Cabbage stem flea beetle has been wreaking havoc on English winter oilseed rape crops for several years now due to the loss of the neonicotinoid seed treatments and resistance to pyrethroid insecticides.
We don’t know the detailed status of pyrethroid insecticide resistance in Scottish cabbage stem flea beetles, but there is likely to be some level of resistance here.
During the current oilseed rape harvest keep an eye on the sides of the trailers. The presence of any blue-black beetles in harvested oilseed rape seed or seen on the trailers is a warning that there could be a problem in the next winter oilseed rape crop from cabbage stem flea beetle. The beetles won’t harm the seed in the store and will be keen to get out of the store and find an oilseed rape crop to feed on,but seeing any is a good indicator of potential problems for the autumn sown crop on-farm.
There are several pyrethroid insecticides approved for cabbage stem flea beetle control, they but may not give full control. Cabbage stem flea beetles are in Scotland, but not at the levels seen in England. It is difficult to distinguish the feeding damage on the leaves at emergence from ‘normal’ flea beetles. The more common turnip flea beetles tend to prefer dry conditions for feeding, whereas the cabbage stem flea beetle is less fussy.
Once the crop gets to the 4 leaf stage, damage by the adult cabbage stem flea beetles is restricted to the older leaves. In addition to causing shot-holing of leaves though, the cabbage stem flea beetle lay eggs near plants and the larvae burrow into the stem which can lead to winter kill, no stem elongation or lodging in spring. However, in Scottish crops over previous years the adult beetle feeding damage tends to be worse than the larval damage – in England larval damage as well as adult damage is severe.
There have been studies in England to try and look at reducing the risk of damage from cabbage stem flea beetle including reducing/increasing seed rate, trap cropping, sheep grazing but all with variable results. Perhaps the best approach is to try and get the crop sown as early as possible into a moist seed bed to try and get rapid emergence and get the crop to grow away from the beetle damage.
Because of the risk of exacerbating insecticide resistance in Scottish cabbage stem flea beetles, sticking to the damage thresholds is recommended. These are:
>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
If damage approaches these levels, then consider applying an insecticide.
The other bane of the oilseed rape crop are slugs who prefer wetter conditions at emergence. Slugs can cause significant damage before crops get to the 3-4 leaf stage, after which they will be happy eating the older leaves. Consider slug pellets if the crop is showing significant damage and has a week or two to go to get to 3-4 leaves.
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