Look for aphids on leaves on warm sunny days, and consider an aphicide if aphid colonies can be found on 50% of plants. Aphids are probably not going to start flying into crops until late April or early May, but there could be aphids on crops that have survived the winter. Note that if you are seeing ladybirds or hoverflies in crops then hold off on any aphicides, as these specialist aphid predators will do a good job of mopping up aphids. Check crops again a few days later and if aphids are still on 50% of plants then consider the aphicide option.
Note that there is evidence of resistance in the grain aphid to the pyrethroid insecticides in Scotland, so use these with caution and only treat crops if thresholds have been exceeded.
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