Adult cabbage stem flea beetle, Psylliodes chrysocephalus, chew holes in cotyledons and early true leaves, giving rise to ‘shot-holing’ symptoms, which can result in stunting and poor plant vigour. If severe, beetle feeding can kill the seedlings even before they emerge. Larvae also bore into leaf petioles and, later, into the main stems, which can also affect plant vigour. The crop is most vulnerable at emergence, as beetles can feed on and destroy the growing point. Once true leaves appear the crop is more tolerant of attack. A warm dry autumn will favour egg laying and early hatch of larvae, coinciding with smaller, more vulnerable plants.
Oilseed rape is a crop which can compensate for substantial levels of damage by regrowth. Therefore monitor the crop. Control is only needed when there is high pest pressure. The AHDB currently suggests treatment if:
- Adults have eaten over 25% of leaf area at the cotyledon–2 true leaf growth stage
- Adults have eaten over 50% of the leaf area at the 3–4 true leaf stage
- The crop is growing more slowly than it is being destroyed.
- There are over 5 larvae/plant or over 50% of leaf stalks are damaged by larvae
An AHDB damage survey (for autumn 2015) estimated that 96% of the winter OSR crop area was under the threshold for
treatment at the three-four leaf growth stage i.e. only 4% was at risk. Winter
oilseed rape crop losses associated with adult cabbage stem flea beetle
activity in autumn 2015 were estimated at around 1% (6,000ha).
Cabbage stem flea beetle resistance to pyrethroids has been confirmed in the UK (See update here).
Neonicotinoid-treated seed cannot be planted following the restrictions imposed in December 2013.
The main natural enemy is the wasp Tersilochus microgaster, which parasitises larvae in spring. All parasitoids may be vulnerable to pyrethroids. This means that repeated applications of pyrethroid may do more harm them good especially when the target pest has evolved resistance to them. Parasitoids remain as pupae in the soil until the next crop, and may be damaged by ploughing. Minimum cultivation after rape favours parasitoid survival.
Cultural control: Early drilling is recommended to get the crop established before migration. Use slightly higher seed rates to compensate for plants being lost to beetles. Providing good nutrition and favourable growing conditions shortens the vulnerable early growth stages and helps plants survive flea beetle attack. Flea beetles are extremely mobile and can easily migrate from field to field which means crop rotation is not usually a viable management strategy for this pest. It has been suggested that crop plants not silhouetted against a bare-soil background, for example when sown into a cover crop and are less obvious to the pest. Yellow sticky traps may help to reduce infestations.
Cabbage stem flea beetles hatch at around harvest time and overwinter in field boundaries before starting to feed and lay their eggs from early September. The presence of suitable field boundary habitats could aid retention of beetles by previous crops where volunteer oilseed rape plants would provide an early food source.
There is a need to develop new interventions for this pest following the loss of neonicotinoid seed treatments. CROPROTECT is making enquires about this. Plant breeders are working on development of OSR varieties with an "early vigour" trait. This is a promising development because if the crop can be established and grow faster than it is eaten by the beetles it might stand a better chance.