Significant cold weather in January and February has increased the likelihood that the emergency derogation for the use of a neonicotinoid seed treatment to control virus yellows in sugar beet will not be triggered this season.
It is reported by Farmers Weekly.
The derogation is based on a Rothamsted virus model predicting that over 9% of the sugar beet crop will be infected with virus yellows by the end of August. It uses a combination of winter temperatures with aphid population dynamics to make a forecast on 1 March.
A preliminary forecast released by the British Beet Research Organisation (BBRO) said there was now only a 16% chance that the 9% threshold will be reached when the final forecast is made. By contrast, last season the model predicted that 70-90% of the crop would be affected by virus yellows.
Up until mid-February the average temperature during January and February was below 3C, which is below the average temperature recorded in the “Beast from the East” year of 2018, when the virus yellow forecast was for 6.5-9.8% of the crop to be affected, BBRO’s bulletin says.
As 2021 is colder, there will likely be a greater suppression of aphid numbers than in 2018. A mild spell during the remainder of February will have a relatively small impact on virus levels, as aphid populations have already been affected by the cold.
A mild spell during the remainder of February will have a relatively small impact on virus levels, as aphid populations have already been affected by the cold.
As well as reducing the number of aphids, the cold weather will also delay first flights and their peak.
Last season first flights were in late March as many beet crops were emerging; this season that will likely be delayed until late May or early June, with the peak not until the end of June or early July compared with mid-May.
That will mean beet crops are likely to be at a more mature growth stage compared with the young, highly susceptible stage in 2020 when aphids arrive, the bulletin adds.
If the neonicotinoid derogation is not triggered, this advanced growth stage, plus the use of foliar insecticides where required, will be key to controlling virus transmission.
BBRO data suggests highest yields come from drilling sugar beet in March, with around a 6-8% yield penalty from delaying drilling into mid-April.