Pest pressures mount in oilseed rape

Wet weather has encouraged slugs and grazing damage can be seen in many oilseed rape crops.

Treatment may not be needed if crops have reached 4 true leaves as they will be able to grow away from any further damage by that stage.  Due to the risk of metaldehyde ending up in water, use of metaldehyde slug pellets should be avoided unless strictly necessary, especially bearing in mind that metaldehyde pellets should not be allowed to fall within a minimum of 10 metres of any field boundary or watercourse. Ferric phosphate slug pellets are a viable alternative.

Damage from flea beetles has been reported in several crops, but as for slugs, once crops have 3-4 true leaves they can usually grow away from any further damage.  Regular flea beetles don’t like wet conditions so will be discouraged.  Cabbage stem flea beetle is still nothing like the issue it is in the south although it has been reported in several Scottish crops.  Resistance to pyrethroids is common in England and the status of the Scottish population is unknown so the SRUC crop clinic would be interested to hear of any reports of poor flea beetle control after insecticide treatments, and may be able to sample these crops for beetles to confirm their resistance through lab tests.

In addition to causing shot-holing of leaves, cabbage stem flea beetles lay eggs near plants and the larvae burrow into the stem which can lead to winter kill, no stem elongation or lodging in spring.  In Scottish crops the adult beetle feeding damage tends to be worse than the larval damage.

There are damage thresholds for cabbage stem flea beetle:

Photo: Martin Cooper, Flickr cc

• >25 % of the leaf area damaged at the 1 to 2 true leaf growth stage
• >50 % of the leaf area damaged at the 3 to 4 true leaf growth stage

However, with the likelihood of beetles exhibiting some resistance to the pyrethroid insecticides, an insecticide treatment should only be considered if these damage thresholds have been exceeded, but bear in mind that control may not be absolute.

If growers had problems with plants in this seasons’ crop that produced extra lateral shoots, or were stunted when you were expecting stem extension in the spring, then that could be a sign of rape winter stem weevil infestation on the farm.  Adult weevils don’t tend to move into crops until late September-mid-October, so would not be controlled by a pyrethroid spray targeting flea beetles or cabbage stem flea beetle.  Consequently, growers will usually have until the end of October/early November to apply a pyrethroid insecticide to control the weevil to prevent them laying eggs, as once the grubs hatch out and burrow into the stem of a plant they are beyond any insecticidal control.  A pyrethroid insecticide treatment can be tank-mixed with the light leaf spot fungicide treatment (check label for compatibilities) and gives good control of rape winter stem weevil if applied before any eggs are laid.  Delaying treatment into November will allow egg laying and hatch to happen, and the grubs will be protected within the rape stem.

Peach-potato aphids may well be carrying turnip yellows virus (TuYV), which, if transmitted into rape seedlings can reduce yields by up to 30%.  Peach-potato aphids are resistant to pyrethroid insecticides, so will not be controlled by the pyrethroid insecticides used against flea beetles mentioned above.  Peach-potato aphids have been caught in suction traps and water traps in higher numbers than average this season, although numbers have tailed off significantly over the last few weeks.  Look for aphids on the leaves (including the underside of the leaves) from crop emergence and if aphid colonies are present treatment may be justified.

Fiona Burnett and Andy Evans, SRUC for the Farm Advisory Service