As we move into October, the question of aphids and barley yellow dwarf virus (BYDV) comes back into focus yet again.
This is a risk that farmers cannot afford to take as bad BYDV infection could take up to 1.2t/ac off yield potential (and even more) over the coming months.
And that could start from as few as one infected aphid.
Aphid multiplication is heavily driven by temperature in the presence of suitable food sources.
Warmer conditions speed up multiplication and they may well also enhance multiplication rates. For this reason, mild weather is associated with aphid multiplication and spread.
Understanding aphid reproduction
Aphid reproduction is much more complex than with many other species. They can produce their offspring as clones, which are exact replicas of the parent and if that individual happens to be resistant to an insecticide, then so are all its offspring.
These offspring are born live and ready to feed, so this is a very rapid process and the young are easily visible on plants.
They can also lay eggs but this reproduction process takes longer to result in offspring that are ready to feed. Both methods can produce winged or wingless aphids.
Winged aphids are the major concern initially because these are the ones that can bring infection into a field and transmit infection from field to field.
Aphids largely multiply on plant leaves by producing the wingless form, but when the density of insects gets too high in any area, two things seem to happen.
Firstly, some aphids fall off that leaf to the ground and move to feed on a neighbouring plant. That process continues and the single infected plant becomes an expanding circle if left unchecked.
The second thing that seems to happen when numbers get very high is that a proportion of the progeny are produced with wings and these feed for a while, get infected with the virus and then fly away to be carried in the wind to another location. This is the process which science and research is attempting to disrupt, prevent or monitor more accurately.
The good summer weather and unseasonably warm September add to the significance of these concerns.
The September 2021 weather summary from Met Éireann states that all tillage areas were between 1.5°C and 2.3°C above normal for the monthly average, with maximum daily temperature only being below 15°C for the last four days of the month. This must signify the probability of significant aphid multiplication for the time being.
This is being borne out by farmers who report significant aphid numbers in catch crops around the country and if they are there, they are also probably present on volunteer cereal plants in these and other stubble fields. If these plants carry the virus, then they act as a potential source of infection for newly emerging crops.
Because of the risk, farmers must spray to protect their crops even though aphid numbers will be very low.
While the need for crop protection is great, there is an increasing desire among many farmers to not spray aphicide if they can be assured that the risk of infection is low or zero.
Work at Teagasc is attempting to assess the risk of infection more accurately by collecting flying insects in new suction towers.
They then look to see if the species are present that can carry BYDV infection on to cereals. If these specific aphids are present, they are then examined to see if they are carrying the virus.
If there are none of the aphid species present that can transmit BYDV then, in theory, the risk is low to zero.
If these are present then the risk depends on whether they are carrying BYDV or not. If they are, it is a numbers game; if they are not then the risk is low to zero.
The unfortunate thing is that the research is not yet sufficiently evaluated to give this advice this year. However, we must also acknowledge that a high-pressure year is needed to help evaluate the risk forecasting system that is being researched.
Assuming the system is found to work well at farm level, farmers can be advised of the times that BYDV-infected aphids are on the move and this information should guide the need for, and timing of, aphicide application.
If infection takes place, then the risk and timing of ongoing infection and internal field spread can be gauged using devices like Syngenta’s aphicide timing app, which estimates the multiplication rate in a field based on geography and weather.
Such devices estimate the point at which aphids on an infected plant are likely to spread from one plant to increase the size of a patch. This is a very different tool but it could be complementary to the suction towers.
While farmers will look forward to improved accuracy in the advice on the need to apply aphicides or not, the fact is that this advice will not be available for another year or two. So, in the meantime, they must respond to the likely pressures that this year will bring.
Based on the summary of the information available, we must assume at least a modest risk of infection unless it becomes very wet or very windy from here on, as these conditions act to prevent aphid flight.
The three big risk factors for BYDV infection are:
Taking these three factors into consideration, the infection risk can extend to later drilling dates in milder autumns like this one.
Higher soil temperatures help get crops out of the ground faster and so be more subject to infection risk. We have some crops already emerged and these are automatically exposed to high risk and likely to require two aphicide treatments.
The other big unknown is the level of total resistance that exists in a population. It would seem that most populations still only carry partial knockdown resistance (kdr) and those aphids can still be killed with full rates of pyrethroid insecticides.
But there are some populations that have additional resistance mechanisms and these would need to be tackled with insecticides such as Transform or Teppeki, which are different to one another and to the pyrethroids.
In the event of having to spray a second time, always use a different product with a different mode of action. The presence of young, cloned aphids on a leaf shortly after treatment would indicate a risk of the presence of resistance to the product applied. Always seek advice if you find this.
Crops that are emerged now might need to be sprayed around the two- to three-leaf stage to slow the movement of any initial infection.
Remember, the aphids will most likely have fed before they are killed so they are still likely to have infected a single plant before they die.
If there are a lot of infected aphids flying in, there could still be quite a number of yellow leaves and plants despite achieving excellent control. That said, always check sprayed fields shortly after application to check on the job that your insecticide did.
The early-sown crops that have emerged by early October may need a second treatment depending on how the year progresses. Even if the first spray does its job well, where a second spray is needed use an active from a different insecticide family.
Crops sown in early October may require a second aphicide after being sprayed in late October/early November but this will depend heavily on the weather conditions from now on.
A simple guide to these possible timings is shown in Table 1.
Many growers will have sown KWS Joyau winter barley early because of its claimed tolerance to BYDV infection. We are told that it is tolerant to the virus and that infection does not result in an impact on yield. This may well be the case but it has yet to be proven here.
If Joyau is found to help minimise infection risk in early sown crops, we also need to know if it could still harbour virus and act as a point of infection for other crops.
While it is possible that the variety will cope well, even where it is infected, how does one deal with the aphids that may be in these crops? If plants are carrying virus, aphids feeding in that crop can still take in, carry and transmit BYDV elsewhere.
If aphids can multiply on the variety, numbers are still likely to reach a level that winged aphids will be produced. So we need to know if a crop of Joyau can act as a transmission hub for the transfer of virus to other varieties and crops, even in spring?
If the variety was resistant, rather than tolerant, this might not be an issue because the virus would not propagate within the plant. But does tolerance enable the virus to exist as a source for further transmission and infection? Perhaps not, but we need to be conscious of the risks for the time being.
This is a serious question that needs to be answered before we can put too much faith in what we think the variety can offer us. Might Joyau still need to be sprayed to prevent potentially virus-infected aphids from exiting that crop and moving to infect others?