Keep an eye out for the flea beetle in oilseed rape crops

Farmers who planted oilseed rape should be keeping an eye out for the flea beetle in oilseed rape crops, or the cabbage stem flee beetle to give it its full name.

The flea beetle leaves behind shot holes in the leaves of oilseed rape plants and has decimated crops of oilseed rape in the UK in recent years since the neonicotinoid seed dressing used to control the pest was no longer permitted for use on oilseed rape crops.

In the UK, the flea beetle has spread from East Anglia to Wales and has made its way to parts of Scotland.

The cabbage stem flea beetle has shown resistance to pyrethroids in the UK. Farmers are advised to use full rates where numbers meet the threshold to spray, and if this does not work they should suspect resistance to pyrethroids in the population and avoid the use of pyrethroids in the future.

In September adults migrate into crops. From October, the adults lay eggs and feed on the leaves of crops. Once temperatures drop activity reduces.

UK trials have shown that a warm autumn favours egg laying and the early hatch of larvae. A temperature drop may help to prevent the flea beetle from migrating into crops and damaging leaves.

The Agriculture and Horticulture Development Board (AHDB) in the UK advises farmers to set traps to get an idea of the population, as well as keeping an eye out for shot holes and leaf damage in plants.

Yellow water traps should be left on the ground and filled with water and a small drop of detergent. The AHDB advises farmers to place two traps on the headland and two in the field. Numbers should be counted and the traps should be emptied weekly.

How to identify a cabbage stem flea beetle?

An adult cabbage stem flea beetle is 3-5mm in length, metallic blue-black or brown in colour. The beetles have long antennae, large hind legs and jump when disturbed according to the AHDB.

The larvae are white with very small, dark spots on the back with a black head and tail and three pairs of dark legs. Larvae can reach 6mm in length.

Larvae of the cabbage stem flea beetle. Image source: AHDB

When is treatment needed?

The AHDB has provided advice to farmers. During germination and emergence, crops should be treated with an insecticide where the risk is high and at the first sign of infestation as the crop is most vulnerable to damage at this stage.

Once the cotyledons have unfolded, treatment should be considered if flea beetles have eaten over 25% of the leaf area or if the crop is being eaten faster than it is growing. Traps should be set up at this stage.

Once three leaves have unfolded, treatment is advised if the flea beetle has eaten over 50% of the leaf area or again if the crop is being eaten quicker than it is growing.

From the time the first leaf unfolds (GS10) to the time when nine or more side shoots are detectable on the plant (GS29), the AHDB advises that treatment with an insecticide should be used if the number of cabbage stem flea beetles in a trap exceeds 96. Traps should be checked weekly.