will sometimes hear not to worry about first generation infestations, and it is the second generation that does the most damage.  This is kind of true only because the second generation is typically larger and more spread out.  However, first generation infestations of SWCB in whorl stage corn can be even more impactful on yield. 

The treatment threshold for SWCB is when 20% or more of plants are infested, and honestly, I think 15% is a better number.  Scouting and treatment for SWCB is much easier during the whorl stage.  I do not scout for SWCB eggs during the whorl stage.  It takes too much time, and larvae are much easier to detect.  Larvae feed in whorls almost exclusively until they begin to tunnel into stalks 7-10 days after hatching.  Leaf feeding will be apparent as whorl leaves emerge.  Typical signs include elongate or irregular holes on leaves and a windowpaning effect (pictured below).  The whorls of damaged plants can be pulled out and unrolled to confirm the presence of larvae (also pictured below).  Small SWCB larvae are white to pale yellow, have a dark head capsule, and rows of dark spots that get more conspicuous as they grow.  Frankly, you can just walk through fields and move on if you are not seeing any signs of feeding on emerging whorl leaves.

Fall armyworm or corn earworm larvae may also feed in whorls.  If in doubt, get some help to make sure you can tell the difference.

Insecticide applications, if justified, should be made before most larvae have begun tunneling or they will be much less effective.  It is possible that two applications will be needed if infestations persist.  My preferred treatment in whorl stage corn is mid-range or higher rates of pyrethroid insecticides such as Asana XL, Baythroid XL, Declare, Karate, Mustang Max or one of the bifenthrin products (link here for a list of recommended products and rates treatments).  The pyrethroids typically work well in whorl stage corn and are economical.  In contrast, products such as Intrepid, Belt SC or Prevathon are preferred treatments for infestations in tasseling corn.

Do you have a problem of SWCB on your farm?  I could give you a pretty good guess based on pheromone traps catches on your farm.  Otherwise, it requires scouting.