By Jeff Bradshaw
Extension Entomology Specialist, Panhandle Research & Extension Center
The Russian wheat aphid, Diuraphis noxia, is a pest of wheat worldwide; however, the last time Nebraska had a serious outbreak of this pest was probably in the late 1980s, after it was introduced to the United States in 1986.
However, as of last June, some fields in Kimball County reported heavy infestations of Russian wheat aphids in their fields and some required treatment.
Additionally, we also noticed the patchy presence of these aphids in our wheat at the Panhandle Research and Extension Center (PHREC) in late April 2021.
This year we again observed a few scattered symptoms in our wheat plots at PHREC.
Although I am not aware of any major infestations this year, I think it is prudent to let you know about this pest if you are a wheat grower in the area. Also, given all the challenges that seem to be happening in wheat so far this year that are beyond our control, I would like to present at least one potential problem that you can handle: the Russian wheat aphid.
Russian wheat aphids do not transmit plant viruses
We often refer to aphids collectively in wheat as “grain aphids.” For aphids such as the English grain aphid, cherry aphid, and corn aphid, virus transmission is usually the biggest concern.
Winter wheat can be at high risk for viruses (such as barley yellow dwarf) when aphids move to wheat after planting in the fall. This risk can be particularly high for fields planted early. That is, early planted winter wheat may receive aphids from hosts that over-summer and may also harbor wheat viruses. Viruses can be transmitted in the spring; however, older plants are thought to have a greater tolerance to virus infection.
Russian wheat aphids can cause direct damage to wheat
A few cereal aphids, such as the Russian wheat aphid and the green chinch bug, more often cause direct damage through their sap-sucking activities alone and their numbers can increase rapidly to cause significant economic damage to wheat. For example, the Russian wheat aphid can live 60 to 80 days and produce up to 80 offspring when the ambient temperature is 68°F. Additionally, a Russian wheat aphid reaches reproductive maturity in about 8 days. This should give you an idea of how quickly a Russian wheat aphid population can grow!
The impact on the yield of these aphids is approximately 0.5% for 1% of infested tillers, from tillering to flowering. Additionally, these yield impacts can be exacerbated by dry conditions.
Symptoms of infected tillers are very visible (Figure 2). As the aphid colony grows and the aphids feed, the edges of the leaves curl up and enclose the aphid colony in a protected tubular leaf.
Once a colony is enveloped in this protective structure, natural enemies and insecticides can no longer reach it.
Additionally, infestations that develop on flag leaves and roll them up can trap the awns of the emerging wheat ear, resulting in poor pollination. These curved heads can look a lot like 2, 4-D wounds.
The situation in Nebraska
In western Nebraska, we began to find Russian wheat aphids again in April of this year. Currently, aphid numbers do not appear to be high enough in western Nebraska to warrant treatment. However, cooler temperatures extend the longevity of these aphids, so conditions may be adequate for populations to grow slowly and then burst once temperatures warm. Watch your wheat fields closely from now until June for the development of aphid populations!
Testing for Russian wheat aphids
Spring thresholds for Russian wheat aphids have been found to vary with plant stage, but generally range from 5-20% infested tillers. However, for fields where a marginal yield is expected (less than 40 bushels/acre), you may want to consider a higher threshold as a greater amount of damage would be required to justify treatment costs.
To sample, select 20 sample areas of a field at random and carefully inspect five tillers at each of these sites for aphids. Use a total of at least 100 tillers per field (5 tillers per sampling area) to estimate infestation in a field.
Control of Russian wheat aphids
It is important to consider the population of natural enemies, as the timing or inappropriate use of certain chemicals can reduce the number of natural enemies disproportionately to the pest and delay biological control activity. I won’t go into details here; however, there are several parasitoids and predators that will attack and keep aphid populations regulated in our wheat fields.
They offer the most profitable management action, so careful consideration of the above threshold is warranted.
Since these aphid populations tend to establish themselves first at the edges of fields, if detected early enough, wheat edge applications of insecticides may be sufficient to reduce Russian wheat aphids to a low. level below economic importance. Such treatment strategies can maintain biological control and suppress an aphid outbreak while saving on input costs.
For more information on thresholds and processing options, see https://cropwatch.unl.edu/2022/scout-your-wheat-russian-wheat-aphids