BYDV control – to spray or not to spray?
Teagasc Crops Specialist, Shay Phelan delves into the advice and research relating to BYDV control over the next couple of weeks, asking growers will they apply an insecticide on current crops or not?
Almost all winter cereals have been drilled at this stage, with many having already received a pre-emergence herbicide. Many growers will expect that they are finished with these crops until next spring.
However, based on the evidence of the harvest gone, Barley Yellow Dwarf Virus (BYDV) did play an important factor in some of the poor yields, particularly in winter barley crops this year. So, the question is do we apply an insecticide on the current crops or not?
While this may seem a simple question, the answer is not so clear. There are many farmers who do not like applying an insecticide and haven’t done so in years. Then there are others who do not like the risk of not applying them in case they get a bad infection like we saw in 2026.
The region (i.e. coastal regions are more at risk), sowing date, variety and the weather all contribute to the levels of infections experienced in any given year.
Milder autumns are certainly allowing aphids to survive and move longer into the winter, therefore increasing the risk. While predators can play a role in keeping the number of aphids carrying the virus down, they are only useful if they prevent the infected aphid from feeding on the leaf.
Infected aphids carry the virus on their mouth parts and, as soon as they begin to feed, the virus is transmitted to the plant. Once in the plant, it cannot be treated.
Aphicides by their nature will kill all aphid species and many beneficials, so they are not as targeted as we would like.
Many aphid species do not carry any virus; even the ones that do, may carry strains that cause very small yield losses.
Aphicides only work in the field where they are applied, but aphids can fly in from anywhere in the environment. They are constantly moving until the weather becomes colder and days get shorter, then they stop flying and only move by walking.
Control
Yield losses are impossible to predict at this time of year. However, where yield losses are observed they can be substantial.
Work conducted by Tom Kennedy in the 1990’s and 2000’s suggested that the best time to control aphids in the autumn was early November at the 2-3 leaf stage, for October planted crops.
This usually coincided with a drop in temperature, which slowed down movement and therefore helped control the spread of the virus. Recent work carried out by Dr. Louise McNamara suggests that a single application of an insecticide for this circumstance is still the best way of spreading the virus in a susceptible variety.
However, with the increased availability of tolerant varieties and the trend to earlier drilling, does this advice still hold?
Recently published work by Dr. Louise McNamara, looking at the amount of virus in both susceptible and tolerant varieties, shows that while both will still show signs of the virus, the tolerant ones will suffer less of a yield penalty.
September-drilled crops will have potentially more virus than October planted ones and remember wheat also can get BYDV as well as barley.
Dr. Louise McNamara has also shown that a January applied aphicide can perform as well as a November treatment, she has also seen no significant increase in pyrethroid resistance in the aphid population.
Figure 1 below shows a trial carried out as part of a wider Teagasc evaluation of BYDV control systems in 2024. The picture was taken in a field that had not received any insecticides in previous years. One tramline on the left of the photo was untreated, while the two tramlines on the right-hand side of it were treated depending on the advice of various IPM prediction tools, one was sprayed in November while the other was sprayed in January. There was no difference between the sprayed treatments, however there was a difference of 1.2t/ha over the untreated plots.
Figure 1: Treated vs untreated tramline
So, while the advice may well sound simple regarding applying an aphicide to control BYDV, it is actually much more complicated. Variety, sowing date, location, weather and most importantly the farmer’s attitude to risk will all play a part in making a decision whether you apply an insecticide or not. We will only know which decision is correct next April.