The mild autumn conditions are extending the BYDV period of infection later into the year than would normally be expected.
Just when you would assume the potential risk from BYDV is receding, it is doing the exact opposite. This is according to Neil Watson, technical support manager for Hutchinsons.
He says: “The mild nights as well as the warm days are allowing for rapid multiplication - aphids can still take flight above 11degC and this is what we are seeing in the field.”
This is not yet being picked up from the suction trap data which might be partly due to the inevitable delay in collecting the data and getting it published, he adds.
The smaller the relative size of the crop infected the greater the impact on yield due to the lack of ability to compensate unlike a larger plant with plenty of tillers, Mr Watson says.
“This risk is normally the preserve of the early drilled crops with emergence coinciding with infection, yet this season it is the later drilled crops at risk as a result of the relatively wet October and resurgence of invading aphids.”
This could mean it is time for a follow up spray for the more advanced crops, he says.
“Yet again, some of the surveys carried out suggest a relatively high proportion of aphids landing in the crop are also carrying virus. “
“There is also some noticeable interaction coming into play from crop colour. The paler the plant the more likelihood aphids are being attracted to them.
"We know this is a real factor from previous studies where invading aphids are naturally draw to already infected individual plants, which enhances virus spread within the crop.”
The pale colouration this year could be as a consequence of nutritional shortages in newly emerged plants, or even the likes of volunteers oats with an wheat crop, of which there is plenty about this year, due to server shedding prior to harvest, Mr Watson says.
“This underlines the importance of being ahead of any potential deficiencies and keeping the crop as healthy as possible.”
“Once aphids have been observed within the crop, start counting for the 170-day degree spray threshold,” Mr Watson advises. “The exception is if large numbers are seen feeding on the crop, spray immediately, then start counting once reinvasion occurs.”
It is important to recognise that frosts cannot be relied on to kill these aphids.
“Firstly, we have had very few so far this autumn and more significantly, the grain aphid needs frost below -5degC to kill half the population. Even with regards to the rose grain aphid this figure is -0.5degC.”