During January larvae can be found feeding within leaf petioles and in severe cases the centre of the oilseed rape plant.
Simple trials work carried out in Denmark considered the impact of temperature on larval survival at -5 and -10°C. You will be pleased to know that once temperatures reached -10 and were sustained for 32.6 hours 50% of the larvae were dead.
When a temperature of -5 °C for 8 days was maintained mortality of first and second instars was 81.2 and 51.3%, respectively. Therefore with the “Beast from the East Mark 2” forecast for this week – half the larvae in your crops will be dead… maybe!
Why maybe? Well when the researchers considered weather data in Denmark from 1990 to 2013 it showed that larvae were rarely exposed to several continuous days at -5 or -10 °C. With Denmark being that much further North than the UK, we have the maybe answer!
Anything else which might help against these pests? Well the oldest leaves do turn yellow and drop off so, is it too much to hope that this will also result in the reduction of larval numbers?
ADAS considered this point and collecting what were classed as 'fallen leaves' from around the base of OSR plants. Upon dissection no larvae were recorded.
Thankfully we all know that last autumn everyone who planted OSR did not have a problem with CSFB and consequently there are no larvae to be found this January..