Accurate prediction of yield loss caused by cereal aphids in small grains involves assessment of the aphid population density on plants, the duration of their feeding, and the growth stage of the crop at the time of feeding. In this research, aphid-day (one aphid feeding on one plant for 24 h) unitage was used as a standard to compare the effects of feeding by greenbug, Schizaphis graminum (Rondani), Russian wheat aphid (RWA), Diuraphis noxia (Mordvilko), and bird cherry-oat aphid (BCO), Rhopalosiphum padi (L.), on the growth and yield of spring wheat, Triticum aestivum L., to measure the degree of additivity of yield loss caused by aphid feeding on the same plants at several stages of plant growth, and to evaluate the adequacy of the aphid-day standard as a predictor of yield loss. Results of greenhouse experiments with a scaled series of aphid-day components (150 aphids ✕ 2 d; 75 ✕ 4 d; 50 ✕ 6 d; 25 ✕ 12 d) of equal total value (300 aphid-days) showed that the three aphid species were similarly damaging to yield but that for RWA and BCO the 25 ✕ 12 d combination was significantly (P ≤ 0.01) more damaging than the other aphid-day treatments for those species. The administration of escalating aphid-day dosages to plants concomitant with advancing plant growth stages (two-leaf, 300 aphid-days; four-leaf, 400 aphid-days; two-leaf + four-leaf, 700 aphid-days; boot, 1200 aphid-days; two-leaf + boot, 1500 aphid-days; milk, 200 aphid-days; two-leaf + milk, 2300 aphid-days) revealed that yield loss caused by aphid feeding at two plant growth stages was usually significantly (P ≤ 0.01) greater than that at a single stage, but was not fully additive simply as the arithmetic sum of the yield losses at the two stages. Yield losses due to aphid feeding (expressed as milligrams per 100 aphid-days) declined precipitously with advancing plant growth stage. These results show that the aphid-day unitage, properly interpreted in relation to the aphid species involved and the growth stage of the crop, is a useful predictor of grain yield loss at harvest. Relating cumulative aphid-days at successive plant growth stages to ultimate yield loss enables producers to make informed decisions about the need for and timing of rescue chemical treatments.
Contribution of the USDA-ARS and Dep. of Plant Science, South Dakota State Univ., Brookings, SD 57007.