Cabbage Root Fly Forecast 2026

The forecast outputs below are based on weather data provided by Agrii. The forecasts predict the timing of fly activity. They do not predict the number of insects present in the crop, as this depends on the size of the local fly population. For control purposes, we consider that the pest has become active when 10% of the insects have emerged/laid eggs and that the peak of activity has occurred when 50% have emerged/laid eggs. The predictions will change over time depending on weather conditions.

Late-emerging cabbage root flies

The forecasts in the table below are for ‘early-emerging’ cabbage root flies. There are some areas of the UK where a proportion of the cabbage root flies emerge later in the spring than would be expected. We call these ‘late emerging’ flies and they are genetically different from ‘early-emerging’ flies. Relatively large numbers of late-emerging flies occur in some parts of Devon (first generation egg-laying 10% 5 May and 50% 28 May) and south-west Lancashire (first generation egg-laying 10% 29 May and 50% 25 June).