What It Tells You
The BYDV day-degree count is how close an autumn-sown cereal is to the second aphid generation. That generation spreads barley yellow dwarf virus plant to plant. Aphids develop with temperature. The count sums degree-days above 3 °C from crop emergence, or from the last aphicide, towards the 170 °C day threshold in the AHDB tool. A field at 40 of 170 has a reason not to spray yet. A field at 150 is the point to start walking the crop. The window closes at stem extension, when a new infection costs little. The count is computed by the BYDV Risk model. It is a thermal proxy for aphid activity. It is not an observation of aphids.How It Works
A daily job accumulates degree-days from a gridded weather archive and forecast. Emergence is inferred from the crop’s own degree-days since sowing. The state moves from accumulating to approaching at 75 per cent of the threshold. One notification is sent when 170 is reached. A new aphicide in the spray record moves the start date and the count begins again.
The degree-day count fills towards 170 while the crop is still young, and the window closes at stem extension.