This tiny (8mm) little friend was making him/herself at home on one of my baby sunflowers, and unfortunately had to be (humanely) relocated to other green stuff a distance away.
Southern armyworm caterpillars are blackish green to green with a uniform light brown or orange head; otherwise they are quite variable in appearance. There is often a narrow white dorsal stripe on older larvae, and a broad yellow lateral stripe on either side, interrupted by the red spot you can see above on the first abdominal segment. The adults are 1cm long, mottled brown moths.
Like many caterpillars, southern armyworms will eat just about everything, including humans’ favorite fruits and vegetables, and my sunflowers. Wikipedia describes this species as “one of the most important defoliators [in the western hemisphere]”, often skeletonizing leaves as they feed in gregarious groups (hence the name “armyworm”). As the larvae grow, they become more and more solitary.