The challenge of controlling BYDV: where do we go from here?

Barley yellow dwarf is one of the most widespread and damaging viral diseases of grasses and cereal crops in the world. It can cause yield losses ranging from 5% to 80%.

The virus is transmitted by many aphid species including the bird cherry oat aphid, Rhopalosiphum padi, the grain aphid, Sitobion avenae and the rose-grain aphid Metopolophium dirhodum all acting as vectors for the disease.

Visible symptoms of a barley yellow dwarf virus (BYDV) infection on barley include stunted growth and yellow discoloration of leaves.

There are many challenges in the control of this virus.

Research suggests that with climate change, damage caused by aphids and BYDV will increase in cereal crops in the future.

As well as this, there is diminishing availability of chemical insecticides for control of aphids.

Therefore, problems with cereal aphids and BYDV can be expected to increase in the future.

There are many elements to this complex story from the perspective of the pest, the virus and the host plant.

We still have many questions to answer in our goal to improve the integrated pest management of BYDV in Ireland.

Many different species

Experiences with this disease in the field have shown variable symptoms and variable consequences. This begs the following question:

Are there likely to be a range of different strains of the virus that may show similar or different symptom intensity but vary in their impact on grain yield?

The group of viruses causing yellow dwarf disease is diverse and consists of different virus species.

There are several species of both BYDV and Cereal yellow dwarf virus (CYDV), eg BYDV-MAV (vectored by S. avenae), BYDV-PAV (vectored by R. padi, S. avenae and others), BYDV-PAS (vectored by R. maidis and M. dirhodium) and CYDV-RPV (vectored by R. padi). In general, B/CYDV infection causes significant economic losses in cereal crops but viruses do vary in their ability to cause severe disease.

Given that disease severity varies with virus species, it is very important that we develop a greater understanding as to what viral strains are circulating in Irish crops

BYDV-PAV is seen as the most important species worldwide due to its disease severity and the fact that it can be transmitted by a range of different aphid species.

Given that disease severity varies with virus species, it is very important that we develop a greater understanding as to what viral strains are circulating in Irish crops.

For example, BYDV-PAS induces more severe symptoms than BYDV-PAV in spring varieties of oat and barley and mixed infection (when more than one viral species is present) can aggravate the symptoms of the disease.

In order to map the predominant viral species across regions and crops, we have started sampling cereal crops in the main tillage growing regions and “reading” (sequencing) the genetic code of the viruses infecting crops.

This genomic surveillance is allowing us to build up a picture of C/BYDV virus species in Ireland that will ultimately help in the development of better diagnostic tools, decision support systems, and it may help with tolerant/resistance cereal development and variety recommendation.

The aphid vectors

A second question that needs answers relates to the ability of aphid species to transmit the different strains. So:

Is there a difference in infection risk between the grain and bird cherry oat aphids? Are some aphids or clones better or worse in their ability to transmit the virus?

The efficiency of virus transmission depends on both the species of the virus and the aphid vector. This efficiency can vary from 0% to 100% depending on the aphid vector and the BYDV species.

For example, BYDV-MAV is only vectored by S. avenae, whereas BYDV-PAV can be transmitted by both S. avenae and R. padi. Research has also shown that R. padi is a more efficient vector of BYDV than S. avenae.

However, even clones within an aphid species can differ significantly in their ability to transmit BYDV. This makes development of decision support systems extremely challenging, because when we see an aphid in a crop we (i) do not know if it is carrying a virus and, (ii) if it is, we do not know what viral species and how efficiently the aphid can transmit this particular virus.

The issue is made even more complex by the fact that after successful infection of the plant, the plant’s physiology changes, which in turn affects the interaction of that plant with insect vectors. For example, R. padi prefers BYDV-infected wheat plants in comparison to non-infected plants.

Also, there is the question of whether insecticide resistance has an impact on how effectively the aphid can transmit the virus, ie are resistant aphids better or worse at spreading BYDV?

