Normalized yield (i.e., yield with injury divided by yield without injury) of cotton, Gossypium hirsutum L., was observed for various levels of plant injury by Helicoverpa zea (Boddie) or Heliothis virescens (F.) at different plant physiological ages (i.e., stages of plant growth). These data were then used to develop functions describing response surfaces composed of insect injury, cotton yield, and plant physiological age. Normalized yield can be predicted from response surface functions when injury and plant physiological age are known. Plant physiological age was either degree-days, or an index of plant development determined using a mechanistic, physiological cotton plant simulation model. The generalized function (i.e., regression equation) describing response surfaces was In Y = b1 ln X + b2 X + b3 X2 + b4 X3 + b5 T + b6 T2 + b7 T3 + b8 XT, where Y = normalized yield, X = injured reproductive organs, and T = plant physiological age (i.e., as degree-days or plant physiological age index). Regressions were highly significant and fit the data well based on coefficients of multiple correlation, mean square errors, Mallows' Cps, predicted residual error sum of squares, and slopes significantly different from zero for parameters of independent variables. The yield response of cotton to injury changed continuously over physiological age. One response surface function quantified an infinite set of two-dimensional insect injury–plant yield response curves. Different sections of the generalized insect injury–plant yield response curve were observed at different plant physiological ages. Cotton yield was increasingly sensitive to injury from one-third-grown flower bud to first week after anthesis, was decreasingly sensitive to injury from first week after anthesis until 30 d after anthesis, and was unaffected by injury of neonates at 30 d after anthesis. Response surface regression functions were significantly different among cultivar–insect species combinations within degree-days or the plant physiological age index, indicating that cotton responded differently to bollworm and budworm injury and cotton cultivars responded differently to injury. For a given set of economic costs and returns, the economic injury level (ElL) is not static nor a straight line, but is dynamic, varying with plant physiological age and forming a complex continuous curve. Overcompensation increased EILs, steep descents in the response surface resulted in low EILs, and tolerance or inherent impunity rapidly increased EILs.