Willow-carrot aphid migration forecast tool

This tool uses weather data to estimate when willow-carrot aphids are likely to migrate into carrot crops.

As part of monitoring efforts, use this tool to identify risk periods and target control in carrot crops.

Learn about the main aphid species of UK carrot crops

How the forecast tool works

Updated daily around 11:30, the tool uses the following weather data sources:

  • Observed data (2021 and historic) used for the Pest Bulletin
  • Official Met Office weather stations (2021 and historic)
  • Citizen-shared data, through the Met Office WOW network (2021 only)
  • Forecast data from MET Norway

These multiple data sources provide a good indication of the year-to-year and region-to-region variation in the start of the willow-carrot aphid migration period.

The forecast is based on accumulated day-degrees (D°) from 1 February (base 4.4°C).

  • After approximately 325 D° aphid migration starts
  • After approximately 451 D° 10% of aphids area caught

The evidence behind the tool

The degree-day estimations are based on work by Professor Rosemary Collier at the University of Warwick. She used Rothamsted Research suction trap data to estimate the mean number of D° from 1 February until the first aphid of the year is caught in a suction trap (the start of the migration to carrot) and when 10% of aphids are caught.

How to use the forecast tool

The tool dashboard has three pages: 'Compare sites', 'Compare years' and 'Forecast' – which can be navigated to by selecting from the top-left menu, followed by clicking the 'GO' button.

Compare sites page

Track and compare the progress of day degrees in 2021 for selected locations.

The map on the left shows 40 locations across England, Scotland and Wales for which temperature data is available in 2021. Clicking a location on the map will:

> Indicate the predicted progress towards the start and 10% migration in 2021 in the colour legend above the map. Note that once the day on which 10% migration is predicted is in the past, the diamond will no longer be shown.

> Display the accumulated day degrees (based on observed temperature data until yesterday and forecast temperature for today and the next two days) for a particular location in the graph on the right.

Select more locations by holding the ‘Ctrl’ button or using the 'Rectangle select' button on the map.

The 'Date range' slider at the top can be changed to see/compare the accumulated day degrees over a particular time interval.

The graph can only display a maximum of 3000 data points (i.e. only 20 locations by end of June) simultaneously. Narrow down the number of locations by 'Source' or by 'Country and region' by selecting from the dropdown list at the top.

Compare years page

This uses historical weather data to display the range in (predicted) dates of the start and 10% migration in different years.

The map on the left shows 38 locations across England, Scotland and Wales for which historic temperature data is available. Click a location on the map to display a comparison of 2021 (amber line) with other years (black lines) in the graph on the right. Note that the other years are projected on 2021 calendar date based on the equivalent day of year, not date, to include 29 of February in leap years.

The default view contains the years 2011–21, as well as selected years for which migration was predicted to be particularly early/late. You can choose different years from the ‘Years’ dropdown box. Note that the graph can only display a maximum of 3000 data points (~20 years) simultaneously.

Holding the 'Ctrl' button while clicking on a year in the legend above the graph will highlight the corresponding line in the graph.

Due to missing/incomplete weather data, not every location is necessarily included in every year.

Forecast page

This uses historical weather data to create a dynamic forecast of the expected date range of the start and 10% of migration.

The map on the left shows 38 locations across England, Scotland and Wales for which historic temperature data is available. Clicking a location on the map will:

> Indicate the predicted progress towards the start and 10% migration in 2021 in the colour legend above the map. Note that once the day on which 10% migration is predicted is in the past, the diamond will no longer be shown.

> Generate a forecast in the graph on the right; the amber line for 2021 includes observed temperature data until yesterday and forecast temperature for today and the next two days. After that, the black lines for historic years are plotted to visualise the range of potential day-degree trajectories for the equivalent days of the year.

Due to missing/incomplete weather data, not every location is necessarily included in every year.

Locations can be filtered by 'Source' or by 'Country and region' by selecting from the dropdown list at the top.

Learn about the main aphid species of UK carrot crops

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