The greatest risk of yield loss from aphids feeding on grains is in the vegetative to boot stages. Significant yield reductions after the onset of flowering could not be demonstrated in research published from South Dakota in 1997 (Voss et al., 1997. Journal of Economic Entomology 90: 1346-1350).
Reasons for these conclusions were that: after heading the only major yield component aphids can affect is seed weight; aphids are unable to sustain the very large populations necessary to achieve significant impact on this factor.
Other components of yield are determined earlier (number of spikelets - determined at jointing; number of seeds - determined at flowering). During flowering, if wheat is stressed with drought (or other stress) and >85% of stems are infested with cereal aphids, we could see some test weight reduction. Observe PHI intervals if any insecticides are applied late in crop development. Two insecticides that have a short PHI are Malathion (7 day PHI) and Mustang Max (14 day PHI) in wheat. Other insecticides registered in wheat or barley have a 28, 30 or 45 day PHI. Please see the 2012 North Dakota Field Crop Insect Management Guide for more information: http://www.ag.ndsu.edu/pubs/plantsci/pests/e1143w1.htm