The key pests to consider in winter oilseed rape this autumn are flea beetle, cabbage stem flea beetle, peach-potato aphid (and turnip yellows virus – TuYV) and later in the autumn rape winter stem weevil.
Slugs are also a perennial problem, and reliance on fine seedbeds and slug pellets (metaldehyde and ferric phosphate) remains for this pest, with careful use of metaldehyde a necessity to avoid contamination of water. See our article on slugs for more information on how to manage them.
Flea beetle damage is seen as small holes in the cotyledons and first true leaves of the emerging rape crop. The beetles are mainly active during dry soil conditions, so be prepared to spray with a pyrethroid insecticide if feeding punctures are present on
germinating plants. Once 3 leaves have emerged, there is no need for treatment.
Cabbage stem flea beetles also cause shot-holing of leaves, but in addition, they 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.
One way to assess the risk from cabbage stem flea beetle is to look at the trailers during the harvest of this seasons winter oilseed rape – the beetles will be caught up in the harvest and can be found on the trailer and in the harvested seed. They will cause no harm to the seed if taken back into the store, but their presence is a ‘heads up’ that the beetle poses a threat on the farm to the next winter rape crop. We would welcome any sightings of these beetles during the rape harvest.
As with the smaller flea beetles, pyrethroid insecticides (see list below) can be applied if the following damage thresholds for cabbage stem flea beetle have been exceeded:
- >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
Note that there is some concern that cabbage stem flea beetles in Scotland may have some resistance to pyrethroid insecticides (alpha-cypermethrin, beta-cyfluthrin, cypermethrin, deltamethrin, lambda-cyhalothrin, tau-fluvalinate and zeta-cypermethrin), as resistance is widespread in English populations. Only use an insecticide treatment if these damage thresholds have been exceeded but bear in mind that control may not be absolute due to resistance.
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. Look for aphids on the leaves (including the underside of the leaves) from crop emergence and if aphid colonies are present there are three options available to reduce the threat from TuYV:
- pymetrozine (various products) – note in use up on-farm stock period, so not available to purchase
- thiacloprid (various products)
- flonicamid (various products)
If growers had problems with plants in this seasons’ crop that produced extra lateral shoots or were stunted when you were expecting stem extension, 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 CSFB. 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 (e.g. Alert, Angri, Chimpanzee, Fury 10 EW, Minuet EW) 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.
Using crop intelligence information to decide whether insecticide treatments are required will be a key decision-making tool this autumn.
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