ECONOMIC INJURY LEVELS AND DEVELOPMENT AND APPLICATION OF RESPONSE SURFACES RELATING INSECT INJURY, NORMALIZED YIELD, AND PLANT PHYSIOLOGICAL AGE

Normalized yield (i.e., yield with injury divided by yield without inj ury) 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 stages of plant growth). Thes e data were then used to develop functions describing response surface s composed of insect injury, cotton yield, and plant physiological age . Normalized yield can be predicted from response surface functions wh en injury and plant physiological age are known. Plant physiological a ge 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 ln 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 orga ns, and T = plant physiological age (i.e., as degree-days or plant phy siological 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 slo pes significantly different from zero for parameters of independent va riables. The yield response of cotton to injury changed continuously o ver physiological age. One response surface function quantified an inf inite set of two-dimensional insect injury-plant yield response curves . Different sections of the generalized insect injury-plant yield resp onse curve were observed at different plant physiological ages. Cotton yield was increasingly sensitive to injury from one-third-grown flowe r bud to first week after anthesis, was decreasingly sensitive to inju ry from first week after anthesis until 30 d after anthesis, and was u naffected by injury of neonates at 30 d after anthesis. Response surfa ce regression functions were significantly different among cultivar-in sect species combinations within degree-days or the plant physiologica l age index, indicating that cotton responded differently to bollworm and budworm injury and cotton cultivars responded differently to injur y. For a given set of economic costs and returns, the economic injury level (EIL) 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 surfac e resulted in low EILs, and tolerance or inherent impunity rapidly inc reased EILs.