The most critical issues we face in managing cereal aphids currently is the lack of local knowledge about the likely impact of infestations on yield and quality (the damage thresholds). (http://thebeatsheet-ipmnews.blogspot.com/2008/09/cereal-aphids-in-wheat-and-barley.html).
In this article, I will not go over aphid basics i.e. identification and sampling. You can follow the links to previous articles to read about these
In this posting, I want to discuss what is known, from overseas research, and what we might draw on from this work to help us make decisions about aphid management and control. This review may provide some useful information, in the absence of any locally generated data on aphid impacts. Surprisingly, there has been very little work done on cereal aphids in Australia.
General points
The literature, largely from North America and Europe, indicates that there can be significant differences in the way different cultivars respond to the impact of aphids. For this reason, it is important to use this information as general information that may assist in understanding how your crop may be responding to an aphid infestation. In the absence of local data, it is a useful starting point.
Early aphid infestations (from seedling)
Root and shoot growth may be impaired as a result of aphids competing for N. Inadequate N for the crop may make the crop more vulnerable to the impact of an aphid infestation.There is no impact on yield after grain has filled and is maturing (soft-hard dough).
Aphids have a requirement for nitrogen (N) to complete development and reproduce. Honeydew is a by-product of their feeding. Essentially aphids compete with the plant for available N, which can impact on the plant in at different stages of crop development.
Prolonged infestation can reduce tillering and result in earlier leaf senescence. Controlling aphids generally results in a recovery of the rate of root and shoot development, but there can be a delay.
Late aphid infestations
Infestations that occur during booting to milky dough, particularly where aphids are colonising the flag leaf, stem and ear, result in yield loss. Generally, the distal grains in the head fail to fill. Infestations at this stage in which aphids colonise the leaves, particularly lower in the canopy, tend to result in grain with reduced N (protein) rather than a loss in yield. Aphids are intercepting the N being relocated from leaves to the filling grain.
The relative impact of timing and location of infestation makes sense if you review it along with what is known about the contribution of different parts of the crop to yield. The figure below illustrates the contribution of the upper leaves, stem and ear to the yield of wheat and barley (GRDC Winter Cereal Crop Growth Guide 2005
Ongoing research
There is currently research being conducted on cereal aphids, by QPIF and the Northern Grower Alliance (NGA).
In 2008, initial trial work by QPIF and NGA showed different results (see the GRDC Update, Goondiwindi, 2009 papers for NGA results. Briefly, NGA trial work showed an overall yield benefit of around 10% from using seed dressings containing imidacloprid. QPIF results showed no difference from seed treatment, but a yield benefit where a foliar treatment (pirimicarb) was applied at head emergence.