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Aphids in Small Grains - June 29, 2009

by Dr. Ian MacRae, U of MN Extension Entomologist

There have been some reports of bird cherry-oat aphids (Figure 1 and Figure 2) in small grains in NW and WC MN over the last week. The populations I've seen are at very low numbers. Add to this, the recent rainy weekend will likely have had a significant impact on those aphid populations, but it's still a good idea to scout for aphids in small grains. The most damaging aphid populations are ones that reach threshold around flag leaf stage, if populations are at or near threshold at this time, delaying treatment until heading may cost you yield.

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Remember, 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 difficult to count (and often messy...), a modified scouting method can be used; if 80% - 85% of stems in a field have aphids, then the average across the field is probably at least 12/stem and maybe 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 usually 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's a later-planted fields (i.e. still in 6-leaf stage) that are up to 2 weeks from fungicide treatment, then there might be a greater potential for yield loss if aphid numbers are already/near 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/stem for 1 day = 20 CAD, 20 aphids/stem 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 at 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, 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...

The Bottom Line - Keep an eye on the populations for now and see what happens, don't treat before threshold, and don't wait for fungicide application at heading if you're at threshold now.

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