Foundation for Arable Research Focus on Cereal Viruses

Yellow Dwarf Virus

YDV, transmitted by aphids is an economically important virus in New Zealand cereals, causing up to 30% yield loss in some years. There are a number of ways to prevent yield loss due YDV; most aim to reduce the risk of aphid colonisation and spread, and often rely on the use of seed treatment and foliar insecticides. As a result, cereal growers can spend around $100/ha, annually, controlling aphids to prevent the spread of the virus. However, alternative approaches, such as Integrated Pest Management (IPM), which combines the use of natural predators and parasitoids with cultural controls and selective chemistry as required, are available.

In 2020-21, as part of the A Lighter Touch programme, FAR put IPM to the test in a trial comparing YDV incidence and severity in wheat crops grown from seed treated with a neonicotinoid (Group4A insecticide) with crops grown from untreated seed. No foliar insecticides were applied to the trial area or surrounding paddock. Results showed no significant differences in yield, thousand seed weight or bulk density between insecticide treated seed and untreated seed treatments.

Monitoring of aphids, beneficial insects and local weather conditions in the trial identified a potential YDV risk period between crop growth stage (GS) 21 and GS 31 (typically between late autumn and mid spring). Despite this risk period, few visual YDV symptoms were observed in the trial post-flowering and no YDV-PAV (the common virulent strain) was detected in randomly collected leaf samples following laboratory testing. This suggests that the threshold for YDV disease expression was likely not reached in the trial between GS 21 and GS 31, regardless of seed treatment, possibly because beneficial insect populations were managing pest populations. More trials are needed to understand seasonal variation in aphid pressure.

BYDV – Mitigating autumn risk

Managing BYDV is not simple. The disease is a moving target. It is linked to aphid pressure, which in turn is determined by weather (especially temperature), the presence of beneficial aphid predators and parasitoids, cultural control methods (time of sowing and green bridge control) and chemistry. One of the biggest challenges for managing BYDV is that aphid pressure is seasonal, meaning that the subsequent impact of BDYV on yield can vary from negligible to catastrophic. Thus, management strategies need to be flexible both within and between seasons.

The most common chemical intervention is the use of an insecticide seed treatment. The main chemical group used for protection against BYDV vectoring aphids is Group 4A, the neonicotinoids. Neonicotinoids include the active ingredients clothanidin and imidacloprid. Typically, an insecticide seed treatment provides protection from planting until the start of tillering at GS 21. Depending on sowing date and temperature, GS 21 can be reached before aphid numbers reduce in late-autumn/early-winter, a situation which may prompt some growers to apply and insecticide. But aphid numbers themselves should not be considered a trigger for insecticide application as there is no aphid threshold. Rather the ratio of aphids to beneficial predators and parasitoids is a more accurate predictor of whether a foliar insecticide is required and which product to apply.

The best way to quantify this autumn risk is through monitoring. A combination of trapping and direct searching can start in the lead up to GS 21 and continue through the winter until around two weeks after GS 39. As part of the A Lighter Touch programme, FAR has established a number of regional paddocks where temperature, growth stage and aphid:beneficial insect ratios are recorded throughout the season and made available to growers and industry through Crop Action (Figure 1). As with fixed aphid suction traps, this monitoring network only provides data from fixed areas, so there are geographical limitations to the information supplied. For this reason, the information should be used as a prompt to encourage growers and industry reps to check their own paddocks.

Insect identification can be challenging, but with a bit of practice and a magnifying glass, growers and industry reps can determine friend from foe. FAR is here to support growers interested in learning more about monitoring, and offers field walks, workshops or help with sticky trap or insect photo identification.

Figure 1. An example of what paddock monitoring can look like. Degree weeks above 5.8°C (the baseline temperature for aphid reproduction), aphid and beneficial insect populations between sowing and two weeks post GS 39 for autumn sown milling wheat cv. Duchess at Pleasant Point in 2020. No foliar insecticides were applied to this crop.

Can we manage aphid pressure without neonicotinoid seed treatments?

Neonicotinoids have been banned in Europe and are under the spotlight in New Zealand, so cannot be relied upon as part of long-term BYDV management strategies. For this reason, in addition to understanding and managing aphid pressure, FAR established a field trial to determine whether aphid pressure could be managed without a neonicotinoid seed treatment.

In Year 1, there were no significant yield, thousand seed weight or test weight differences between bare and insecticide seed treatments. Monitoring data collected for this trial suggested a potential risk period between GS 21 – 31 (Figure 1); however, the yield and quality data suggested the risk of BYDV infection during this period was reduced by the ratio of aphids and beneficial insects (18:8). It is important to note that these results are from one year of trial data only. Additional trials are needed to provide more robust information as aphid pressure varies seasonally.

Upcoming work will continue to investigate whether insect and degree week modelling can allow growers to mitigate risk based on conditions at the paddock level and will include more work on bare seed as a treatment.

Table 1. BYDV severity, BYDV-PAV incidence, yield and quality for autumn sown milling wheat cv. Duchess sown as bare seed or treated seed under dryland conditions at Pleasant Point in 2020-21.

Note. a.i. imidacloprid 600 g/L, Group 4A insecticide) * Visual assessment of 1m² quadrats, ** incidence determined by ELISA (Enzyme Linked Immunosorbent Assay) by Plant Diagnostics Ltd. TSW: thousand seed weight.

Also included is a link to a recent FAR podcast (3 June 2021) featuring Jo Drummond discussing the BYDV component of A Lighter Touch can be found at www.cutthecrop.co.nz

Trial harvest in operation

Article by Jo Drummond, Senior Researcher – Cereals, Foundation for Arable Research