This question forms the basis of ongoing research at Teagasc where we aim to determine how important resistant grain aphids are as a pest.

The complexity of these interactions is shown in Table 1.

Improving our understanding

Against this knowledge background, research at Teagasc is seeking to improve our understanding of the interaction of all these factors and how they impact. So:

Why are we monitoring aphids and how does the suction tower network work?

The virus is transmitted by cereal aphids in a persistent manner. This means the virus remains infectious within the body of the aphid for a long time.

The virus is not transferred from infected aphids to their offspring, meaning the virus can only be acquired from feeding on infected plants.

When no infected plants are available, BYDV cannot persist within the aphid population. So, to survive, the virus has to move multiple times a year and find new hosts.

To achieve this, the virus uses an aphid vector, which ultimately determines how the virus is transmitted and where it is found.

As winter approaches, winged aphids develop and migrate to their primary host plant. For S. avenae, its primary hosts are grass and cereal species, whereas R. padi and M. dirhodum migrate to woody primary hosts.

The woody primary hosts of R. padi and M. dirhodum are not hosts for BYDV, meaning the aphids emigrating from these plants are virus-free.

Virus-free S. avenae aphids have a preference for BYDV-infected host plants and quickly acquire the virus in infected small-grain fields.

Aphids with more than one host are more likely to be captured in suction towers as they need to migrate between hosts. In order to understand the disease, it is crucial that we develop an understanding of the life-cycle and movements of cereal aphids.

We can do this by monitoring both long distance and local aphid movement.

The Teagasc suction tower network consists of three 12.2m suction towers based on the design by Rothamtsted research.

At greater heights (like 12.2m), insect density tends to decrease in general but a higher proportion of the insects caught will be aphids. These traps run all year and generate daily catches of insect flight.

Aphid species that change hosts (like R. padi) tend to have a longer flight season, are found late in the year and have larger annual counts. This is in contrast to S. avenae which does not change hosts.

In addition to identifying and counting aphid numbers, we also carry out PCR-based diagnostics.

In the first instance, aphids are tested to see if they carry mutations in their DNA that confer resistance to insecticides.

We also then carry out PCR tests to determine if the aphid is carrying BYDV and we will expand these tests in the future so that we can determine what species of virus the aphid is carrying, all in the same test.

Verifying tower results

We are now using the suction towers to give us an indication of the aphid species that are in flight at different times and if they are carrying insecticide resistance and which virus species but can we be confident that these results are accurate for within the full footprint of each tower? So:

How do we verify that the towers are indicative of what is actually happening on the ground and in fields? What happens if we find virus in fields where grain aphids were not found to be in flight?

BYDV epidemics may not be predicted by suction trap catches alone as crops can be infected by both primary infection (aphids flying in and landing) and secondary infection (subsequent generations of aphids spreading virus throughout the field).

As part of new Teagasc research, and in collaboration with ADAS and Harper Adams in the UK, we are expanding our monitoring network to include shorter suction towers (2m to 6m) and an in-field network of yellow trapping across major tillage growing areas.

We will use this in-field network to identify when aphids are starting to infest the crop and what proportion are carrying virus and what species of virus they are carrying.

We will then see how this relates to the suction tower data and visual searching for aphids to determine which method, or combination of methods, provides the most practical advice for farmers and which can be used to develop decision support systems for farmers.

As temperatures are expected to increase due to climate change, problems with cereal aphids and BYDV are expected to increase.

In parallel, insecticide availability is reducing, and aphids have the ability to develop resistance to insecticides using several different mechanisms. Therefore, robust and sustainable integrated pest management options are needed. A reliable decision support system could help to accomplish this.

To achieve this though we need to understand what is driving BYDV infection. Research tells us that:

So we have a good understanding as to how these factors influence the proportion and density of cereal aphids but not on the proportion of aphids carrying BYDV. This is where the Teagasc monitoring network and associated diagnostics platform can help. This allows us to test aphids for both their resistance and viral status and ultimately support the cereal sector to reduce insecticide inputs.