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. WORDS AND IMAGES SUPPLIED BY FAR

IMAGE: A sticky trap, used for monitoring the number of pest and beneficial insect species in a crop. Monitoring is the key to successful integrated pest management (IPM).

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. 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. A Lighter Touch is a $27 million, seven-year, programme jointly funded by government and industry. It will address the challenge of meeting consumer demands for safe food that is produced under sustainable pest management programmes while also being gentle on the environment. Government funding is via the Ministry for Primary Industries Sustainable Food and Fibre Futures Programme. It is led by Horticulture NZ. FAR is a funding partner and research contributor.

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.

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

TREATMENT

Bare seed Poncho® Mean P value LSD (p=0.05) CV (%)

BYDV SEVERITY* (%)

0.9 0.8 0.85 BYDV-PAV INCIDENCE** (%)

0 0 0 YIELD (T/HA) TSW (G) SCREENINGS >2 MM (%) TEST WEIGHT (KG/HL)

8.5 38.6 1.8 63.2 8.9 38.3 1.5 61.7 8.7 38.5 1.7 62.5 0.1 0.4 0.2 0.3 0.5 1.2 0.6 4.3 2.5