Surveys in the region showed very low but relatively wide spread populations of cereal aphids (mostly Bird Cherry Oat aphid). About 50% of plants sampled had aphids but the numbers are low (~2-5/plant). The proportion of plants with aphids is rising and the numbers in the SW MN and South Dakota are high. These are sources for aphids. Added into that, they’re reporting Barley Yellow Dwarf in SW MN. There’s still a lot of mortality factors that can kick in and keep the numbers low. Overall, it’s a good idea to keep an eye on the cereal aphid populations.

The action threshold is when a field reaches 80% - 85% of stems with at least one aphid per stem. Previous thresholds have been set at 12-15 aphids per stem, but as aphids are often tedious to count a modified scouting method can be used; if 80% - 85% of stems in a field have aphids, then the average for the field is at least 12/stem or more.  Sample 100 stems from a field, ensuring you get good representation of the entire field, and calculate the percent infested.  The best time to scout and treat cereal aphids is at the flag leaf stage, but nature doesn’t always cooperate and we can get later season infestations.  Most research indicates that treating for aphids at and after heading doesn’t reliably provide an economic return.  Exceptions have occurred when aphid populations are very high, but in those years, greater returns were seen with earlier treatments (i.e. flag leaf stage).
There are always questions about waiting to treat the aphids until fungicides are applied.  This depends largely on how long the wait is….  If it is a later-planted field (i.e. still in 6-leaf stage) that is up to 2 weeks from fungicide treatment, then there might be a greater potential for yield loss if aphid numbers are already at threshold.

Aphids damage plants by sucking sap, so yield loss depends not only to how many aphids are on the plant, but how long they’ve been there.  Entomologists use the concept of cumulative aphid days (CAD) – 20 aphids on a plant for 1 day = 20 CAD, 20 aphids on a plant for 5 days = 100 CAD, and so on.  The concept was borrowed from heat unit calculations.  Yield loss in cereals from aphid feeding has been estimated to be approximately 0.6 bu/ac/100CAD.  Potential minimum yield loss can then easily be calculated by calculating the average number of aphids per stem and multiplying by the length of the wait.  I say the potential minimum yield loss because if 6-leaf plants are already at threshold, the aphid populations will likely increase over the next 2 weeks.

Total CAD isn’t the whole story, however. The rate of yield loss decreases as the plant matures. There are a number of reasons: physiological changes in the plant and maturation of the grain make it less susceptible to aphid damage; after heading the plant starts to become less suitable as a host and aphid populations start to decline; and, natural mortality factors, such as predators, start to impact the population so that aphid populations generally start to decline within 2 weeks after heading.  From heading on, there usually isn’t enough time to accumulate sufficient aphid days to cause the amount of yield loss that would economically justify an insecticide application.  

Data suggests that the way CAD accumulate also influences the amount of resulting yield loss.  Lower populations of bird-cherry oat aphids that fed over a longer period caused greater yield loss than did higher populations feeding for a short period even though the CAD were about the same.  So, generally speaking: the longer they feed, the more damage they do…