Diamond-back moths (DBM) with resistance to pyrethroids may now be able to overwinter in the UK, according to AHDB Horticulture.
In 2016, three DBM samples, from Lincolnshire, Suffolk and Scotland, tested by researchers at Rothamsted Research in 2016 were found to be resistant to the pyrethroid class of insecticides.
Pests with resistance to pyrethroids often have reduced fitness levels and don’t usually survive through winter.
Tests conducted in 2017, however, suggest that this season’s moths are probably direct descendants of the 2016 population, indicating they probably overwintered in the UK.
Diamond-back moth may infest crops throughout the UK and the larvae can cause damage to the foliage of cabbage, cauliflower, broccoli, Brussels sprout, kale, Chinese cabbage, swede, turnip, oilseed rape and radish.
Large infestations can cause damage to up to 100% of plants but yield losses will depend on the impact of damage on plant growth and quality. Good control is particularly important where the marketable part of the plant is damaged (cabbage, cauliflower, broccoli, Brussels sprout). Plants with low levels of damage usually survive but this may affect uniformity within the crop.
Infestations are sporadic, so control may not be necessary in every crop in every year. Weather that favours migration from continental Europe increases the risk of infestation, as the diamond-back moth does not overwinter in the UK in large numbers at present. The moths are relatively poor flyers but may be transported long distances by the wind.
The diamond-back moth is an important pest worldwide, particularly in tropical regions. Over the years, populations in these areas have developed resistance to almost all of the insecticide groups to which they have been exposed repeatedly.